Posts Made By: Jared Willson

StellarVue 80S (LOMO f/6) on a Tak TeeGul SP mount--I just hate alt-az! I needs my tracking mount!

Best size fto dollar ratio has got to be with some of the smaller scopes. I would say an 80mm is the best value and would complement your XT10i quite well---easy to transport, easy to setup, no collimation required, minimal cool-down time, and, of course, dramatically dimmer views. 80mm won't get you the best scope, but it proabably will get you the best value and you'll find you use it more than you might think. The Orion 80ED, for example, is inexpensive and (because of its small aperture) is very nearly apochromatic. Keep in mind that as your aperture increases, if your focal ratio stays the same, color error increases too--another advantage for smaller scopes.

If you go to 2x2 binning, your exposure time to get the same signal strength on diffuse objects would indeed go to (1/4)X. With 2x2 binning, each pixel is effectively four times the area of a 1x1 binned pixel, so four times as many photons will be hitting it. The trade off, of course, is resolution. You are giving up resolution.

Point sources, by the way, do not behave in quite the same manner. Let's take an example where you are significantly undersampling your image (not unusual for many imagers). I have 9 micron pixels on my camera and a 715mm focal length--significantly undersampled. That means that dim stars often cover only one pixel when I bin 1x1. If I bin to 2x2 or higher, the same number of photons from that point source are going to impact my larger pixel, so I won't get a "deeper" exposure. I will only see exposure time benefits from binning with diffuse objects.

Unfortunately, you are missing on key element. Knowing the efficiency of your filters doesn't tell you the quantum efficiency of your imaging chip! Different chips have significantly different efficiencies at different wavelengths. In the best of all worlds, the filters are matched to the particular chip being used so that you will get perfect "white" (G2V) balance with identical exposure times. For example, Astrodon makes both E-series and I-series RGB filters for use with Kodak's blue enhanced chips and the interline transfer chips respectively.

Based on your assumptions, you would expect that you would need to collect more color data than luminance data since the luminance filter will let in more light. In practice, that isn't true. The physiology of the human eye/brain enter into this as well. The eye/brain combo is more sensitive to noise in luminance than in chrominance channels, so most users find that collecting lots of luminance data is the key to a good image. As a starting point, you might try a 3:2:2:2 ratio with the L:R:G:B channels and see how that works for you. There is no single right answer, though, and getting the same signal strength in all four channels does not necessarily result in the best image.

That's actually a fairly generous budget for a small refractor! The SV90T is right at $2,000 and has an excellent reputation (though I have never used one). Any of the LOMO 80mm f/6 triplets would be great, though you might need a field flattener even with the ST7. The optics can be purchased in A&M tubes, APM tubes, Stellarvue tubes, and sometimes in William Optics tubes. I believe the Stellarvue version is around $1,800. The William Optics version (sometimes available from Teton or from APM) is less expensive but the focuser is not as nice as the Feathertouch.

The M-110 is a good choice if you are going to include much visual use since you get a lot of aperture for the money. It wouldn't be my first choice for an astrograph, though, just because of residual spherochromatism.

I'm sure there are other choices as well, but these are some scopes I can recommend from personal experience.

- Jared

Superb as always, Dietmar! I may even like it better than your original, full frame image.

Wow! I don't think I've seen the outer nebulosity before.

- Jared Willson

Well done!

Very nice result, Floyd! I love the color contrasts between with the two foreground stars and this lovely globular.

Another one? Dietmar, slow down! You're embarrassing the rest of us!

Seriously, congratulations!


- Jared Willson