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Are my flats working correctly?


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My imaging session on Saturday night had a few issues, thus is one of them.

This is the result of stacking (AA5) 36 x 5 minute subs + darks with just a stretch:

post-21918-13387764124_thumb.jpg

This is the result of stacking (AA5) 36 x 5 minute subs + darks + flats + dark flats with exactly the same stretch:

post-21918-133877641247_thumb.jpg

This is a stack of the flats and dark flats with an auto-stretch:

post-21918-133877641234_thumb.jpg

Is this the kind of result that would be expected from using flats? It seems to me that the flats have just inverted the background gradient rather than removing it.

Cheers,

Chris

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I would say no - it looks like they are over-correcting....

Which camera was it with..?

Check the individual flats and see if there are any changes flat to flat (start with the first and last) .. in case its a dewing issue...

Peter...

Edited by Psychobilly
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I'm puzzled as to why you are applying both flats and dark flats. If you really are doing that then the flat is being applied twice which is why you appear to have an over calibrated image. My advice would be to re-process the image by calibrating with just flats and darks if the darks are at a similar temp to your lights.

You also seem to have some field rotation as shown by the 'circular' star trailing towards the corners. Field curvature problems would typically give rise to radial star trailing

HTH

Steve

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I would say no - it looks like they are over-correcting....

Which camera was it with..?

Check the individual flats and see if there are any changes flat to flat (start with the first and last) .. in case its a dewing issue...

Peter...

The camera used is a QHY8L. It has a very big chip and I think that is the reason for the strong vignetting.

I have been through the subs and did check the equipment as I packed up and am sure there was no dewing. The background sky does get slightly brighter with time as dawn approached. I have tried stacking just the first hours worth of subs and still see the same overcorrection of vignetting problem.

I'm puzzled as to why you are applying both flats and dark flats. If you really are doing that then the flat is being applied twice which is why you appear to have an over calibrated image. My advice would be to re-process the image by calibrating with just flats and darks if the darks are at a similar temp to your lights.

You also seem to have some field rotation as shown by the 'circular' star trailing towards the corners. Field curvature problems would typically give rise to radial star trailing

HTH

Steve

My understanding is the dark flats are effectively bias frames and are used to remove the bias noise from the flats and the dark frames are used to remove the bias and thermal noise from the lights. I tried a stack without the dark flats and got the same result. If and when I get this sorted I want to try stacking with all combinations of flats and darks to see just what effect each type of calibration frame is having.

One thought I have had is the flats were taken with the lightbox directly on the OTA instead of on end of the of the dew shield. Could this be a problem?

I am still trying to figure out the trailing in the corners of the image (http://stargazerslounge.com/imaging-image-processing-help-techniques/150570-stretched-stars-edge-image.html). I am unsure if it is field rotation because if I stack the full 3 hours worth of subs without alignment the trailing does not get any worse than in the individual subs.

Thanks,

Chris

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Chris

I think you may have misunderstood something. A dark flat is not the same as a bias frame. A bias frame merely captures noise which is fixed and independent of exposure time. The reason why dark flats are used is to remove the bias component from a flat. In practice, they are not used very often. Also, you may need bias frames only if the darks have to be scaled.

What software are you using to calibrate the images? THere may be something unusual in the way it sequences the processing

Having your dewshield on the scope shouldn't impact on the quality of your flats.

If the star trailing is independent of exposure time then you may need to look elsewhere. Something may well be moving in your imaging train. I'd start be checking all the connections especially any which have a compression fitting

Steve

Edited by SteveP
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I am still trying to figure out the trailing in the corners of the image (http://stargazerslounge.com/imaging-image-processing-help-techniques/150570-stretched-stars-edge-image.html). I am unsure if it is field rotation because if I stack the full 3 hours worth of subs without alignment the trailing does not get any worse than in the individual subs.

One simple way to check whether it is field rotation is to take a set of individual subs at different exposure times eg 1 min, 2 mins, 3 mins and see whether the trailing gets progressively worse

Steve

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I'm puzzled as to why you are applying both flats and dark flats. If you really are doing that then the flat is being applied twice

Steve

This is incorrect. Applying a flat and a flat dark isn't the same as applying the flat twice. The flat dark is applied to the flat as a dark frame, removing any hot pixels etc. The purpose of a flat dark is not to remove any vignetting, dust marks etc, which it cannot do anyway as it's taken with no light entering the system. It therefore cannot act as a flat frame, and cannot, if applied, act like you've applied flats twice.

Flats should be bias subtracted, or flat dark subtracted, but never both, as the flat dark contains the bias information and applying both will double subtract bias.

I have seen the effect in the above images on many occasions, and it's almost always the result of not bias subtracting (or flat dark subtracting) the flats. If it's not done the software being used cannot make the correct calculation when applying the flats, and tends to overcompensate.

Rob.

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Chris

I think you may have misunderstood something. A dark flat is not the same as a bias frame. A bias frame merely captures noise which is fixed and independent of exposure time. The reason why dark flats are used is to remove the bias component from a flat. In practice, they are not used very often. Also, you may need bias frames only if the darks have to be scaled.

What software are you using to calibrate the images? THere may be something unusual in the way it sequences the processing

Having your dewshield on the scope shouldn't impact on the quality of your flats.

Hi Steve,

You could well be right in that I have misunderstood calibration frames. My dark flats are about a 2 second long exposure (the same length as the flats obviously). I thought that at that short length, for a cooled CCD camera, the only significant noise contained will be the fixed, exposure independent bias noise. Hence why I considered them the same as bias frames for removing bias noise from the flats.

The software I am using for stacking is Astroart 5.0. I had a quick try with DSS but got a similar result with the over correction of the vignetting.

Okay, so I do not need to worry about removing the dewshield. Thanks for that.

One simple way to check whether it is field rotation is to take a set of individual subs at different exposure times eg 1 min, 2 mins, 3 mins and see whether the trailing gets progressively worse

That is a good idea and I shall give that a try. But doesn't stacking all my lights without any alignment amount to a 3 hour exposure? This should show extreme trailing if it is occurring.

Thanks,

Chris

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My dark flats are about a 2 second long exposure (the same length as the flats obviously). I thought that at that short length, for a cooled CCD camera, the only significant noise contained will be the fixed, exposure independent bias noise. Hence why I considered them the same as bias frames for removing bias noise from the flats.

Chris

That's right Chris....using bias instead of flat darks is what most of us do....it saves lots of time and is effectively the same.

Rob

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That's right Chris....using bias instead of flat darks is what most of us do....it saves lots of time and is effectively the same.

Rob

Thanks Rob, that makes complete sense. The main reason I opted for dark flats was the QHY8L camera automatically operates in a double exposure mode for short (< 3.5 sec I believe) exposures and I wanted to ensure the bias frames would match the flat frames. Since my flats are ~2 sec I guess there is no reason not to use bias frames instead.

see here Astrophotography for a primer on calibration. Some software doesn't actually help you understand this business especially when the incorrect terminology is used. I suspect the most likely problem is one of over correction, in which case re-visit your Flats average ADU.

Dennis

Thanks for that link Dennis, that is an excellent summary and has now been bookmarked. This is the data for one of my flat frames:

post-21918-133877641327_thumb.gif

I was aiming 20000 to 22000 average ADU.

Cheers,

Chris

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Now I'm confused!

Chris - When you refer to 'dark flats', do you mean 'dark subtracted flats' ie you have taken a flat and from it deducted a dark to remove any dark current? This is what I had assumed you had done. If that is the case than deducting (strictly speaking dividing by) a flat and also a dark flat will result in over calibration

Rob on the other hand uses the phrase 'flat dark' and I don't then disagree with anything he then says about using a flat dark.

Can youjust clear up this point?

Steve

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Now I'm confused!

Chris - When you refer to 'dark flats', do you mean 'dark subtracted flats' ie you have taken a flat and from it deducted a dark to remove any dark current? This is what I had assumed you had done. If that is the case than deducting (strictly speaking dividing by) a flat and also a dark flat will result in over calibration

Rob on the other hand uses the phrase 'flat dark' and I don't then disagree with anything he then says about using a flat dark.

Can you just clear up this point?

Steve

I called them 'dark flats' because that is what DSS calls them, I have seen them called 'flat darks' elsewhere - I assume these are the same thing. What I am talking about are exposures taken with exactly the same settings as the flats but with the cover on the scope.

Cheers,

Chris

Edited by cgarry
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Glad that's sorted !

Now Chris, what on earth is intruding into the frame top left ? Can you see anything when looking down the tube ?

By the look of it, to me at least, it seems to be in the filter area. Are the filters held in by plastic washers ?

Dave.

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Glad that's sorted !

Now Chris, what on earth is intruding into the frame top left ? Can you see anything when looking down the tube ?

By the look of it, to me at least, it seems to be in the filter area. Are the filters held in by plastic washers ?

Dave.

That is an easy one to answer, it is the prism from the OAG. I deliberately set it off centre because I wanted to see how much it would show up in the final image - which is quite a lot if I can't get these flats to do their job!

Cheers,

Chris

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Have you still got your kit setup?

Just as an experiment can you retake your flats at say 10000 ADU 20000 ADU and 30000ADU

I've got a similar problem and I am going to try the above and try to work out an ADU which works for me

Steve

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Can't you reposition the pick-up prism so that it's over the short side of the sensor?

This imaging run was really an experiment with some new pieces of kit, an OAG and a lightbox I deliberately moved the prism away from the short side of the chip to see how much the flats would clean up the part of the image the prism dimmed. The idea is I will know in the future how much I can move the prism around to find a guide star once the target image has been framed.

Have you still got your kit setup?

Just as an experiment can you retake your flats at say 10000 ADU 20000 ADU and 30000ADU

I've got a similar problem and I am going to try the above and try to work out an ADU which works for me

Unfortunately the OTA is packed away after each session so I can't take new flats for this set of subs. When I get a chance I will get a few subs quickly and then try flats at different ADUs as you say. I haven't thought of anything else to try yet!

Cheers,

Chris

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Now I understand !

Putting the prism on the long side is a good idea. It also helps to be as central as possible.

The down side of hiding the effect via flats is that it doen't put photons back so get it out of the way as much as you can.

Having said that, your tracking is great. Not once but 36 times. It's a good image in the making. Impressed.

Dave.

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