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Old 04-09-2019, 01:58 PM
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ZWO ASI1600MM microlensing examples

Looking ahead to the future a bit, I can see the time coming when I am likely to move from my OSC camera to mono imaging. I was just wondering if anyone has any examples they could share of tightly zoomed in images showing the microlens reflection issues the ASI1600 is noted for?

I saw early on some fairly spectacular examples of a similar effect with my ASI294 OSC camera which were more or less entirely resolved with a UV-IR cut filter. I was just wodering if there is a visible difference in the reflection issues seen with the ASI1600 between different filters?

It will probably be a while before I am likely to move on to mono and I am not sure if I should just hang out for the next generation of cameras.
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Old 11-09-2019, 09:23 PM
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I think ZWO has a general problem with reflections that is not just specific to the 1600 and 294. Some feedback I’ve seen on various ZWO Facebook groups would lead me to believe this is not a focus for their product improvements. Although not mono I’m still looking at the 294. So try not to worry too much or otherwise get the equivalent qhy cam which I may well do.
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Old 12-09-2019, 12:12 AM
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Quote:
Originally Posted by SuperG View Post
I think ZWO has a general problem with reflections that is not just specific to the 1600 and 294. Some feedback Iíve seen on various ZWO Facebook groups would lead me to believe this is not a focus for their product improvements. Although not mono Iím still looking at the 294. So try not to worry too much or otherwise get the equivalent qhy cam which I may well do.
The problem is that the cover glass on the sensor is not anti-reflection coated. As ZWO does not make sensors they can't really solve this problem, apart from changing the sensor.
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Old 12-09-2019, 02:30 AM
glend (Glen)
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Paul, there are plenty of old threads here, and on Cloudy Nights forum, providing examples and discussion on the ASI1600 mono in this regard. Learning to use the Search function would give you plenty to read on the subject.

As far as the 294 comment below is concerned, these are two entitely different sort of cameras, with different market segments. Reading the Cloudy Nights 294 Beta Test thread should be mandatory for any prospective purchaser. The 294 has some substantial issues that affect its traditional long sub capture use, and requires Flats and processing gymnastics in long sub roles, things not required by the 1600. ZWO markets the 294 as a EAA camera these days.

Search is your friend.

Last edited by glend; 12-09-2019 at 03:27 AM.
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Old 12-09-2019, 08:31 AM
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Just to add to this the QHY version (QHY-163) of the ZWO-1600 suffers from the same problem as it uses the same Panasonic sensor.
The only solution is to remove the cover glass on the sensor and replace it with an anti-reflection version.
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Old 12-09-2019, 09:54 AM
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I know there are plenty of examples about of the microlensing issue and have searched extensively, what I was looking for are some example raw LRGB subs which display the issue as I am specifically interested in something I have not seen really delved into. I am interested to see if the reflection issues in the ASI1600 are wavelength dependent and if so, what portion of the spectrum are they most evident at. I did get hold of some RGB subs but they are relatively short exposures without many really bright stars so the issue is not evident enough in any of them to be useful.

As for the ASI294. I think they have a fairly undeserved bum rap regards calibration. I recently, finally uncovered something like the issue that they are supposed to be notorious for in an image of mine, but it took an image where I have over 100 subs to stack and average the noise smooth to be able to profitably stretch the data hard enough to see it. I don't do anything particularly special by way of calibration, I shoot about 50 darks at the gain, temp and time I plan to use to create a master dark in APP (I used the same one for six months before I saw calibration issues slipping through and made another) Likewise I shoot about 100 flats and the same number of dark flats to create a master flat, making sure to keep the exposure time long enough to have the off chip electronics doing the work. By what I have seen the biggest calibration issues arise when exposures below about 4 seconds are used to create flats, when the SOC takes over the timing and circuity is active on chip that is not active for a traditional light frame, changing the noise profile.

My bigger issue with the 294 was reflections which were in some instances staggering but which were 99.99% resolved with a UV-IR cut filter. My interest with the 1600 is if that turns out to be a similar effect and similarly controllable with filtering, but I have not found that discussed as such.

I have some difficulty in smashing the ASI294 up too hard for deep sky when I have done the two images below with one without any particularly tangled processing steps. Most of the defects I would criticise in these two amount to shooting them through a $450 refractor with visible bloat in the blue end of the spectrum, at least when focussed using an undebayered RGGB sensor output. If it was not so many steps I would be tempted to focus then refocus while displaying only the blue channel to see if the blue can be tightened up, if so I could work out a focus offset and shoot two sets of subs, one where the blue channel was used and the other where the red and green were, splitting them out in APP and recombining the bits I want. But it is a lot of work to get around a dirt cheap frac's limitations.

https://www.astrobin.com/full/yypxc4/B/

https://www.astrobin.com/full/fvqi6n/B/

Last edited by The_bluester; 12-09-2019 at 11:28 AM.
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Old 12-09-2019, 11:33 AM
AnakChan (Sean)
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There’s a pretty good explanation about the micro lens diffraction on CN:-

https://www.cloudynights.com/topic/6...ect-explained/
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Old 12-09-2019, 09:02 PM
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It can and does show up at all visual and near-IR wavelengths, broadband and narrowband.

As I mentioned in a similar thread, the number of targets where it becomes an issue are few and far between.
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Old 13-09-2019, 08:59 AM
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Which ties in to the data I was able to download (FITS files) of the Trifid, I went looking for it and could not find any evidence of the issue in those files though they are only 2 minute subs, I am used to 5+ minute subs with the ASI294 but obviously have no experience with the 1600 to compare results between sub lengths.

What it really leaves me torn about is saving up and buying one with filter wheel and filters, or save up and wait for it's replacement which would hopefully have much greater pixel well depth and is probably lurking in the wings given how quickly all things tech move on. I can probably move in steps, my current rig has space between my rotator and camera to squeze in the 20mm of the ZWO 7 position filter wheel. I just have to be sure my next guider is thin enough to make that work or I have to lose the rotator.

I have had no chance to do more than a quick browse of the link above from Sean, I look after a fibre optic cable network and someone put a backhoe through part of it around lunchtime yesterday.
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Old 13-09-2019, 04:11 PM
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What scope would you be using? I wouldn’t suggest the 1600 as a good fit for your C9.25, probably a bit too oversampled and slow at native focal length, unless you regularly get excellent seeing. A 294 would probably be better suited, depending on your preferred use case.

I wouldn’t get too hung up on well depth, the 1600 is capable of a full 12-bit dynamic. What gain do you typically use with your 294?

I’ve not seen any signals that there is a substantially superior mono sensor in the wings, besides the 61MP full frame chip from Sony. That will certainly be in a different price bracket...
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Old 13-09-2019, 05:46 PM
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Had a look at the Bintel calculator, it gives the 1600 "Slight oversampling" both with and without the 0.63 reducer, and the same for my ASI294. I am awaiting arrival of an SCT thread UV-IR filter to see if it has as dramatic an effect on reflections on the SCT as on the ED72.

The ED72 would also be another prospect though it will probably be in line for an upgrade later, I am sure mono would uncover it's weakness in the blue end of the spectrum. The 1600 would be less undersampled than my 294 on that scope though.


Edit, gain wise I run the 294 at unity, for deep sky stuff nothing else really makes sense to me as one way will increase the apparent dynamic range but at the cost of non linearity (If one converted photon equals less than one electron it can not be linear) and the other way crunches dynamic range AND makes it non linear aside from gains that are whole multiples of 1. I used to run 0 gain but once I worked that one out I went to unity and have stayed there ever since.
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Old 13-09-2019, 06:15 PM
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I’d expect that 0.33” is more than slight oversampling the vast majority of the time

The advantage of the mono sensor (if it is an advantage at all), is that if the 72 has some blue haloing going on, you can refocus with the blue filter to minimise it. The colour filters have UV/IR cut built-in.

Regarding the gain, unity (120?) once HCG is enabled is probably a good spot, although a little higher wouldn’t hurt.

FWIW, unity isn’t actually true unity on this camera...it is ~5e/ADU since the well depth is substantially higher than the ADC resolution by about 4x...the wells may be able to accomodate ~63k electrons, but the 14-bit ADC can’t express that full range. That’s a very similar situation to the 1600. FWIW2, there’s no magic or purity in unity
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Old 13-09-2019, 06:39 PM
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More or less I am aiming at as close as I can get to 1 photon equals 1 electron equals one step in the ADC conversion (Too simplistic I know given I am ignoring QE completely there, I should call it as I did above, 1 converted photon)

I was thinking of doing some focus tests to see if I can defocus the red and green and get better blue focus. Even with the ASI294 if that is workable, with sufficient time invested in shooting and processing I can achieve similar by splitting out the channels to separate FITS files in APP then recombine the separately focused blue files with the normally focused red and green. I will have to fire up APT again and see if I can kill the red and green channels in the preview (Given APT debayers the image preview) and if I can work out a good focus offset from the normal and tighten the blue up in separate subs just to extract the blue channel from then it would hopefully indicate that the scope would work OK with a mono cam.

I am glad you mention the RGB filters being UV/IR cut, I had not tracked that bit of info down yet and I take the "Typical passband" pics on websites as marketing guff more than anything else. I do wonder what the passband of the ZWO red is, the RGB sample data I downloaded included something that was considered to be an artifact on a similar image in the Malins this year but I have worked out they are stars that are very rich in red (And presumably NIR) and almost invisible in the green data and totally absent in the blue, so in the final integration they show up as bright red stars and look like processing artefacts, miss-registered stars or similar. I had an image shot with my cam of the same target with the UV-IR filter and the ASI294 and they were not visible at all on my shot.
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Old 14-09-2019, 05:26 PM
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R/G/B filters are typically of the order around 100nm bandwidth. Check their spectral analysis carefully, and be cynical of those that don’t offer any

I’d also be sceptical of stars in an image that really are a deep deep red. There are relatively few that are brake-light red, and these are typically carbon stars. Outside of that, sufficient care likely hasn’t been taking when pushing the saturation...
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Old 14-09-2019, 06:29 PM
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I actually found these ones myself in the data, they are visible before any sort of pumping up of the saturation levels. I took it in to photoshop as a combined RGB image so I could flip the red, green and blue on and off quickly while zoomed in on the star in question (This was an image of the trifid and there were only a couple of obviously red stars in the data) I was a bit tempted to approach Ken and see if I could specify the star he may be able to grab it's spectrum.

Comparing what data is really easy to get, the Astrodon RGB set has a lot narrower red filter bandpass than the ZWO, Astrodon is about 625 to 670nm at the 3db points and ZWO is 590 to almost bang on 700. If you look at the cutoff points the ZWO extends (With obviously lower transmission) out to about 725nm where the Astrodon is at zero before 700.
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Old 14-09-2019, 08:57 PM
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This is getting a bit off my own topic but anyway..

I managed to get in a quick focus offset test tonight before the clouds rolled in. I ran an auto focus routine in SGP, took ten 30 second subs then auto focused again and manually offset the focus outward by an arbitrary amount before taking another ten 30 second subs, I integrated them as separate runs in astro pixel processor and saved them as TIF files so I could open them in photoshop and have a look at the blue channels only, the offset integration had noticably tighter focus on the blue stars at the expected expense of redder stars.

I more or less conclude from that, that even with the OSC I can improve the images some more by investing the time to work out the best focus offset for the blue channel and shooting two lots of subs to pick the red and green out of one set and blue from a focus offset run. SGP makes it easy enough to do, I set up a filter wheel simulator with a blue and lum only, the focus positions are set with the appropriate offset between them, the auto focus routine focuses using the "Lum" filter and the blue has an offset automatically applied. I can then set up two lines in the target sequence, one for the lum "Filter" and one for the blue, the blue will focus as many times as the normal setup would before applying the offset value.

It also means that even this little cheapie scope would likely benefit from a mono cam and RGB filters.
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Old 15-09-2019, 08:52 AM
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So, a quick animated gif, it shows some promise though it is a little harder to see now it is downscaled to get it under the 200KB limit to post here.

This is a process of the run described above, 10 subs stacked as auto focused by SGP, the second frame is the red and green from the first run combined with the blue of a second run with the focus offset by 70 steps, which might be too much, or too little, I did not get enough cloud free time to experiment more, I could see things that were obviously toward the green end of the blue pixels softening focus wise so it is probably too much of an offset.

A few processing steps to get this, basically to generate the better focused version I calibrated all 20 frames ("In focus" and offset focus) with a dark frame and then in APP split the calibrated frames colour channels and saved them individually, then restart APP (It might be doable without restarting by just deselecting the "Force bayer CFA option and clearing the file list) load the appropriate files as R-G-B lights and process from there like mono imaging.

For a proper run I assume for best results I would need to shoot flats at each focus point, just like you would for mono imaging. I did not bother with flats last night for the sake of an experiment.
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