I think I'm going to like this camera

[DavidTrap]

First light with my QHY600M over the weekend.

Just a bit of testing really - much to learn about optimal settings with the camera.

10 x 300 sec subframes of B33

AP130 with Quad TCC reducer

Chroma 3nm Ha filter.

Minimal processing in Pixinsight - haven't had time since coming home from Leyburn.

There's a bit of tilt - haven't worked out how to use the tilt corrector yet ;)

Link to Astrobin (https://www.astrobin.com/3hy7h4/)

Thanks for looking.

DT

[gregbradley]

That looks terrific especially for only .8 hours.

That looks to me more like a spacing issue with the TCC rather than tilt as all 4 corners seem affected. The TCC is fairly intolerant and needs +/- 1mm correct spacing. There is a note by Roland about how to check the spacing if its under or overcorrecting. Focus in the middle and note the focus position then focus on a corner star and note if you had to rack the focuser in or out.

Greg.

[multiweb]

That's a massive fov with good image scale. Good fun ahead for sure. :thumbsup:

[Peter Ward]

A great start. :thumbsup:

I've found the QHY600 is incredibly terse in highlighting focal plane tilt or off axis aberrations....hence have numerous adapters being made to tie things down a much as possible.

That said, once sorted, I can see this camera being responsible for some benchmark imagery in the future.

[DavidTrap]

Thanks Gents,

Greg - the back focus specs of the TCC are +/- 2.5mm. I’m within 0.5mm of the recommended figure, depending where the tilt corrector is within its range of movement. Analysis of tilt in ASTAP shows a definite skew towards the top right corner - it’s definitely visible at full resolution. I’ll be disappointed if it doesn’t resolve with adjustment of the tilt corrector.

Mark - I’m loving the massive FOV. I always envied the amazing wide field shots taken with big sensors on modest focal length refractors.

Peter - I’ve made up an alignment collar, on the 3D printer, to try to overcome the lateral movement of the dovetail interface to the QHY filter wheel. I’m hoping that will afford a degree of reproducibility in alignment for this itinerant imager. It’s definitely not going to be a point and shoot camera.

DT

[irwjager]

Congrats on the new gear! That bodes very, very well.


It'd be interesting to see how close you can get to describing one unit of detail per pixel with your gear under ideal circumstances (seeing etc.).


Clear skies!

[Placidus]

Very fine. Those cometary knots toward the top of the image are excellent. Much promise.

[Ryderscope]

Well done. Let the full frame journey begin:)

[gregbradley]

Greg - the back focus specs of the TCC are +/- 2.5mm. I’m within 0.5mm of the recommended figure, depending where the tilt corrector is within its range of movement. Analysis of tilt in ASTAP shows a definite skew towards the top right corner - it’s definitely visible at full resolution. I’ll be disappointed if it doesn’t resolve with adjustment of the tilt corrector.



DT[/QUOTE]

The +/- 2.5mm is for the flattener. The TCC is +/- 1mm.

https://www.astro-physics.com/quadtcc-ap130

https://www.astro-physics.com/13035ff

You most likely have both issues to some degree. Roland has an article (I'd have to look for it on their site) about spacing for the correctors. But the intro is interesting as basically he says that there can be minor variations between scope's lenses. So with +/- 1mm as a tolerance there is a chance a particular scope may need a slightly different spacing. Something to know if things are not round after correcting for tilt. You could make some spacers with your 3D printer.

Greg.

[DavidTrap]

The +/- 2.5mm is for the flattener. The TCC is +/- 1mm.

https://www.astro-physics.com/quadtcc-ap130

https://www.astro-physics.com/13035ff

You most likely have both issues to some degree. Roland has an article (I'd have to look for it on their site) about spacing for the correctors. But the intro is interesting as basically he says that there can be minor variations between scope's lenses. So with +/- 1mm as a tolerance there is a chance a particular scope may need a slightly different spacing. Something to know if things are not round after correcting for tilt. You could make some spacers with your 3D printer.

Greg.[/QUOTE]

You are correct on the specs - my apologies.

I'll try correcting tilt first, then worry about spacing.

DT

[DavidTrap]

Thanks also to Ivo, M&T, and Rodney for your kind words.

DT

[ChrisV]

That's looking really good

[DavidTrap]

Thanks Chris,

Just need to find time to process some more images I took last weekend...

DT

[Peter Ward]

I am curious as to what readout mode you settled on (plus gain/offset) ?

Secondly, I have to say those Chroma filters are pretty awesome as...minimal halos. :thumbsup: makes the Baader H-alpha I am currently using for my QHY600 look sad.

[DavidTrap]

I’m hoping someone like yourself is able to enlighten me on as to what mode, gain and offset I should be using. This was shot in readout mode 1 (high gain), gain 300 and offset 76. Any guidance would be greatly appreciated.

Very happy with the Chroma filters, except for the price tag - although at least they’re less than the Astrodons...

DT

[strongmanmike]

Been watching this thread Dave, a good looking result there mate, sounds like you are moving in the right direction, look forward to the high quality results that seem likely to come your (and our) way :) :thumbsup:

Mike

[DavidTrap]

Thanks Mike,

It’s a good beginning, but I realise it’s a long process to really get this thing humming.

DT

[alpal]

Hi David,
that's a great start.
I read that the IMX455 sensor has about 72% QE efficiency
at the 656 nm Ha wavelength which is very high.
https://www.qhyccd.com/uploadfile/2019/0925/20190925055202120.png

Why did you chose 5 minute exposures?
I'm not familiar with CMOS cameras.
Is there a problem doing longer exposures with CMOS?
I found with my QHY9m CCD that a 20 minutes exposure
was an ideal amount of time when using Ha.

cheers
Allan

[DavidTrap]

Thanks Alan,

It was more of a testing weekend. The wells on this camera are relatively deep, so I’ll definitely be trying longer exposure. That said, there is a trade off with CMOS between exposure duration and read noise that means you do well with relatively short exposures compared to CCD - I don’t fully understand that, a bit like my lack of understanding about gain, offset and readout mode. Many “knobs to twiddle” and much to learn!

DT

[Peter Ward]

:lol: I find it weird that you can even vary these parameters let alone have insight to how they affect data gathering. If I have a Eureka moment I'll let you know :)

BTW I have found that QHY's stated back focus and thickness dimensions are rubbish. AP's Quad compressor has a +/- 1.00mm tolerance, which I found wasn't being met when I actually measured everything with a micrometer. I expect my AP spacer set is now correct but have yet to test it on the sky.....

[alpal]

Hi David,
I spent a bit of time investigating CMOS cameras

over the last few years but didn't get very far.
In your case it would have been interesting to take 4 of your 5 minute Ha frames
and stack them and stretch them -
then compare with say one 20 minute Ha frame.


I believe you're right and the CMOS cameras have too high a

read out noise to take long exposures.
It would be nice to see that demonstrated rather than a page
full of formulas such as here:
http://www.astrosurf.com/buil/CMOSvsCCD/index.html


cheers
Allan

[TrevorW]

As far as I know the for CMOS cameras is multiple shorter exposures say 3 minutes also gain and offset lower values :)

[DavidTrap]

Looking forward to your enlightenment!

Not impressed about the backfocus distances being out. Any hints as to where they’re wrong. Are you using the QHY filter wheel?

DT

[DavidTrap]

Thanks Allan, I’ll have a look at that link - I like to understand the theory, rather than just follow a recipe.



Thanks Trevor - just need to know what values to use and when.

DT

[gregbradley]

CMOS cameras have way less read noise compared to CCD. For example at high gain mode the 600 has read noise of around 1.2 electron whereas a good quality 16803 sensor is 9 electrons.
Dark current is also typically lower.
Think of gain as ISO and offset as the black point. Higher ISO on a digital camera means less dynamic range but the amount it drops gets complicated as these Sony sensors often have a 2 step gain where the read noise drops significantly at a certain gain(ISO).

High gain reduces well depth but lowers noise and also weakens Dynamic range.

Too small wells at high gain and you'll blow out your stars and get these all white star type images with no star colour.

The reason shorter exposures works is because the read noise is a fraction of CCDs so you get your signal above the read noise much sooner than you will with CCD.

If you want to do long exposures then you may want to use the mode that has the deepest wells (ie. best dynamic range).

I did 10 minute exposures in narrowband no problem with myASI183mm at a relatively high gain (111). I use 10 minutes with my CCDs as well. But I did start using 5 minute subs for narrowband as it seemed the sweet spot.

QHY gain and offset numbers seem to be different to ASI numbers so the settings aren't interchangeable.

Greg.

[alpal]

Hi Greg,
"I think you meant;
The reason shorter exposures works with CMOS is because the read noise is a fraction of CCDs so
you get your signal above the read noise much sooner than you will with CCD. "

Also - if the read noise is well below the sky noise from
a suburban location then I don't see how a long 20 minute subframe
would make any difference using CMOS?


cheers
Allan

[gregbradley]

Yes quite right.

Well you probably could but I think you'll find that often bright areas will blow out due to the super high QE of these sensors.

Greg.

[Peter Ward]

The run of inclement weather in Sydney has made it difficult for me to explore the QHY600M's capabilities further, but I am coming to the conclusion the intrinsic noise of its CMOS sensor is about an order of magnitude higher than my KAF16803.

Read noise looks to be a red herring. It is simply the consistency of the signal your read-up, and says nothing about the pixel to pixel variations.

Problem is CMOS have outliers that while consistent in value, are spread over a much wider bell curve....

I've put some more test data here (http://www.atscope.com.au/BRO/tutorials/QHY600.html).

Don't get me wrong...I like this camera a lot. The resolution is awesome.
But for ultra dim targets from my urban sky....humm...no revelation so far.

[alpal]

Thanks Peter,
that's a good write up -

especially the 2 pictures shown with these words:








The CMOS sensor is superior.


cheers
Allan

[Peter Ward]

Well, swings and roundabouts. I have yet to use this sensor on my RC16, where the sampling would favour larger pixels. The big difference between this chip and the KAF16803 is the noise...note that the pixel values of the median combined CCD vary just 8 ADU, the CMOS however has a bell curve well over 100 ADU wide. A bit like 1600 grit sandpaper vs 80 grit...the CCD gives you a much smoother finish...

[alpal]

Hi Peter,
you are getting a very narrow range of ADUs and
also a very low value too.
I'm not getting that with my KAF8300 CCD.
See the pic below.
From a dark frame I get a range of values from 898 to 1149
which is 251 ADUs wide.
You are getting only 8 ADUs wide with your KAF16803 CCD.
You are also way down at an average 100 ADUs out of a possible 65536
whereas I'm at 1019 ADUs.
You're an order of magnitude better in base noise.

What's going on here?


cheers
Allan

[Peter Ward]

As I mentioned on the web page, the ADU levels shown are from 12 median combined, dark calibrated darks taken from both sensors (if you just used uncalibrated darks, all you'd be looking at is thermal noise).

I used ALL of the KAF16803 pixels for the upper graph, but only a 190k pixel sample from the CMOS....not entirely valid sampling I know, but when I increased the sample size for the CMOS the bell curve actually got wider.

I am not aware of any systemic error I would have made, but if there has been a blunder on my part, please let me know.

P.S. You can download my KAF16803 median combined dark calibrated dark here (http://www.atscope.com.au/BRO/tutorials/MasterDarkCCD.fit) (right click, save as)
for completeness the CMOS calibrated dark is here (http://www.atscope.com.au/BRO/tutorials/MasterCMOSdark.fit) right click, save as.. careful it's 122Mb

[alpal]

OK thanks Peter,
I think it's because you're using 12 median combined, dark calibrated darks -
they are processed - not the RAW data coming straight off the sensor.
The median combine is smoothing out your real values.


cheers
Allan

[Peter Ward]

The rub is: I did the same with the CMOS and got a spread of 100ADU

The goal was to see how consistent calibrated data would be.

[alpal]

Hi Peter,
that's true.
I think the reason that my average value is 1019 ADUs and yours is at 100 ADUs
is the offset in my camera.
I set it to 110 which I don't quite understand but it has probably placed
the average where it is. ( I copied a known good value)
see here:
http://www.stark-labs.com/help/blog/files/GainAndOffset.php

By the way - my gain is set to 10 which is supposed to mean 10%
but that's not true either as my maximum peak is at 54,570 ADUs
(not 2 to the 16) = 65536.

cheers
Allan