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QHY268M Field Notes and First Light: M51 (in progress)

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#1 BenKolt

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Posted 12 April 2021 - 07:45 PM

Greetings!

 

Like many reporting on this forum, I have recently received a new QHY268M.  This is just part of a whole set of new equipment that I ordered in early January:

  • QHY268M: 16-bit APS-C sized CMOS, 3.76um pixels
  • QHYCFW3M-US: thin medium-sized FW
  • QHYOAG-M: medium-sized OAG
  • 36mm Round Unmounted Astrodon LRGB + 3nm NB Filters (Ha and SII filters not available yet)
  • Filter Centering Masks from Buckeyestargazer Shop (I highly recommend these!)
  • Various Adapters and Spacers from QHY, Moonlite Focuser, many other vendors (not all available yet)

Prior to collecting enough gear to mount the new camera, I characterized it and came up with some devations in my measurements from the specifications posted by QHY.  I posted some graphs in case you are interested.

 

After a long wait, I have finally been able to mount my new gear on my Teleskop Services 130mm / 910mm Photoline Apochromatic Refractor.  I also have the TSFLAT 2,5 flattener for this scope, but unfortunately the adapter required to connect it to the QHYOAG is not yet available.  For the time being I have to be content with slanted corner stars. The focuser is a Moonlite CHL, which takes 68mm threaded connection.  I am using the OAG's 54mm-sized spacers, and I had an adapter made by PreciseParts.  Only later I discovered that this adapter is available elsewhere, however it's yet another one that has a long, unspecified wait time.

 

Over the past couple of days I've been spending much time adjusting the pickoff prism of the OAG, calculating spacing, mounting, dismounting the imaging gear, but I finally have things assembled in working order.  As many on this forum have previously reported, care must be taken in adding sufficient spacing between the FW and the OAG so that the guiding camera can be focused in tandem with the imaging camera.  This was not a trivial exercise.  For guiding I use a QHY178M, and I could only find vague specifications on its back focus.  I think it's on the order of 12mm, although it could be 10mm.  I also couldn't add too much spacing between the FW and OAG in that the longest screws provided by QHY still needed to thread sufficiently through all the spacers, OAG and into the front plate of the FW.

 

Here is a picture I took of my assembly with annotations showing the items and their back focus.  I made this for my own records, but also wanted to post it here as it may be of use to others:

 

QHY268M-FW-OAG TS 130-910 2021-04-12-1.png

 

At first I thought that I'd get by with a 10mm long M54 spacer between the FW and OAG, but when set it all up, I quickly discovered that I couldn't quite focus the guider with the imager.  I had the guider seated as far down as it could go in the holder of the OAG, but it wasn't close enough.  I then added an additional 3mm long M54 spacer, and this was now sufficient.  I have a small amount of play now with the OAG's helical focuser where I can bring the QHY178M into focus.  The rest of the gear is labeled in the picture.

 

When I get the last adapters to attach the scope's flattener, I should still have plenty of room to fill the 88mm or so back focus budget.

 

Next I'll post my first light image (still a work in progress) along with some reflections on how this camera is operating for me.

 

Best Regards,

Ben

 

 


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#2 BenKolt

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Posted 12 April 2021 - 08:08 PM

Here is a combined wide FOV image of M51 using just the L filter.  It is comprised of just 31 x 120s = 62 min integration time.  I have a rare set of clear nights this week and intend on collecting more integration time on this and perhaps another galaxy target.  Again, I wasn't able to include the flattener for the TS 130/910, hence the corner stars are slanted.  This image has had no noise reduction, no convolution, only a very mild gradient removal (almost not needed due to the New Moon) and stretching.

 

M51_L_137-02_20210412.v002B_HT1-1.jpg

 

I have to say that this is the cleanest, simplest calibration that I've every done.  I am even more impressed with this camera than I thought I would be.  My experience with CMOS imaging has thus far only been with my little QHY168M, which is 12-bit, small full well capacity, and a large, angry amp glow.  This new camera acts much more like a CCD.  I operated in Mode 1 with gain setting 0, offset setting 15, temperature -10C.  According to my measurements, this corresponds to a gain of 0.8 e-/ADU, read noise of 3.5 e-, full well capacity 50 ke-, dynamic range of about 14 stops.  (Take a look at my plots I linked before if you want to see more.)

 

I arrived at 120s exposure times for the light subs in that I was only saturating a few stars in the field, and M51 itself was not at all saturated.  I am also taking RGB frames, but I didn't want to post any color yet as I need more integration time to do it justice.  RGB frames are 240s exposures.  I'll post a final LRGB image later.

 

There is an ongoing discussion about optimal number of flat frames.  I tend to shoot for total flat exposure on the order of 500 ke- to 1 Me-, and in this case I used 25 frames with peak at the middle of the bit range, about 30,000 ADU or so, and this corresponds to about 750 ke-.  I used a flat panel to expose the flats, using 4s exposures.  I subtracted a 4s dark master from each flat sub.  I used 25 dark frames for the flat-dark.  For the 120s and 240s frames I created master darks of comparable exposure, using about an hour's worth of total integration time each.  I note that the dark current on this camera is so small that I doubt these master darks are introducing any real comparable amount of noise.  I measured about 0.0017 e-/px/s at -10C.

 

As I said before, this has to be one of the cleanest, easiest calibrations I've every done.  I expected there to be many issues since it was first time with a new camera and filters, but it all worked out well.  I'm happy to go into more detail about all this if anyone is interested.

 

One more post on this to come ...

 

Ben


Edited by BenKolt, 12 April 2021 - 08:25 PM.

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#3 BenKolt

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Posted 12 April 2021 - 08:16 PM

Finally, here is a cropped image around M51.  Again, this is only about an hour of integration time so far.  Note that resolution is about 0.83 "/px.  Seeing conditions were pretty bad last night, but I am still pleased with the outcome.  I look forward to applying noise reduction and deconvolution to this later when I've acquired more integration time.  I think the seeing is to improve over the next several nights.

 

M51_L_137-02_20210412.v002C_Crop1-1.jpg

 

With such a low read noise, I probably can get by with even shorter subs, and that will saturate even fewer stars.  However, I wanted to follow the 10 x RN^2 rule, and these exposures are not too far beyond that.

 

Thank you for reading through this and looking at the product of my efforts.  And thank you for indulging my excited loquaciousness about the new camera.  I look forward to seeing what else I can do with it in the future!

 

Best Regards,

Ben

 

 


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#4 polslinux

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Posted 13 April 2021 - 02:33 AM

Very interesting, thanks for sharing!

One question: do you know what's the best ADU for a light frame in order to not saturate (too many) stars? And with which gain/offset?



#5 SilverLitz

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Posted 13 April 2021 - 06:16 AM

How far down is your OAG prism?  (with reference point)

 

If you 178 is like the QHY5L-II-M, my reading is the BF is ~11mm with the CS attached and 6mm without the CS.  The CS will unthread, but then the sensor is totally uncovered. 



#6 Shannon Foye

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Posted 13 April 2021 - 09:04 AM

I also couldn't add too much spacing between the FW and OAG in that the longest screws provided by QHY still needed to thread sufficiently through all the spacers, OAG and into the front plate of the FW.

The screws I received are 27.5mm long. At this length and with 55mm of backfocus to the sensor (according to QHY's manual), the screws have 3mm of exposed thread for securing the spacers and OAG to the filter wheel. The threaded holes on the filter wheel cover appear to be 3.5mm deep.

 

I believe I need 3mm or more of additional back focus, so I ordered the 30mm and 35mm M3 stainless screws below. I specifically ordered the harder stainless versions because small screw heads made cheap soft steel are easy to strip.

 

If I need screws longer than 30mm which I probably will, I'll cut down the 35mm screws to a length that gives me just under 3.5mm of thread for securing the spacers and OAG to the filter wheel.

 

M3x35mm:
https://www.amazon.c...0?ie=UTF8&psc=1

 

M3x30mm:
https://www.amazon.c...1?ie=UTF8&psc=1



#7 BenKolt

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Posted 13 April 2021 - 09:39 AM

Very interesting, thanks for sharing!

One question: do you know what's the best ADU for a light frame in order to not saturate (too many) stars? And with which gain/offset?

polslinux:

 

Thanks!

 

That's a great question, one that I've wondered myself. The answer will vary quite a bit depending upon your sky fog conditions, brightness of the stars, camera gain and offset settings. There's also a choice here in that you may be interested in pulling out detail of low intensity features, say, of a galaxy, and not be interested in the surrounding foreground stars. In those circumstances, one may be willing to let some of the stars saturate since they will likely be near saturation anyway after doing a final stretch of the image.

 

One thing I've played with in the past is High Dynamic Range Combination, where one combines long and short exposures together to increase the dynamic range.  I've had mixed results with that.  I've mostly been content to let my brightest stars saturate since I'm usually interested in galaxies.

 

Best Regards,

Ben

 

 

How far down is your OAG prism?  (with reference point)

 

If you 178 is like the QHY5L-II-M, my reading is the BF is ~11mm with the CS attached and 6mm without the CS.  The CS will unthread, but then the sensor is totally uncovered. 

Mark:

 

I meant to measure that accurately before (finally) mounting the gear on the scope for my current imaging series but forgot.  I'll get back to you about that, likely this weekend when I pull my gear back inside.  This was something that I needed to adjust in order to get the QHY178M to focus well with the imaging camera, but I also wanted to insert the prism as far into the imaging circle as I could to minimize edge distortions.  It sounds like my camera is similar to yours in that there is a CS attachment, and removal of that completely exposes the sensor unprotected.  That wasn't a choice, of course.  It would be nice to have a shorter windowed cover that doesn't have any internal threads.  This could be quite short and return several mm of back focus.

 

Best Regards,

Ben

 

The screws I received are 27.5mm long. At this length and with 55mm of backfocus to the sensor (according to QHY's manual), the screws have 3mm of exposed thread for securing the spacers and OAG to the filter wheel. The threaded holes on the filter wheel cover appear to be 3.5mm deep.

 

I believe I need 3mm or more of additional back focus, so I ordered the 30mm and 35mm M3 stainless screws below. I specifically ordered the harder stainless versions because small screw heads made cheap soft steel are easy to strip.

 

If I need screws longer than 30mm which I probably will, I'll cut down the 35mm screws to a length that gives me just under 3.5mm of thread for securing the spacers and OAG to the filter wheel.

 

M3x35mm:
https://www.amazon.c...0?ie=UTF8&psc=1

 

M3x30mm:
https://www.amazon.c...1?ie=UTF8&psc=1

Shannon:

 

Thank you very much for the links!  I considered purchasing longer screws as well but was finally able to assemble with the longest 27.5mm screws.  I did not plan to achieve 55mm backfocus with the QHY spacer plates as my needs will be mostly for longer back focus.  For this scope, for example, I'll be wanting to hit about 88mm.  I have a collection of 54mm and 68mm spacers that ought to get me to what I need.  I have a future plan for this camera with a 10" RC as well with its flattener, and I will need to revisit this point again at that time.  But I will think about getting those longer screws to assemble the whole Camera+FW+OAG system at 55mm and then add any additional spacers as needed.

 

Best Regards,

Ben

 

 



#8 SilverLitz

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Posted 13 April 2021 - 01:25 PM

Here is my estimates for my EdgeHD925 using my QHY5L-II-M with CS removed, and 11mm of spacers between the OAG & CFW.  This gives me a total of 56mm (55+1 for filters).  With the CS attached, I do not think I can get it to work.  It would be great if the OAG also came with an M48 OAG Thread Adapter (3mm thick, like their M54 & M42 plates), then you could move 2mm from before the OAG to after the OAG, and the QHY5L-II-M would work with the CS attached.''  Without the CS, I could also move the 1mm spacer to be before the OAG.

 

At my preferred 4mm Stalk Retraction, I estimate that the bottom of the prism will project at ~8mm above the center at the image sensor, ~right above the top of the QHY268M's long edge, getting the most of the light cone without shadowing.

 

EdgeHD925 + OAG.png

 

For those of you that are curious, the COAG column, puts Celestron OAG at its "normal" distance, but the problem is the opposite, e.g. getting the guidecam far enough away w/ extra spacers on the OAG.



#9 BenKolt

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Posted 13 April 2021 - 08:41 PM

Sorry that you are having this trouble, Mark. This design is certainly a tricky one. Have you examined the possibility of using the small version of the QHY OAG, or would that have such a smaller opening to affect your image with vignetting?

 

Ben



#10 elmiko

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Posted 13 April 2021 - 09:24 PM

Nice first light images Ben! Great job on the imaging train diagram.



#11 SilverLitz

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Posted 13 April 2021 - 09:53 PM

925

Sorry that you are having this trouble, Mark. This design is certainly a tricky one. Have you examined the possibility of using the small version of the QHY OAG, or would that have such a smaller opening to affect your image with vignetting?

 

Ben

I do not expect vignetting to be a problem.  The problem is the minimum distance to the guidecam while still having 56-56mm from the output of Celestron T-Adapter (M48), but eliminating the CS will solve that problem.  ASI mini cams would also work with 10mm between the OAG and CFW as their BF is a few mm shorter.  

 

I will actually put everything together tomorrow for my EdgeHD 925, as I transferred my existing filters (have not yet gotten new filters) from the ZWO EFW to the QHY CFW3.  I only got the QHY268M last week, but I was still working on a project with my Esprit 100 & ASI183mm-Pro.  For the next 1-2 months, I will be hunting the small galaxies with my EdgeHD 925 and the QHY268M. 



#12 BenKolt

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Posted 15 April 2021 - 07:36 PM

Some folks may be interested to see my updated M51 image after acquiring more frames.  This is the combination of the luminance data of 150 x 120s = 5 hr integration time.  This constitutes about 70% of my total stack accumulated over the past several nights.  Inevitably, some frames come out more blurry than others, either from my mediocre seeing conditions, focusing issues, an owl landing on the telescope, whatever. This image is a simple stretch with linear background gradient removed.  At this point there is no deconvolution, noise reduction or other steps that I may wish to take later when putting together a final LRGB image.  I also didn't do anything fancy with the stretching to rebalance the brighter and dimmer parts - just slide the histogram around a few times.

 

M51_L_137_20210412.v001C_HT-1.jpg

 

The rendering on CN doesn't look as good as it does on my own screen - I suppose there is some kind of compression going on with the upload.  I am impressed with the final detail of the tidal streams that come off of M51 and its companion galaxy.  I'll try to process those for better clarity.

 

 

 


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#13 BenKolt

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Posted 15 April 2021 - 07:42 PM

And here is M51 cropped:

 

M51_L_137_20210412.v001D_Crop-1.jpg

 

I'm looking forward to what a mild application of deconvolution can do to this image.  I'll see if I can process further this weekend.

 

As I posted before, I am quite pleased with this camera.  It has actually performed beyond my expectations, which is a rare occurrence!

 

I'll return to this thread later with a measure of my prism location inside the OAG, but that will have to wait a few more days when I disassemble.  I should have 4 more reasonable nights to go before clouds and rain return, and it's my plan to move on to M106.

 

Ben

 

 


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#14 Lucca V

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Posted 20 April 2021 - 09:39 PM

From your work, I can learn that this is a really good camera worthy getting.



#15 rockstarbill

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Posted 20 April 2021 - 10:16 PM

You have some star elongation at the edges, which scope is this? FSQ?

 

Ignore: I see its the TS and no flattener yet.


Edited by rockstarbill, 20 April 2021 - 10:18 PM.


#16 BenKolt

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Posted 21 April 2021 - 09:30 AM

From your work, I can learn that this is a really good camera worthy getting.

Lucca:

 

That's ultimately only a decision that you can make for yourself, of course, but I'm pleased the information several of us have posted here is of use to you.  Good luck!

 

Best Regards,

Ben



#17 BenKolt

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Posted 21 April 2021 - 10:37 AM

You have some star elongation at the edges, which scope is this? FSQ?

 

Ignore: I see its the TS and no flattener yet.

 

Bill:

 

Yes, that's right.  The scope is a TS 130/910.  I've had it a little over a year now and remarkably have imaged with it 76 nights to date!  I originally purchased it to explore its use with my new small-pixel QHY183M.  That was my very first CMOS camera.  Now a year later I've upgraded to the QHY268M, which comparatively, of course, is superior in all ways.

 

I do have the flattener for the TS scope, the "TSFLAT 2,5", however, of all the adapters and spacers I needed to order, one in particular has been on backorder for months. In hindsight, I should have paid the extra money to have one made for me, but I didn't know about the long delay at the time.  Now I'm told it'll be available in about 2 more weeks.  In the meantime, I do have all the connections that I need without the flattener, which I posted above.

 

I'm looking forward to adding the flattener.  The elongation around the edges is quite pronounced, and in fact I set my autofocus in SGP to only use an interior portion of the FOV so as to ignore the slanted stars.  Our latest string of clear weather is now over, so I'll turn my attention to processing my first light data.

 

Ben



#18 MarcoItaly

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Posted 21 April 2021 - 11:34 AM

Interesting thread, I was just testing my brand new 268M on M51 as well last night!

I put together 1h L with L-Pro filter and RGB 20min each with a TS ONTC 10"f4 Newton.

I used a different camera setting, Mode 1 with gain 80.

Here the result (heavily compressed):

Attached Thumbnails

  • M51_ONTC_268M_LR.jpg


#19 MarcoItaly

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Posted 21 April 2021 - 11:34 AM

I know I have a problem with one filter, should be slightly dirty. I have to find which one.



#20 BenKolt

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Posted 21 April 2021 - 06:37 PM

An unusually long stretch of clear April nights in the Pacific Northwest just ended.  I have brought all my equipment inside and was finally able to measure where I ended up placing the OAG prism in the QHY-OAG-M.  Measuring from the bottom of the prism (closest to center of light path) to the point where the prism stalk disappears into the OAG's outer ring, the distance is 13 mm as shown here:

 

QHY-OAG Prism Spacing-1.png

 

Ben

 

 


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