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ZWOASI1600MM at longer focal lengths

17 Mar, 2018 23:49
I am in the process of buying my first AP set up for mainly nebula and galaxy. Pretty much set on a CGX mount, 80mm refractor and a ASI1600MM. I'm happy with that, however i would like a little bit of future proofing. If i were to buy a longer focal length telescope later on down the track (e.g 1200mm'ish focal length) would the ASI1600 handle it ok or is it asking to much of  it. suggests that it is slightly oversampled for OK seeing with a resolution of 0.64'/pixel. I understand that guiding would have to be good, but would images remain crisp? Any feedback from anyone with experience with a similar set up or advice would be greatly appreciated.

Edited 18 Mar, 2018 00:36
18 Mar, 2018 11:49
What about using it in bin 2x? I know binning isn't the same thing on cmos as it is on ccds but doing so you should get the right sampling. Or at least that's what I'd do with my ccd.
18 Mar, 2018 16:02
I shoot at 1625mm with my RC, with a resolution of .48"/px and never had any issues. The mount is the biggest factor at these resolutions, it has to track very well and be very responsive to guiding corrections to make up for seeing. I set up every time I image and I ALWAYS make sure my mount is level and that PA is good. If your mount supports a PEC curve, you should have one programmed. Use the Guiding Assistant in PHD to help with settings. These things combined with average or better seeing will help you a long ways in getting sharper images with round stars.

You could also bin 2x2 and half the resolution down to 8Mp, but you won't get the benefit of lower read noise that CCD users get with their cameras. Not that it matters, since the 1600MMC already has very low read noise. I have not binned b/c I've never come across a situation where I needed to do so.

  Perhaps binning isn't such a great idea, read this thread:
Edited 18 Mar, 2018 16:21
18 Mar, 2018 23:36
slightly oversampled for OK seeing with a resolution of 0.64'/pixel
0.64 is a good image scale for 2 a-s seeing, which is good but not amazing seeing.  A good rule of thumb is Seeing / 3 = best image scale.

Being a little oversampled is not really a problem, under-sampling is worse.  Even if you have 10 pixels describing a feature when 3 pixels will do it's not going to produce any problems or need better guiding.  The down-side to over-sampling is you are giving away SN ratio/Dynamic range because the smaller pixels each catch fewer photons.  Imagine one 9 micron pixel that can catch 100 photons for a given exposure/object - if you have 3 micron pixels each will capture about 11 photons each.   Statistically, those smaller sample sizes will mean more variation and thus noise.

But at 1200mm you are still really close to a good match for image scale so I wouldn't worry about it.

19 Mar, 2018 04:07
Thanks everyone. and thanks Kevin for the explanation. Gives me much more confidence in my upcoming purchase. Amazing images too, by the way.

19 Mar, 2018 12:40
The camera does a simply fantastic job at longer focal lengths.   You really can take advantage of the shorter exposures made possible by these CMOS cameras.   I have been shooting at 1490mm with my SCT at f/7 and have been more than happy with how it performs.    In fact I’m coming up on 2 years with the camera and I still wouldn’t trade the setup for anything new and improved, yet.

You can check my gallery for examples.
21 Mar, 2018 00:17
Thanks for the feedback, Jason. Gives me a lot of confidence in the purchase.
Spectacular images too.
21 Mar, 2018 00:47
I am just about to buy a second ASI1600MM for my 10" F/8 SCT at 2000mm F/L and I can use it with a F/6.3 FR at F/5 with good results. I am just starting to fire up the SCT for galaxy season. I have been mostly useing my EON 115mm APO for nebula at 680mm but I have tested my current ASI1600 previously on my SCT and it works great. It is MUCH more about the mount than it is about the camera…
23 Mar, 2018 12:46
Has anyone tested an ASI 1600 with focal length around 2000mm (for example SCT 8" f/10 or RC10" f/8 ) or 2350/2430 mm (C9.25 f/10 or RC 12" f/10)?
I suppose that in conditions of medium seeing (3"/pixel) results will be not so good due to big oversampling.

Generally speaking, as the marked trend is to have more pixels and smaller, does this means that owners of scopes with long focal length will be forced to decrease focal length by means of focal reducer? Or move scopes in places with good seeing? smile
Edited 23 Mar, 2018 12:46
29 Mar, 2018 19:59

I imaged M82 the 23 march 2018 with an ASI 1600 and anRC GSO 250mm f/8
The best seing was from 2.5" to 3.6" during the night (L 600x10s 1x1, RVB 60x10s 2x2)

This is not a very goog image, but not so bad
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