Planetary Imaging

[Owmuchonomy]

1 hour ago, Gina said:

Thank you.   I guess the ASI385MC could have other uses as well as planetary.  This could change the balance in favour of the 385.

Very important is to match the camera to the scope for planetary and solar.  Pixel size dominates in this respect.  So the 120 (and 385) is 3.75 microns and thus a scope arrangement that gives a f/ of approx 5 times this (f/19) would be optimal.  For example this would work well in my f/10 SCT 9.25" with a 2x barlow.   This in turn gives a focal length of 4.7 metres.  I use the 174 chip which needs a f/ of about 25 to 30 and thus I use a 2.5 x powermate which gives a focal length of 5.9 metres in the same scope.  I also use the 290 chip which runs very well at 2x (f/14) in my ED refractors particularly for the Moon.  Same scenario apples to my Lunt scope.  The 385 chip is much bigger than the 120 which may sway you if you want to do lunar or solar?  Try the FLO FOV calculator for this calculation.

[Demonperformer]

If you have a scope/camera that don't give you a nice easy barlow value, is it better to use too high a f ratio or too low?

[vlaiv]

1 hour ago, Demonperformer said:

If you have a scope/camera that don't give you a nice easy barlow value, is it better to use too high a f ratio or too low?

This is tricky question and it very much depends on circumstances - I'll just list things to consider without going into too much math detail.

As with all imaging SNR is important. On its own, SNR is quite easily achieved for Sun, Moon and major planets, simply because all are bright and one stacks large number of frames. There is other side to SNR in this story. We tend to sharpen up images we get from stacking. Sharpening is essentially amplifying high frequency components of the image. When we amplify signal we end up amplifying noise as well (noise is uniform in frequency domain so there is a lot of noise "signal" in high frequencies that we amplify) - that puts a limit on how much we can sharpen without making noise too noticeable. Using higher F/ratio spreads light over more pixels (over sample) which leads to less signal per pixel and less SNR out of given total capture time.

On the other hand, there is something called pixel blur. Ideal sampling is point sampling, but pixels integrate signal level over whole pixel surface and this introduces certain blur. From this perspective sharper image can be obtained when we "shrink" pixels to be more point like - or if we keep pixels the same and "enlarge" image instead (this will make pixels look smaller compared to image). This is very important when doing OSC imaging, because red and blue pixels are in "sparse" matrix (every other pixel) - which makes those channels be sampled at twice lower rate effectively - so you want to boost F/ratio to twice of that used with mono cameras and filters (and you get "smaller" pixels as a bonus). Using interpolation when debayering OSC is just keeping lower sampling resolution and adding more blur (if bilinear interpolation is used for example).

Whole point of planetary imaging is this balancing act - get enough SNR to be able to do good sharpening and on the other hand get decent sized image and shoot very short subs to freeze the seeing - both working against good SNR.

Back on the question - in good seeing, you will be able to capture more subs suitable for stacking and overall blur level will be low so you won't need much deblurring - use higher F/ratio because you can afford lower SNR. In poorer conditions go for lower F/ratio because you probably won't be able to recover highest frequencies anyway due to poor seeing and you will need very good SNR for best sharpening results.

 

[Gina]

I think I understood that ?

[Demonperformer]

Gina, that is word for word what I was going to post!