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 found 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.
I feel like I’m ready to learn the next part.