Planetary lucky imaging, coma corrector or additional equipment needed?

[mandragara (Richard)]

Hi all,

I had great fun last year imaging the planets with a 6" f/8 Dob and since then I've upgraded to a manual 12" f/5 Dob and am really keen to have a good stab at the planets again this year. Managed to get my 6" results on the Bintel FB page, so a high bar is set!

I plan to image with a QHY5III462C camera (2.9um pixels) and an Explore Scientific 3x focal extender, producing an effective focal length of f/15 - which should be good according to the "optimal FL = 5*pixel size in um). Collimation will be important, so I've invested in a Catseye Autocollimator to make sure everything is as good as can be.

My plan is to do a luminance layer with the UV\IR cut filter off and then a colour image with the filter on and merge them in post.

Now we finally come to the questions at hand:

1) Given the tiny FOV of the camera at f/15, is it worth adding a coma corrector to the optical train? Or will the coma always be less than the airy disc at this distance from the centre of the optical axis? Not sure how to calculate that

2) Any general feedback on my plan? Any glaring issues? I know it will be a nightmare keeping the planets in the camera's FOV at f/15

Thanks,

Richard

[Merlin66 (Ken)]

Richard,
For planetary imaging you won't need a coma corrector.
Your plan sounds good. You're already aware of the issue with dobbies and exposures....
Best of luck!

[Tulloch (Andrew)]

You will certainly have issues keeping the planets on target with a manual Dob and effective focal length of 4500mm - my advice would be to invest in a Goto mount . Achieving accurate focus will be extremely difficult!

Not too sure what you plan to achieve by combining an image without the L filter, note the focus point in the high IR will be significantly different to the visible part and will probably adversely affect the result. I've never seen anyone do this before...

If you haven't seen these before, I'd recommend watching these tutorial videos, they cover everything from the equipment to the software.
http://planetaryimagingtutorials.com/

[mandragara (Richard)]

Originally Posted by Tulloch

You will certainly have issues keeping the planets on target with a manual Dob and effective focal length of 4500mm - my advice would be to invest in a Goto mount . Achieving accurate focus will be extremely difficult!

Not too sure what you plan to achieve by combining an image without the L filter, note the focus point in the high IR will be significantly different to the visible part and will probably adversely affect the result. I've never seen anyone do this before...

If you haven't seen these before, I'd recommend watching these tutorial videos, they cover everything from the equipment to the software.
http://planetaryimagingtutorials.com/

Don't quite have the cash on hand needed for a GOTO sadly, plus no one has stock anyway!

For focus, we have a Bahtinov mask, so hopefully, that'll do.

As for grabbing data without the UVIR filter for an L layer, no idea! Worst case scenario I just have my regular stacked colour data anyway

That tutorial has some good stuff on gain etc but I'm not sure why he uses RegiStax over AS!3 - will flick through it sometime

Here are my pix taken with a 6" dob and 2x barlow (3x for mars).

[Tulloch (Andrew)]

The tutorials are pretty old, and at the time as2 and registax were both used. Nowadays everyone uses AS!3 and only uses registax for sharpening.

Nice images with the 6”, the 12” should give you more resolution if you get enough frames on target.

[mandragara (Richard)]

Actually now I think about it, won't visible and IR be focused on the same plane because I use a reflector?

[Tulloch (Andrew)]

Pretty sure that they will still have slightly different focus points. And you are using a Barlow ...

[Outcast (Carlton)]

From my own limited experience doing planetary with the ASI224MC the resultant colour is much better with an IR/UV cut filter...

Cheers

First image is with the filter, second is without...

[mandragara (Richard)]

Cheers Carlton,

What I was thinking of doing (no idea if it works) is to have the filter off and shoot a luminance layer, then get the RGB with the UV\IR filter on and blend in post.

It might just be stupidity, we will see!

This is an example using your two images:

[Stonius (Markus)]

I understand that you don't need a coma corrector for planetary photography with a newt, because theoretically, the planet stays in the center of the field of view, where there effects of coma are not apparent.


Which requires a tracking mount, of course.


I have tried using a manual dob the way you intend to and gotten some reasonable results, but it is a bit frustrating. Obviously, there is the constant centering of the planet in the field of view, but I could also see the image degrade as the planet passed into the edges of the field, but this may well have been because I was doing eyepiece projection because it was easy to locate the planet by swapping out to a lower power EP.


Anyway, I think the main difference here is that without a tracking mount, the planet will spend a fair bit of time on the edge of frame. I don't know whether coma is an issue for the very small fields required for planetary photography, but I do know that you can increase your chances by editing the videos or limiting the frames in autostakkert, so as to exclude the frames where the planet is not in the optimal position.


Cheers, hope that helps


M