Imaging Uranus and Neptune

[pmrid (Peter)]

I am in a suburban location so light pollution is an issue. As a result, I'm looking at shifting my imaging focus away from DSOs to planetary and lunar. I'm using a combination of scopes and cameras and looking for the best combination I can achieve with what I have.

I have a 10-inch astrograph (with a Suchting conical f3.8 mirror) and a North Group ED127 refractor. Both have roughly similar native focal lengths although the refractor is a f7.5 scope and the oother f3.8.

I don't see much future in using conventional CCDs (such as SBIG and StarlighXpress) but plan using ASI290MC as a dedicated planetary cam using either SharpCap of FireCapture. Whichever scope I use, I'll have to include my x2 Barlow. Although I do have a GSO x5 Barlow as well but I think that will be difficult to use in practice. But boith scopes have their down side: the f3.8 will have a fair bit of coma and it isn't practical to use a barlow with an MPCC; and the refractor is going to have some CA despite being flogged as a triplet.

My question therefore is what others are using to image these two planets.

With Uranus at only 3.3 arc-seconds and neptune a bit less, I'm going to struggle to get any decent image scale with just a x2 Barlow - the ASI290 has an image scale of 0.31 arc-secs/pixel and the refractor something similar.

Any suggestions to improve on these numbvers?

[Tulloch (Andrew)]

Hi there, the ASI290MC will be excellent for imaging the blue planets, and the 10" will also be the best scope (aperture always wins with planetary). The best focal ratio to use with planetary is about 5x the pixel size of the camera, so for the 290MC you will be looking around f/15.

I imaged both planets 2 nights ago from my backyard in the Melbourne suburbs (see my post here ) using my C9.25" SCT, 2.5x PowerMate and ASI224MC camera.

The planets are small and will be in the center of the frame, so you don't need to worry about coma.

If you've never done planetary before, these tutorial videos will help.
http://planetaryimagingtutorials.com/

I'd also start with Mars, it's much bigger, and easier to frame and focus on.

Andrew

P.S. I should also point out that the preceding Neptune was one of my worst ever - there is a better version here, when it was in a much better position in the sky.
http://www.iceinspace.com.au/forum/s...d.php?t=184973

[pmrid (Peter)]

Thanks Andrew. I was conscious of the 5 x pixel size guideline but more or less reconciled to being unable to reach that with my f3.8 newt unless I also used my x5 Barlow. I guess the main reason was (is) the difficulty of getting the planets in the FOV with such a small object. But I can see that I will have to fine-tune my visual finder and find a bit of patience. Story of my life!

Peter

[Tulloch (Andrew)]

No worries Peter, actually a 3x Barlow will probably get you the magnification you need. When imaging, barlows tend to give a high mag than quoted due to the extra spacing out the back end, so if you have one (or can get one) this would be better than the 5x.
https://www.televue.com/engine/TV3b_...=52&Tab=_photo

[pmrid (Peter)]

Originally Posted by Tulloch

No worries Peter, actually a 3x Barlow will probably get you the magnification you need. When imaging, barlows tend to give a high mag than quoted due to the extra spacing out the back end, so if you have one (or can get one) this would be better than the 5x.
https://www.televue.com/engine/TV3b_...=52&Tab=_photo

That's interesting and good to know. I'll give a x3 a whirl. I just ran up the x5 and to be honest, wasn't comfortable with the results. So a x3 sounds promising.

[Tulloch (Andrew)]

Most Barlows work like this, the cheaper ones at least (that aren't set up to be equal mag with back spacing). I have an Bintel 8x50 RACI finder, and can spot Uranus quite easily with it (when the moon isn't too full). For Neptune I need a star map (I use SkySafari) to put me in the right location, then just slew around until I find something that looks about right.