CCD imaging problems

[cgarry]

Last night I managed to get a second go with my new QHY8L camera. I ran into some of the same problems as I had the first time I used the camera and was hoping for some advice. This time I know there were no dewing problems.

Anyway, I took 21 x 300 sec subs but only stacked 11 out of the first 12 as the later subs had a very bright background. This is the result of the stacking (with DSS) with a stretch applied and converted to jpeg:

The issues I have are:

(1) The vignetting has distinct rings in it, I have never seen this using my DSLR.

(2) The DSO itself is very dim, barely above the background glow and I cannot bring it out to produce a final image.

I thought these problems could be due to the sky not getting dark enough, so I plotted a graph of the average (mean and median) pixel value for each of the subs. This is the graph:

From the graph it looks like I only got 3 subs before the sky started to brighten (I assume the increase in the average pixel value is due to the sky brightening).

Is it possible that the problems I am seeing is just down to the sky not getting dark enough for long enough? Does this mean that I just need to give up on imaging for a while until the nights get darker for longer? Is my CCD working as it should?

I can make the original fits files available if anybody wants to have a play with them.

Cheers,

Chris

[harry page]

Hi

Ok lets have a look at your files

You can upload them to my FTP server if you cannot post them anywhere

Harry

[roundycat]

last night I worked from about 22:30 to near dawn without a problem. Admittedly I was using Narrowband but it most certainly is not to bright to work between 11 and 2am.

Dennis

[harry page]

Hi

Had a quick stack with your subs in AA5 and could not find anything that would cause your steps in the vignetting;)

The only time I have seen this is when you are not processing in 16 bit depth, but a couple of comments

1) m101 is quite faint and high up near the twilight and dark skies are needed for this object , I do not image at this time of the year as its to light for normal RGB

2) There is quite severe vignetting and flats will be a must , can you improve this ?

3) The stars in the corner are elongated as if the sub is rotating during exposure , how are you guiding , what was your polar align like ?

4) CCD images usually are 16bit depth unlike a dslr this can make the dslr look more sensitive , but in fact it is not

Regards Harry

[cgarry]

Hi

Had a quick stack with your subs in AA5 and could not find anything that would cause your steps in the vignetting;)

The only time I have seen this is when you are not processing in 16 bit depth, but a couple of comments

1) m101 is quite faint and high up near the twilight and dark skies are needed for this object , I do not image at this time of the year as its to light for normal RGB

2) There is quite severe vignetting and flats will be a must , can you improve this ?

3) The stars in the corner are elongated as if the sub is rotating during exposure , how are you guiding , what was your polar align like ?

4) CCD images usually are 16bit depth unlike a dslr this can make the dslr look more sensitive , but in fact it is not

Regards Harry

Many thanks for that Harry, it does seem that either I or Deep Sky Stacker are doing something wrong at the stacking stage to get the steps in the vignetting.

1) I agree M101 is not a great target at this time. I am really just trying to perfect my guiding skills and the use of this camera before the real dark nights return rather than produce any stunning images.

2) Yes the vignetting is severe, I guess this is down to a combination of the C9.25 + x6.3 focal reducer and the large (near DLSR sized) chip in the QHY8L. I do not have any idea how to reduce the vignetting as it appears on the chip, but flats will be required and I have yet to decide on a plan to produce them yet.

3) I am guiding with an ST80 + QHY5 mounted beside the C9.25 with a side-by-side saddle. Although PHD keeps the guide star exactly on its spot, the actual images taken through the main scope do seem to shift slightly during the session. I need to investigate the cause of this, my guess at the minute is flexture in the setup.

I have not checked the polar alignment of my pier in quite a while and it could be out. I will check it out before trying again.

4) Thanks for pointing out the difference between the DSLR and CCD cameras, I will keep that in mind when comparing the 2 in future.

Many thanks for your help Harry, I have a much better handle on what is going on now.

Cheers,

Chris

[Beyond_Vision]

What capture program are you using and what gain and offset settings are you using for the QHY8?

Regards

Kevin

[cgarry]

What capture program are you using and what gain and offset settings are you using for the QHY8?

Regards

Kevin

Hi Kevin,

I am using the software that came with the camera, EZCAP (v2.4), for capture.

Gain was set to 2 and offset was set to 125. These values were chosen after following the procedure in the user manual.

Cheers,

Chris

[Beyond_Vision]

I have only used Maxim and Astro Art so cant relate the figure I use to EZcap. I was wondering if the offset was too high I would try around 110-115.

Regards

Kevin

[harry page]

Hi

I also use astroart so I am not sure what these settings are:confused:

The subs I looked at have a full 16 bit range in them

Harry

[cgarry]

Thanks guy, Astro Art looks pretty good. I shall have to check it out.

Chris

[martin_h]

Here is a link to a document download that I use to set Gain and offset for my QHY ccd's.

gain and offset.pdf

[cgarry]

Thanks for that Martin, now I have something to do during these light evenings.

Chris

[ollypenrice]

Most people imaging with SCTs use off axis guiders because flexure is hard to avoid and very punishing at long FL. (The mirror itself can move, making an OAG the only solution.)

I also use AstroArt and feel it has a lot going for it. I feel with DSS that you have less of a clue what is going on. In AA it is easier to check where a problem might lie.

It is amazing how well flats wioll deal with vignetting. You certainly need to take them, especially with the very uneven illumination of the pre-Edge SCTs. I use a Neumann panel Gerd Neumann jr.-Aurora Flatfield Panels-

but in the large sizes they are a bit minty.

For your tests I'd suggest that a target high in the milky way, a cluster, would give you the most feedback on distortions etc and give you enough SNR to work with.

Olly

[Cloudwatcher]

That's really weird!

I'm no imager so have no idea what causes the problem but you can reduce the effect in Photoshop using the Exposure tool on a separate background layer..... a quick tweak shown below,not brilliant but most of the concentric circles are gone.

[MikeWilson]

Hi Chris,

May I ask why you chose "2" for your gain value? I use "0" for gain and "120" for offset.

All the best,

Mike

[cgarry]

Hi Chris,

May I ask why you chose "2" for your gain value? I use "0" for gain and "120" for offset.

All the best,

Mike

I followed the "Setting the GAIN and OFFSET" section in the QHY8L user manual and 2 + 120 were the values that I came away with.

Thanks for the advice Olly, very useful. I had thought that an off axis guider would be the way forward for accurate guiding but was worried that it might be hard to find a guide star. If I could get that working it would be ideal as it is easier to set up one scope than two.

Cheers,

Chris

[cgarry]

Looking at my guiding problems now, I seem to get a slight shift in each and every sub. As an example here is what I get if I use Startrails on 21 x 5 minute subs:

Does this look like a flexture problem? I do not see any field rotation so I do not think my polar alignment is far out.

I am really just looking to get a handle on the current problems before I splash out money on an off axis guider.

Cheers,

Chris

[roundycat]

So long as each sub frame is sharp do not worry about a slight shift between them. That will act like a dither and go towards ensuring your master frames are noise free.

Dennis

[cgarry]

So long as each sub frame is sharp do not worry about a slight shift between them. That will act like a dither and go towards ensuring your master frames are noise free.

Dennis

I agree the slight shift is acting like a dither and a good thing but to be honest my engineer's mind will not allow me to just accept this without understanding it!

The stars in the individual subs are actually quite round and show no sign of smearing, so it does seem that the shift is happening between images not during them. My current theories are that I have inadvertently enabled dithering or the downloading of the image from the imaging camera is taking all the USB bandwidth and stopping the tracking/guiding for a few seconds after each exposure. I shall investigate further when the skies clear again.

Cheers,

Chris

[roundycat]

A lot of SBIG's (both of mine) have a mechanical shutter so cannot guide during image download as the shutter is closed.

Dithering should not all be in the same direction. If it is to be any use the dither moves should be made randomly around a nominal centre point.

I also have a feeling that MaxIm gives priority to downloads so even with a separate guide camera guiding could be comprmised. It happens to a very small degree with my AP1200 and separate guide scope/camera and I am not bothered by it.

Dennis

[Tim]

Does this look like a flexture problem? I do not see any field rotation so I do not think my polar alignment is far out.

I am really just looking to get a handle on the current problems before I splash out money on an off axis guider.

Yep, flexure I would say, or possibly the mirror shifting a little. Either way an OAG will sort you right out, I used to struggle to take 5 mins subs with my SCT (C9.25), since using an OAG I can happily take single subs over an hour in length with no sign of movement. Definitely recommend one. You do need a sensitive guidecam to get the best out of them.

[Beyond_Vision]

If it was flexure would it not show up on the individual subs?

I have wondered myself in the past if this movement is more down to the guiding suffering during the imaging camera download so I would tend to agree with Chris and Dennis

[cgarry]

I do want to move over to using an OAG, I like the idea of a setup with a single scope without flexure problems. Is the QHY5 sensitive enough to use with an OAG? Also, are there any particular OAGs which should be avoided?

Cheers,

Chris