For reviewing purposes, I skipped over the short Chapter 6, focused on Sky Portal operations, as I’ll do that chapter after I have a chance to connect to my tablet and test some of the operations. I thought of doing the same for Chapter 07, Connecting a PC, Mac, Tablet or Smartphone to Your Tablet, but it’s a short chapter, and easily dispensed with here.
Some of the highlights:
P.165 – Wired Connections for RS-232 Hand Controls…I knew that most of the wired connections used a USB to Serial adapter, and plugs in to the RJ-22 Jack (I thought it was an RJ-45, but apparently not!). However, one “new” thing in the guide is that there is a way to do a wired connection to a tablet or smartphone using SkyWire + Sky Safari with an iOS device. I had no idea there was an option for a physical wire connection. I might have skipped the dongle wifi if I had known that earlier, as there can be challenges maintaining connections.
P.165 – Wireless connections…As with the wired one, I didn’t know people had attempted doing it with BlueTooth but it sounds way more exciting than the wifi connection. Out of my abilities, most likely, but I’d be willing to buy one if it is ever perfected. I’ll stick with my wifi connection (SkyPortal module) although good to see the range of other options available.
P.168 – Software to Control Celestron Telescopes…I don’t have a lot of interest in this aspect, to be honest, and I have a copy of NexRemote that came with my scope back in the day. However, what I found incredibly useful was the suggestion that when any software asks you for your type of scope, if it doesn’t offer you an option for your specific model, it suggests using NexStar GPS or CPC…very useful to know! I also like the multiple refs to more materials on the author’s website.
P.169 – Troubleshooting Serial and USB Connection Problems…I have had problems with my wifi connection in the past, but I read the other connection section anyway. Interesting that NexStar Observer List software (NSOL) is only able to connect through the ports on the bottom of the hand-controller, not the extra ports on the mount. Under the Troubleshooting WiFi Connection Problems section (P.171), I was intrigued to see that when connecting to Android, it may note that the internet is not available (yes, I’ve had this!). It says you have to say “YES” and “DON’T ASK AGAIN” or Android will block everything. Nice. I also didn’t know that I could connect both my phone and the telescope through my router at home (if I was in the backyard) and that connection works better. While that would be helpful at home, I’m curious if that method (called Access Point mode) would work with my iPhone set as a hotspot. Something to keep in mind for the future.
A good short reference chapter. Hopefully, I won’t need the trouble-shooting part as much.
Chapter 5 (of Michael Swanson’s “The NexStar User’s Guide II”) is entitled NexStar and StarSense Hand Control Operations and you would be right if you assumed this was going to more like a software manual than a telescope manual. It is a highly specific reference manual for the use of the hand controller and reading it page by page is for the hard-core geek only. But it is full of fantastic info that I didn’t know enough about, even though I’ve worked with the hand controller for years. These are the highlights for me:
Pg. 128 – Adjusting slew rates…I knew how to manually adjust the slew rate (MOTOR SPEED then press 1-9 where 9 was the fastest). I really only use 3, 6, and 9 for rates though. 9 if I’m going a great distance, 6 if I’m adjusting for centreing, and 3 if I need very fine adjustments while doing astrophotography and looking at centreing something on the screen. However, I didn’t know that if you were slewing in one direction (say left) and also held down the opposite button (i.e., right), it would temporarily speed up to rate 9. I have no idea when I would ever use that, but it’s good to have. I suppose if I knew I was going from rate 6 to something that was farther away and wanted 9 temporarily and then back to 6, I could do the temporary speed boost.
P. 130 – SAO stars…I knew the list was there, I didn’t know that you could type in a few digits and that served as a “filter” so that 0004 would give you a list of 000400, 000401, 000402, etc. with only the visible ones at that time showing up in the list. Not sure I would ever use it much though.
P.130-131 – Sky Tour…I use the sky tour regularly, and have always wondered about the methodology it uses. While there isn’t much detail on that in the manual, it does note that it is the same Sky Tour in my son’s 4SE as well as my own 8SE, with no adjustment to the software to recognize that some objects might not be suitable for that particular scope.
P.132-133 – Anti-Backlash/Backlash Compensation…The previous chapter already covered this from a physical setup perspective, this gives you the walk-through for the software side. Still need to do this.
P.135 – Slew Limits…I have had an occasional problem with my slew limits, and I didn’t really understand how they were working. The Slew Limit tells you when something is too far up or down for your scope to see, perhaps because of what you have attached to the scope (like a camera) or could be for permanent horizon. I set my scope up so that nothing CAN hit the base any more and I almost NEVER go to zenith anyway. But I occasionally get slew limits for something that is too low. What I didn’t realize is that my setup is throwing off my limits. I physically install my tube with the mount vertical and pointing up; my son does his vertically, but with the mount pointing down. We install the tube, turn on the scope, and go to horizontal. I didn’t realize that the scope THINKS you start horizontal. So, we need to make it horizontal, and then restart the system to reset the counter to horizontal. We haven’t had any problems, but there were a few objects here and there that I thought we should be able to see and the scope said “Nope”. Might be why. Need to adjust that as part of my setup technique.
The Filter Limits (P.136) is the temporary version of slew limits, in a sense, and is usually related to horizon. I wish it would allow you to just point to your horizon and say “Use this as your lower horizon limit” and to allow you to do it in multiple directions. I have zero viewing sites that have all 4 directions with the same horizon. My backyard is great to the South, maybe 15 degrees. East and West are more like 60 degrees with houses. North is perhaps 45 degrees. If I set it by the limits East and West, the highest, I’ll miss out on a ton of objects to the South and North. At the Carp site, great views to the West, but North and East has a hill and South has some trees. Not huge problem, they would work okay there. The FLO site has trees to the East and North, but better views to South and West. AstroPontiac is fantastic in almost 225 degrees, but the Gatineau Hills right behind it for the other 135 degrees is another story. And at the in-laws’ cottage, West and South are awesome, while East and North are about 85 degrees straight up for trees. I know some of the software options for tablets and planetariums will sometimes allow you greater flexibility for simulating your view, not sure it changes much for scope control though.
P.137-138 – GoTo Approach…This was covered for the physical side in Chapter 4 on alignment, and notes the default settings for back-heavy and front-heavy scopes, with the recommended settings over the defaults. I want to try this soon too.
P.140-141 – User Defined Objects…I had seen this in the menus but really had no idea what it was about, nor did I understand the sub-menus much. I loved the explanation on these two pages to know there are basically 99 memory spots to remember UDOs. If I choose my own object in the sky, and pick SAVE SKY OBJECT, the computer will add that location’s coordinates to memory, but unfortunately with no option to edit the name. SAVE DATABASE OBJECT does the same but for objects already in the main database, and so it retains the name too. Interestingly, you could program your night’s viewing ahead of time using this method as the scope doesn’t need to be aligned. You would essentially be creating your own personalized star tour. ENTER RA AND DEC would do the same, albeit without a name. When you want to use the list, pull up UDOs and choose GOTO SKY OBJECT. I’m curious to see if I can pre-program those with software, but I guess if I’m using the apps, I don’t even need to use that as they allow custom lists / catalogs anyway.
P.141 – Display settings…I need to check to see if I have LCD Contrast and Bold Font set for my hand controller, I sometimes have trouble reading it. And I can do the same for my son’s.
P.142 – Cord Wrap Prevention…It sounds strange to say, but I hate the Cord Wrap Prevention. Why would I hate it? Because while I like the general premise of stopping the scope from getting all tangled up and pulling on the power wires, my Sky Tour starts near M110 and the Andromeda Galaxy. Then, as I go along, and as I get close to due North, the scope can get a bit confused…it hits the cord wrap limit (North), and then to get to the next object (across the limit line), it goes ALL the way back around, as it’s supposed to do. I would much prefer if that limit line was close to M110 so that for a sky tour, I could do almost all of it before running into that problem. Particularly as a couple of elements shift a bit in the order, and one will be past the limit (all the way back around) and then an object or two later, will be back across the limit (so all the way back around again). If I could adjust it closer to the right (East/South) of M110, I would be able to do almost the whole tour without that problem. And since I am rarely in that section of the sky in most of my sites, it would be a good limit anyway. Except the only setting seems to be ON or OFF. Sigh. I’ll check with the author to see if there is any way to edit this.
** Update: I heard back from the author, who was very polite considering I missed something he had already explained in the book. I just mis-read it, as I was looking for something that says “here’s how to edit”, and that isn’t quite how it works. Apparently, the default setting is due north (I thought was “near North”, but no, it is actually North) and since I always leave it on, I’ve never see an option to change it. But if I turn cordwrap OFF, and then turn it back on, it will then ask me to point to the exact opposite point of where I want cordwrap to be. At present, the default of SOUTH means the mount thinks my cable starts to the south, so when it gets half-way around to NORTH, it activates the cord-wrap. So, if I want to activate cordwrap “east of M110” which has RA of 0h, I need to toggle CORDWARP and then point the scope to an RA of 12h (directly opposite M110). That initially seemed ludicrous to me, but the author further explained that it is asking you to point to the centre of the available range of motion. Ah-hah! That makes sense! Except then I found out that if I’m using Sky Portal, I can’t change the setting, it’s always due South. Sigh. Cuz I want to start using that! **
P.147 – Direction Buttons…I already mentioned this in the physical setup / alignment section as it messed me up. I had inverted controls set ON by default, and it meant that my UP and RIGHT should have been DOWN and RIGHT, so my alignment process always included altitude slippage. FOR FIVE YEARS. Arghh!
P.147/148 – Hibernate…I have wanted to try to see some planets during the day for a long time, just to see what you could see. But it’s impossible to do an alignment, right? Well, in addition to the option to do a solar system alignment (even on the moon or sun), you can also align during the night, and then put your scope in “hibernation” mode like a laptop. Then, when you want to use it during the day, you can wake it up, don’t MOVE anything, tell it the current time, and voila, you’re back in business. It already knows your location, and where it is in relation to the stars a few hours before when you did the first alignment, now it just needs to know how long it was sleeping. If you do ANYTHING else before entering the new time, the alignment is lost. I really want to try this, except for one little thing. Well, two things. First, dew would be a pain. Second, it would work best with a great alignment in the wee hours before dawn…yet I am NOT a morning astronomer. I even struggle to stay up much past 1:00 a.m. But I do want to try it sometime.
P.150 – Version…I figured SOMEWHERE in the handset there must be some indication of what version of firmware is running, so I will have to look that up, now that I know how!
As I said at the beginning, this is the hard-core manual side of the software. Most people would likely use it as a reference guide, “How do I….”, but it was fascinating to read the whole thing at once and see other settings that I might use. Great chapter.
Chapter 4 of Michael Swanson’s “The NexStar User’s Guide II” is entitled Alignment and is the chapter that made me want to buy the book and read it cover to cover. Readers of the blog know that I struggled with alignment for my NexStar 8SE (Solving alignment problems with the Celestron NexStar 8SE) and Michael’s online resources were helpful in resolving some of them, or at least narrowing the problem. I even held off buying the book because he said Guide II was coming and I should wait over Guide I. Then I managed to solve most of the alignment issues, and so when his book arrived, I didn’t devour the whole book right away. I just cruised through the Alignment chapter. Then this past summer, I went back and re-read everything in order.
Here are some highlights from Chapter 4:
Backlash compensation (P.87): A great element for those of us with slightly older scopes whose mounts are no longer “factory fresh”. My son’s new 4SE? Dead tight regardless of the direction I’m slewing. My 8SE? There’s a bit of a lag. I tried adjusting this 3 years ago and just got confused. The new guide has it very clearly laid out, and I’ll be attempting a fix later this month.
GoTo Approach (P.88): Tied to the backlash compensation is a setting that depends on whether your scope is back-heavy or front-heavy. I have always felt that my scope was “front-heavy” the way I pushed it forward, but I reached out to the author to ask, and he confirmed that the SE series is still generally back-heavy given the weight of the mirrors, unless you are using a metal dew shield to make it more front-heavy. I need to change this too.
At the time of this writing, the default settings for the NexStar+ and the SkyPortal app are negative for altitude (up/down). The default settings for the NexStar version 4 and StarSense hand control are positive for altitude. When used in alt-az mode (no wedge), back-heavy scopes perform best with this set to positive, while front-heavy scopes should have it set to negative. This helps to minimize the effects of gear backlash on GoTo accuracy.
Accurately Center Alignment Stars (P.89):The end of the section outlines the doughnut method for doing alignment, i.e. defocusing a star until it is a large disc that makes it easier to see how well it is centred in the eyepiece/field of view, a great method.
Final Movements When Centreing Alignment Stars (P.89/90): Regardless of backlash compensation, or your GoTo Approach above, the scope is always going to have its drive tighter in two directions than the other two. In other words, for azimuth at least, going right is always going to be tighter than going left. When you go left, there will always be a bit of slippage. So you always want the “last” movements when centering to be to the right to keep the drive as tight as possible, no slippage. Any slippage will increase your margin of error with the alignment.
The PROBLEM comes with the up/down movements. And the source of multiple years of alignment problems that I couldn’t figure out for the life of me. All the online tips say that the final movement should be: Right and Up. In other words, the tightest last movement should be to the right (as above) and then up. In theory, this means that DOWN will have slight slippage, UP will be tight. And that’s what I was doing.
Except I wasn’t. Here’s the bombshell from the guide:
To help minimize the effect of gear backlash, your final movements when centering the alignment stars are essential. The default settings with the NexStar+ hand control and the SkyPortal app require you to center each star with final movements using only the right and up arrow buttons. The default settings with the NexStar version 4 hand control require you to center each star with final movements using only the right and down arrow buttons.
While the bolding is mine, I would like to THINK that if I had read that, I might have saved myself a few years of pain. My handset did indeed have the settings reversed — so I was doing “RIGHT AND UP”, but my handset was inverted, and I should have been doing “RIGHT AND DOWN”. Son of a biscuit. Which meant that while my azimuth settings were relatively fine (left and right were tight), my altitude was consistently off. Only when I was talking to the guy who programmed the hand controller one night and I mentioned that the setting was inverted did we together have a eureka moment. Every single site I found online, every reference, everything I saw said NexStars should go RIGHT and UP. It never occurred to me that my default settings were different. I have no idea if I had read that bolded section above if my brain would have sparked or not. It sparked when I was out with the programmer trying to solve my alignment issues. Alas, I read it in the guide almost three years later. Grrrr.
Cancelling a Slew (P.91): This is one of the most useful things I learned in the whole book, and it is so simple. Once you press enter on the hand control when selecting an object to view, the GOTO features take over and it starts slewing there. I’ve tried hitting BACK, a few other buttons, nothing seemed to stop it until it got to its destination. One time that was a zenith location and my diagonal was hanging down low, and yep, it smacked the mount base. It ground so hard, I needed Celestron to fix it and my mount was gone for 2 months. I got to this section, and literally I was like, “Are you kidding me? All I have to hit is one of the directional buttons? Really? How did I *not* figure that out before?”. Sheesh.
Various alignments (P.92-99): While I feel that I knew most of the details about Sky Align, Auto Two Star Align, and Two Star Align, mostly from reading various posts online (some by the same author), I knew very little about One Star Align or a Solar System Align. I’m itching to try the Solar System align to see if I can do some stuff during the day (with appropriate solar filter in place, just in case it passes the Sun). I confirmed with the author I can even do this on the Moon or the Sun. Woohoo! 🙂 Maybe I’ll try it the day of the Mercury transit this year (November 11th) as I’ll be outside anyway!
Using SkyPortal (P.99-103):I have the wifi dongle, and I have found the connection with the tablet or smartphone to be a bit finicky at times. Nevertheless, I am inspired to try it again, partly because I hadn’t realized you could add more stars to the alignment process — up to 10 in total to improve GoTo and tracking. I desperately want to try that. I’ve thought of buying a StarSense for improved alignment, but I am willing to try this first. Might want to upgrade firmware though.
I have always wondered what stars are in the database for possible use during alignment, and a DLable manual on the author’s website notes that
Two Star alignment has 247 possible objects [a somewhat eclectic list, as some go as faint as 5.4 (Polaris Australis) but even a full list of magnitude 4 would well surpass 250 objects];
Auto Two Star and One Star are limited to 56 possible objects (again, a bit odd since there are 88 constellations and you would think at least the brightest in each would be available); and,
Sky Align (the generic three star approach) has 82 possible objects.
And here’s the weird part…you would think it would be cascading / shrinking lists — 247 for 2-star, 82 of those for Sky Align, and 56 of those for Auto 2-star or 1-star. Except Albireo and Algenib are available for 2-star and 1-star but not the more general Sky Align. Albireo is a double-star, but there are other doubles in the list (Mizar, for example); I don’t know what’s unique about Algenib. Like I said, weird. I assume it has something to do with the math for Sky Align, some sort of conflict if you use those two.
Sync (P.123): I had learned how to do this, as well as Precise GoTo (P.124), when I was out observing with the hand controller programmer, but it was good to see how to use a semi-permanent “additional alignment star” for a series of viewings or a local star for a one-time alignment boost.
One thing I have never seen listed anywhere (and it’s not in the guide) is what to do when something happens and your alignment changes during the night. You can replace alignment stars, sure, but my question was more specific — what happens if you “kick” the tripod by accident (or more likely someone passing by kicks it) — do you reboot the scope and start over? There’s no menu option to start a new alignment from the beginning. I reached out to the author of the Guide II to ask. He confirmed that while you may have to do a reboot if the legs are moved considerably (i.e., the alignment may fail when you do the first replacement), you can always try just repeating the existing stars again rather than starting fresh or saving two new alignment stars. Good to know.
And this concludes Chapter 4. This chapter alone was worth the purchase price.
I like to blog about non-fiction books as I read them, as it helps me both synthesize and retain the info. Chapter 3 is a general overview on all the computerized scopes in Celestron’s arsenal, and I found myself liberally highlighting as I read it. Here are some highlights:
P.57 — a great overview of the different processors in the mount, including some of what the processors in the mount do (drive the motors) and the hand control (determining the correct tracking rate)…I was initially confused about something in the manual, as it said the hand controller sends the update to the mount every thirty seconds. I wasn’t sure if this meant it sends a “movement” command every 30s (as that seemed too slow) or a rate adjustment every 30s (adjusting the existing tracking speed), and the author confirmed the latter. Now rereading it, it was already clear, I think I just confused myself;
P. 69 — I was surprised to see that there are CPC mounts that are actually quite well-designed for accurate tracking and thus support astrophotography. Maybe something to aspire too, instead of going Equatorial some day;
P.78 — Good overview of the various types of hand-controllers, …I think I have the NexStar+ serial (Celestron button + RJ45-like jacks). I updated when I first got it seven years ago, but nothing since. From the author’s website, I clicked over to the Celestron list of bug fixes and updates…the only bug fix that seems really significant to me are the GPS rollover ones this year (like a Y2K problem), but I should probably do those. Fingers crossed I don’t brick my mount (I’m always nervous updating firmware for anything!);
The volume of material in the chapter is huge, although more of a reference chapter when you need specific info about different scopes, in my view.
I mentioned previously that it helps me to blog about NF books as I read them — kind of like transcribing my notes into something more coherent that my brain can recognize. Chapter 2 on Astronomy Basics in the NexStar User’s Guide II is a similar outcome. Here are some thoughts:
P.12 — Constellations…There are 88 constellations that divide the sky…I always assumed there were WAY more than that. I’m curious if each of them will fit in an EP at 50x magnification which is my smallest / lowest power option;
P.13 — Meridian…I knew horizon and zenith of course; I had not heard the term “meridian” to indicate a line going from northern celestial pole to southern celestial pole through the zenith to divide items west and east;
P.15 — Magnitude…I wasn’t aware that a 1 magnitude increase in power equaled a 2.5x increase in brightness…I guess I just assumed a flatter linear scale. My son’s 4″ has a magnitude limit of 12.5, while my 8″ has 14, an increase not of simply 1.5 but of closer to 1.25 x 2.5 = 3.125 i.e. I can see way fainter stars. However, I have never figured out what a reasonable limiting magnitude in my backyard is, or anywhere else. Now that I see that magnitudes of clusters or galaxies are total luminosity of the object, without adjustment for diffusion across the object, I can see why supposedly bright DSOs do not show up clearly in my EP;
P.17 — Seeing conditions…I confess I have always been somewhat fuzzy on the meanings of seeing and transparency, etc., and lumped them all together. I like seeing the definitions clearly explained as seeing = distortion caused by the layers of air; transparency = particles in the air; and light pollution = man-made light that is directed upward;
P.19 — Observation…everybody includes descriptions of “averted vision”, but rarely have I seen it explained so simply and easily i.e. that the central retina is good for straight-on but the off-centre parts are good for detecting fainter objects, so looking to the side of the target lets the off-centre “see” the fainter areas. Equally, I like the description of using higher-magnification for bright, compact objects (like planets, globular clusters) while lower is better for fainter, distributed objects (like nebulae, galaxies, and open clusters);
P.21 — Observation log…I already designed my own sometime ago, and I also am hoping to create my own app version some day. In the example given, the only thing I see that I don’t have is limiting magnitude, degrees above horizon, field diameter for observing and the constellation, and I generally ignored the first and the last while I didn’t have the info for the second and third;
P.23 — Astrophotography…I was hoping for a bit more on this, but honestly, it could be a whole book on its own, and the suggested materials were good enough starters (i.e. ebook on SkyAndTelescope.com);
P26+ starts to talk about key elements of basic observing, and most of it is stuff I already knew:
Get a moon filter;
The only thing you can see for Mercury is phases;
Similarly for Venus, no real details can be seen;
Mars is mainly pink- to red-colored disk, with lighter-colored polar caps and various dark surface markings;
For Jupiter, 4″ scopes (like my son’s) can see banding, while larger scopes can see details in the bands (supposedly) and moon transits;
For Saturn, you can see shadows of the rings on the surface as well as five of Saturn’s moons;
Uranus will be a fairly bright disc, blue in color;
Neptune can be distinguished from surrounding stars;
Pluto is only seeable in 6″ scopes and above, and then only as a pinpoint dot;
DSOs i.e. anything outside of our solar system are often very faint fuzzies;
Open clusters are stars that were born together and moving apart (didn’t know that);
Globular clusters are balls of stars held tightly together by their mutal gravity (didn’t know that either) and look good under higher power;
Nebulae are clouds of gas and dust, but bright nebulae have stars in them (reflecting nearby light or emitting their own) while dark nebulae block background stars;
Planetary nebulae are similar to planets in size and appearance, and are the remains of old stars, and higher power in higher scopes can show colour;
Galaxies are the bane of my astronomy experience as everything is always just a big or small faint fuzzy, mainly as I never get to a truly dark site…spirals (arms around central bulge), elliptical, dwarf/irregular … hard to tell what I’m seeing;
Once you get into the equipment, my knowledge drops off past the names. Sure, I can calculate that my 8″ scope has 4x the light-gathering power as my son’s 4″ version. And I get that longer focal length are good for moon, planets and similar DSO while shorter/faster focal lengths are good for open clusters and large nebulae.
But what I was excited by was the explanation of exit pupils. Basically you divide the aperture by the magnification, sure, all the explanations say that, right? So, since my scope is an 8″, and an f/10, it ends up that exit pupils are exactly the EP size divided by the f/10 so a 41mm EP gives me a 4.1mm exit pupil. But what does that mean?
So the light coming from your scope is focused to a certain size image, i.e. it’s exit pupil. If my exit pupil is larger than the telescope’s, it’s all good. If the telescope’s size is larger than mine, I’m going to have to move my eye around inside of the EP to see the whole image. The explanation in the book isn’t any more awesome than previous ones, but the formula was better presented with examples, and it got me pushing harder to figure how it works for ME!
Well, it is like looking at a larger image through a small circle like a magnifying glass (without the magnification factor). If the hole I’m looking through (i.e. MY exit pupil), is bigger than the image (i.e. the telescope’s exit pupil), then I can see the whole image at once. If the hole I’m looking through is smaller than the image, I’ll have to move the circle around to see the image in sections.
And, it just so happens, I was testing my EPs about six weeks ago and figured out that in the 42mm BIG eyepiece that I was using, I was having to move my eye around a bit. In other words, that EP produced about a 4.2mm exit pupil, and my exit pupil was probably about 4.1 or 4.0 at that point. As the book points out (p. 41), “As we age, the muscles in our irises stiffen, and it is common for older observers to have a maximum dilation of about 5mm”. I would say without full dark dilation, I’m about 4mm-ish. But here’s the part that has ALWAYS confused me…I assumed that if my exit pupil was bigger, that was bad. Nope, it’s the opposite. As long as my exit pupil is 4mm, I can see EVERYTHING that is smaller than 4mm. So on my f/10 scope, everything below 40mm is going to work just fine for me. Above 40mm will still work, I just won’t be able to see the whole image at once. My son’s f/13 at 40mm will produce about a 3mm image, so everything will easily work for him or me. Finally, I get it!
Similarly, pages 43-48 run through all the different types of scopes in basic terms, and discusses some pros and cons:
Refractors – Classic design, long tube with lens at front and EP at back — objective lens up to 6″ at the front collects light and focuses it at the back, and good for moon and planets, crisp and clear;
Newtonian Reflectors – Two mirrors, with parabolic at back reflecting to small mirror at the front side — almost all large amateur scopes are Newts, with great bang for the buck yet open tube design;
Schmidt-Cassegrain Catadioptric – like my 8SE, most compact for any aperture, uses front corrector plate at the front bouncing light off mirror at the back to mirror at the front that focuses on EP at the back again, and allows for closed tube, but can have excessive image shift when focusing;
Maksutov-Cassegrain Catadiaoptric – like my son’s 4SE, MCTs have a curved corrector plate, and is great for contrast on moon and planets, but smaller FoV overall for large nebulae objects;
The chapter also covers the basics for different types of mounts such as alt-az, modified alt-az Dobsonian for Newtonian reflectors, and German Equatorial Mounts (GEM).
And finally, I like the ending about eye-pieces:
Eye-relief (the distance your eye can be from the EP and still see the whole image);
Apparent Field of View; and,
Using Barlows to step down from say 20mm with decent eye-relief to 10mm with decent eye-relief, rather than switching to 10mm plossl that frequently doesn’t have good eye-relief.
I am surprised in some ways for the chapter. Lots of stuff that I already knew in general terms, but I like the way it was explained and/or reading it for the seventh time in this format suddenly “clicked”. I finally get some of it. And it helped me make my decision on getting my son his 4SE, which is a Maksutov design.