ZWO120MC Camera

[Barv]

Hi.

I have one of these cameras and I'm having trouble with imaging Jupiter at much over F15. F15 is fine but I'm not getting any improvement by extending the focal length out much further than this. I would have expected focusing to be tougher going out further but I am finding that even when I am nailing the focus bang on I can't keep the gain down lower because of the increased picture size etc etc. Are other owners finding this? Any feedback would be very helpful!

Thanks

Harvey   

[JamesF]

What sort of gain setting are you using?  And what OTA?

James

[Barv]

What sort of gain setting are you using?  And what OTA?

James

Thanks James.

I have an 8" F6 scope, am using it with a 2.5X powermate and that's fine. If I try and put in my 4X imagemate I seem to lose contrast too much and I'm not really gaining anything! I think that this is probably too much. I just wanted some feedback from other experiences with the same camera/or chip. Have tried various gain settings from low to high without success!

Cheers

Harvey

[riklaunim]

ASI120 has smaller pixels than previously used cameras. Their optimal-maximal f-ratio is around f/14-15 and not around f/20. That's the reason

[Barv]

ASI120 has smaller pixels than previously used cameras. Their optimal-maximal f-ratio is around f/14-15 and not around f/20. That's the reason

I thought this might be the case, thanks for clarifying this. Have any users got decent results past F15, on those still, clear nights with excellent seeing, or won't that make any difference?

[knobby]

Interesting, I've tried with a 2.5 x Barlow on my mak with no joy. Just ordered a 1.6 x that will give me about f18 ...

[Barv]

Interesting, I've tried with a 2.5 x Barlow on my mak with no joy. Just ordered a 1.6 x that will give me about f18 ...

I'd be interested in how you  get on with that Knobby on really good nights. Feel free to PM me with some results!

[JamesF]

My feeling is that the desirable focal ratio for the ASI120 should be higher than f/15.  I think f/25 is nearer the mark and f/20 to f/25 is what I aim for.  I believe that Stuart and Neil are both working at around the f/20 to f/25 mark, too.

James

[JamesF]

By way of some examples, these are a bit noisy because I was testing out software rather than working on coming up with a decent image and I was a bit heavy-handed with the sharpening, but they are from the ASI120MC, 127 Mak and 2.5x barlow:

I've not measured the actual multiplier for the barlow, but some people are claiming that it's only about 2.3x, which would mean the focal ratio for these was f/27.

James

[knobby]

By way of some examples, these are a bit noisy because I was testing out software rather than working on coming up with a decent image and I was a bit heavy-handed with the sharpening, but they are from the ASI120MC, 127 Mak and 2.5x barlow:

I've not measured the actual multiplier for the barlow, but some people are claiming that it's only about 2.3x, which would mean the focal ratio for these was f/27.

James

You really are good at this Imaging lark James ...

One day I'll catch you up ;-)

[Barv]

My feeling is that the desirable focal ratio for the ASI120 should be higher than f/15.  I think f/25 is nearer the mark and f/20 to f/25 is what I aim for.  I believe that Stuart and Neil are both working at around the f/20 to f/25 mark, too.

James

By way of some examples, these are a bit noisy because I was testing out software rather than working on coming up with a decent image and I was a bit heavy-handed with the sharpening, but they are from the ASI120MC, 127 Mak and 2.5x barlow:

I've not measured the actual multiplier for the barlow, but some people are claiming that it's only about 2.3x, which would mean the focal ratio for these was f/27.

James

Oh, I can't understand Riklaunim's comment then? I shall have to try myself with my C11, that will give me F25 with the powermate. I'll post results on here!

[JamesF]

Oh, I can't understand Riklaunim's comment then? I shall have to try myself with my C11, that will give me F25 with the powermate. I'll post results on here!

It's quite possible that a wider range of values work well and that Piotr prefers to stay near the lower end of the range.  I don't think there is necessarily an absolute wrong or right here, more varying shades of grey.

I originally worked out my figures for the SPC900 and similar cameras which have a 5.6um pixel size and came up with around f/35 to f/40.  To the best of my recollection that was the sort of range that many of the "hardcore" planetary imagers were working in at the time.  The linear dimensions of the ASI120 pixels are very close to 0.67 times those of the SPC900 (3.75um vs. 5.6um), so I'd expect the desirable focal ratio to change in the same proportion, which would actually give f/23 to f/27.  f/20 to f/25 suits my kit depending on exactly what I'm using, so whilst I aim for f/25 I'm happy with a bit less if that's how it works out on the day.

James

[riklaunim]

If you have super seeing you can push it more (like f/30 for DMK21 with 5.6 pixels), but that usually only makes images bigger, but not more detailed. I don't see a point of imaging with such smaller pixels at f/ratios above 20. You get dimmer and more fuzzy image and you won't get extra resolving powers. I would suggest using something below f/20 by default. (and be sure you are not imaging with 2x binned pixels, but with smaller subframe ).

[Barv]

If you have super seeing you can push it more (like f/30 for DMK21 with 5.6 pixels), but that usually only makes images bigger, but not more detailed. I don't see a point of imaging with such smaller pixels at f/ratios above 20. You get dimmer and more fuzzy image and you won't get extra resolving powers. I would suggest using something below f/20 by default. (and be sure you are not imaging with 2x binned pixels, but with smaller subframe ).

I agree. I originally put the post up because I was struggling. My F24 image was passable shall we say but resolved less than the F15 one. (And was obviously much more difficult to get).

I think that the direction I need to go in is to stay around the F15-20 mark and minimize the frame size down as much as possible, with a view to increasing the FPS rate as much as I can. I feel after extensive use of this camera that the more frames the better is the way to go! 

[ArmyAirForce]

I'm still waiting for those "still, clear nights with excellent seeing"

[JamesF]

I'm still waiting for those "still, clear nights with excellent seeing"

I'm not sure we get those any more

James

[ArmyAirForce]

What; Ever? oh! bummer.

[SnakeyJ]

Great set of Jupiters from James - I'm expecting big things from that C925 when the weather allows!

Incidentally, I also think Stuart and Neil were talking up to F40 - though with bigger pixels on the DMK's.

typed on my mobile with Tapatalk

[JamesF]

Great set of Jupiters from James - I'm expecting big things from that C925 when the weather allows!

Incidentally, I also think Stuart and Neil were talking up to F40 - though with bigger pixels on the DMK's.

Thank you.  You may well be right about Stuart and Neil.  I'm struggling to keep up with who has which camera these days

James

[lensman57]

Hi.

I have one of these cameras and I'm having trouble with imaging Jupiter at much over F15. F15 is fine but I'm not getting any improvement by extending the focal length out much further than this. I would have expected focusing to be tougher going out further but I am finding that even when I am nailing the focus bang on I can't keep the gain down lower because of the increased picture size etc etc. Are other owners finding this? Any feedback would be very helpful!

Thanks

Harvey   

Hi Harvey,

This image is the last one of Saturn that I took before going into DSO imaging last March. It was taken with an ASI 120 MC, SW 100 ED DS PRO and a Powerrmate 2.5X making it F22.5 The seeing was the usual god awful Manchester affair. The capture shows good detail for such a small aperture and not so great seeing. I don't know of the brand of your 4X and as not all barlows are created equal the loss of contrast and definition may have something to do with it. BTW to my shame I can not find my original tiff files from the start of my imaging last year , so I had to find my post and copy the image and paste it so there is considerable image degradation.

Regards,

A.G

[michael.h.f.wilkinson]

With my SPC900/ and DMK21 (both 5.6 micron pixels) I find pushing beyond F/25 rarely helps (F/30 only on objects bright enough to gather enough photons per pixel). This makes sense in terms of the so called Nyquist sampling frequency. For 3.75 micron pixels the value should be 3.75/5.6 = 0.67x the F-ratio of the DMK21, i.e not beyond F/16.7 (roughly). F/15 should be close to optimal. If I do get an ASI 120, I am also going to look for a quality 1.4x to 1.6x barlow.

[michael.h.f.wilkinson]

Just noticed that Siebert optical has Telecentric Barlows (like powermates) down to 1.3x

http://www.siebertoptics.com/SiebertOptics-barlows.html

[Barv]

Hi Harvey,

This image is the last one of Saturn that I took before going into DSO imaging last March. It was taken with an ASI 120 MC, SW 100 ED DS PRO and a Powerrmate 2.5X making it F22.5 The seeing was the usual god awful Manchester affair. The capture shows good detail for such a small aperture and not so great seeing. I don't know of the brand of your 4X and as not all barlows are created equal the loss of contrast and definition may have something to do with it. BTW to my shame I can not find my original tiff files from the start of my imaging last year , so I had to find my post and copy the image and paste it so there is considerable image degradation.

Regards,

A.G

Hi A.G.

You are right, that is a great image considering the conditions. I do tend to use powermate's too. The scope I have been trying to use is an Intes Micro MN86. (8" Mak Newt). The central obstruction is miniscule and for all intents and purpose's they are reckoned to be as good as an Apo of the same aperture. Collimation is always bang on aswell as it is permanently sited in an obsy. The Mak is only F6 so the 2.5 powermate is only giving me F15. - Which works fine. The other barlow I was using was an Astro Engineering '4X Imagemate' which I know are also reckoned to be quite good. (see Mick Hyde's website). 

I also have a Celestron 11" scope with an F10 F Ratio. I will put a good quality 2X barlow in that I think to see if I can improve things when/if the seeing gets a bit better! There must be something in the points that the other guys have been saying on this thread though, - the night I tried to image at F24 and couldn't get much I also took a cracking image at F15, (unless the seeing went off which I think was unlikely)!

Thanks for your input.

Harvey

[SnakeyJ]

With my SPC900/ and DMK21 (both 5.6 micron pixels) I find pushing beyond F/25 rarely helps (F/30 only on objects bright enough to gather enough photons per pixel). This makes sense in terms of the so called Nyquist sampling frequency. For 3.75 micron pixels the value should be 3.75/5.6 = 0.67x the F-ratio of the DMK21, i.e not beyond F/16.7 (roughly). F/15 should be close to optimal. If I do get an ASI 120, I am also going to look for a quality 1.4x to 1.6x barlow.

Interesting reading, though the wiki entry on Nyquist went rather over my head and will have to be studied.     I have seen/read quite a lot of posts where this F16/17 is mentioned, but also a number of excellent images where it has been exceeded, in good seeing and usually with bigger apertures (10"+).   Presumably higher QE and sensitivity from mono cameras can extend this?

[michael.h.f.wilkinson]

Interesting reading, though the wiki entry on Nyquist went rather over my head and will have to be studied.     I have seen/read quite a lot of posts where this F16/17 is mentioned, but also a number of excellent images where it has been exceeded, in good seeing and usually with bigger apertures (10"+).   Presumably higher QE and sensitivity from mono cameras can extend this?

Exceeding F/16 -F/17 will not instantly degrade the image. However, if I go from F/15 to F/30, I will lower the photon count per pixel by a factor of 4, halving the S/N ratio, which will hurt the image quality (wavelets will tend to explode the noise).