20 years in the wilderness ... questions mostly about planetary imaging

[ijontichy]

I've been out of the amateur astronomy game since the late 80's, and evidently a lot has changed since then, so much that I am completely lost again. What has me really excited are the extraordinary images of Jupiter and Saturn that amateurs are generating with their CCD cameras and rather modest (aperture-wise) telescopes. I would like to try something like that.

After reading a lot of stuff on the internet, some out-of-date, I still have many questions, and I'll just have to dump them all here and hope that some will get answered and for the rest I'll get some references:

1) For planetary imaging, would an 8"-10" Schmidt-Cassegrain suffice (like the standards from Celestron and Meade)? Would a high-quality 5"-6" apochromatic refractor be better? Cost is not a major concern (until I add everything up and get a heart attack).

2) For planetary imaging, the CCD cameras like the Imaging Source DMK31 and the Lumenera SKYnix2 seem to be the go (ignore the expense, for now). These cameras take 30 or 60 frames per second and planets are real bright, so my question is: is it still necessary to have your telescope polar-aligned and continuously correcting for the rotation of the Earth? Does the processing software help you with that sort of thing?

3) I think I'm right in believing that light pollution isn't as huge a concern for planets as it is for other sky objects ("seeing" is the big killer). But I live in a very restricted environment: an apartment with a large balcony that faces the direction where the sun rises. My azimuthal range is about 100 degrees (I don't think I can see the South Pole). My altitude range is about 70 degrees, i.e. I have a clear horizon, and can see up to about 20 degrees from the zenith. I need to practice "silent astronomy", because I do not wish to awaken my neighbours. Big question: is my goal of doing planetary astrophotography compatible with my residential situation?

That's enough for now. Any help or pointers would be much appreciated.

[iceman (Mike)]

Hi and to IceInSpace! (and welcome back to astronomy!)

A lot sure has changed in the last 5 years, let alone the last 20 years.

I'll see if I can answer your questions:

1) You really want the focal length for planetary imaging - so the 8" or 10" SCT's will be better for planetary imaging than the refractors. I use a 12" Newt Reflector. Anthony Wesley uses a 14.5" Reflector. Trevor Barry uses a 16" Reflector. Others use 14" SCT's from Celestron and Meade.

So it's really up to you and your budget. If you're limited for space, a 10/12/14" SCT would be the best.

2) With those webcams, you're right - you'll likely use 30fps normally, and sometimes 15 or 60fps depending on the object you're imaging.
You do still need the telescope/mount to be tracking the object. But it doesn't need to be perfectly polar aligned because your exposures will be short. So if you have to make adjustments from time to time to keep it in the field of view, that won't be a problem.
If you get an alt/az mount you may have some problems with field-rotation over the length of a night, but if you get an EQ mount that won't be a problem.

3) Planetary imaging is perfect for suburban/city imaging because the planets are bright enough that light pollution isn't really a concern. The biggest noise from your setup is likely to be you (apart from the whirring of the your mount doing the tracking or slewing).

Good luck and keep asking questions! Sounds like you're on the right track already.

[Kal (Andrew)]

1) I own a 5" Apo and a 11" SCT. The 11" SCT is far better for planetary imaging. I do planetary imaging at around 7 metres focal length, many others are up over 10 metres focal length.

2) Field rotation over the length of a night is a non issue - the real issue is field rotation in the length of any video clip that you capture to process. For Jupiter, you will be looking at around a 1-2 minute video clip to process to produce a single image. You do not want any noticeable field rotation in the clip. The amount of field rotation, of course, depends on where the planet is located at the time of capture. Low and rising from the East is minimal field rotation, near zenith is alot of field rotation.

3) Yep, light pollution doesn't play a major factor in planetary imaging. Some mounts are more silent than others, but those details can be found by reading product reviews when you have narrowed down your choice.

[renormalised (Carl)]

In addition to what Mike said there, with field rotation using alt-az mounts, you can get field derotators for some SCT brands that pretty much negate that problem. Especially for short subs on objects like planets and bright DSO's. Most of the latest image processing software for astropiccies will take care of some field rotation as well. Ultimately, you would want to have accurate polar alignment if you were going to take piccies of faint fuzzies. It's much better if you do. But the field derotators do a good job.

[astro744]

Tele Vue Powermates are available in 2x, 2.5x, 4x and 5x with the 5x unit capable of 7.7x when imaging.

See http://www.televue.com/engine/page.asp?ID=237

and http://www.televue.com/engine/page.asp?ID=42

No need for eyepiece projection as in the past.

eg. 2000mm f10 C8 will give 15400mm at f77. C9.25 perhaps even better choice since secondary amplification ratio is less than in C8 and C9.25 tube fits nicely on GM-8 and has manageable weight. Note fork tubes will not give clearance for long accessories.

A 2.5x or 4x Powermate may be more suitable more often depending on seeing.

Good tracking highly recommended due to high magnification and small imaging sensor size.

Whatever you choose, enjoy!

[ijontichy]

Thanks guys for your responses. I've been reading a lot more about CCD imaging over the past few days, so I have a much better idea now of what's possible and what's required. I'm not going to rush into it just yet, though.