Strange Flats - but only with Red Filter

[kirkster501]

First off, I am certain all my filters are fine since I do not get this issue with refractors.

Below is a flat I took with Red filter with my RC8 scope.  Blue and green filter and luminance flats are fine.  The mean ADU of this flat is 21921 so that seem OK.  Yet this flat shows a hideous shadow of the secondary on my RC8 scope.

Anyone got any ideas please?  These have ruined my data set from the other night.

[kirkster501]

.. and this is what I get after RGB combination after calibrating with said flats (stars are eggy a bit - I know why).

[ollypenrice]

It looks as if the makers didn't use the right black paint somewhere. https://www.sbig.com/about-us/blog/flat-fields-the-ugly-truth/  The gist of the article is that many 'black' surfaces reflect strongly in IR and CCD cameras can pick this up. This would account for it afflicting only the red, which would be transparent to the IR. The other filters would block it.

I find no difference between L,R,G and B flats and have stopped taking them, as have one or two friends. We just take and apply L. All I can say is try it. Use the 'wrong' flat for the red lights. If your filters are really dusty you may have a problem but the worst bunnies to deal with are closer to the chip. It works for me. Or try the red without flats at all. It can't be worse than the Himalayan red flat you've used. That will make things worse.

Now what is throwing that IR around, if such is the problem? I'd say the inside of tube. There's an incredible amount of it and the spider is in shadow, not in bright reflection, which makes me doubt it's that.

Olly

[kirkster501]

Olly let me replay that.  So you just use the luminance flats to calibrate the LRGB channels right?  My filters are spotless so if I read you correctly that could be a goer!

[Gina]

I use the Ha filter for all flats and just use a white T shirt over the dew shield in daylight.  It needs a NB filter because the light is too bright for LRGB at minimum exposure of 1ms.  I too keep my filters clean (and everything else) and rarely suffer from dust bunnies.

[kirkster501]

Wow, great info.  I will retry with the luminance flats then - thanks for that fabulous bit of advice, I would never have thought of that!   I must admit, taking flats for LRGB, and potentially narrowband as well is a PITA.  Since one is only using one flat type do you take a lot more of the L flats then to kind of "compensate" ? 

The dust bunnies on my flat above are either on the CCDT67 reducer or on the Hutech LP filter. Not 100% sure which.  I am thinking of dropping the 2" Hutech LP filter out of my imaging train to see if it is truly required if I am using LRGB filters.

[Gina]

To find out which surface your dust resides on you can use this calculator - CCD Dust Shadows.

HTH.

[ollypenrice]

Olly let me replay that.  So you just use the luminance flats to calibrate the LRGB channels right?  My filters are spotless so if I read you correctly that could be a goer!

Yes, just try your L flat to calibrate all channels. See what happens. I started doing this about 6 months ago and it has worked fine. As you say, all those flats were a PITA.

However, I'd still think about why your red channel is so affected. There's no need to do extra flats. If you have enough to eliminate most of the noise (I generally do about 30) ten enough is enough.

Olly

[lensman57]

Hi,

How did you take your flats and what did you use for illumination?

A.G

[davew]

I don't think I've seen such an effected flat before. Some sign but not that much. Due to the fact the L filter lets through red, doesn't it show slightly in that ?

Would IR still get passed an IR blocked filter ? What filters do you use ?

if you find a solution let us know. Anyone with grubby filters can't really do the L flat trick,

Dave.

[ollypenrice]

I don't think I've seen such an effected flat before. Some sign but not that much. Due to the fact the L filter lets through red, doesn't it show slightly in that ?

Would IR still get passed an IR blocked filter ? What filters do you use ?

if you find a solution let us know. Anyone with grubby filters can't really do the L flat trick,

Dave.

Yup, good point. Try green in the mid spectrum, then.

Olly

[kirkster501]

Hi,

How did you take your flats and what did you use for illumination?

A.G

I used a FL panel on the front of the scope and adjusted the exposure of the CCD to give a ADU in the generally accepted 20-23000 ADU range.  I do this all the time with refractor and it is A.OK.  First time I have used my CCD on the RC8 scope.  With my Canon it was fine.

I have yet to try Olly's L trick.  Will have a look at that later.

If the Red filter is letting this muck reflect about though I'd expect the red lights to be similarly hideous.  But they are not, they look the same as the LBG lights - i.e. they do not have that secondary shadow.

[lensman57]

I used a FL panel on the front of the scope and adjusted the exposure of the CCD to give a ADU in the generally accepted 20-23000 ADU range.  I do this all the time with refractor and it is A.OK.  First time I have used my CCD on the RC8 scope.  With my Canon it was fine.

I have yet to try Olly's L trick.  Will have a look at that later.

If the Red filter is letting this muck reflect about though I'd expect the red lights to be similarly hideous.  But they are not, they look the same as the LBG lights - i.e. they do not have that secondary shadow.

Looks like you are doing everthing by the book, It is fascinating to see what the outcome would be. BTW my EL panel gave up the ghost after only  two months and a half dozen sessions of use, please do not buy Chinese, there is absolutely no QC standard of any form  in whatever they manufacutre. I have other hobbies and interests but AP , some are even more high tech than AP, my experience has been the same.

Regards,

A.G

[kirkster501]

Thinking about this.  It would mean I would have to do the luminance flats in both 1x1 (obviously for the 1x1 luminance lights) but also in 2x2 or whatever binning level I was using for the RGB channels right?

Not sure there is a way to calibrate using calibration frames from a different level of binning?

EDIT: In other words do the flats at all binnign levels of the light set

[opticalpath]

Sorry to ask an obvious question, but is this the first time you've taken flats with this setup - is there any history of the same equipment having worked OK before?  Have you tried again since and got the same result?  Just to eliminate the possibility it was an environmental one-off.

I had very weird flats once that turned out to be caused by dew on the filter (or on the camera window - can't remember which) , though admittedly they didn't show sharp shadows like that.

I thought that most LRGB filters these days had UV and IR cut-off filtering built-in, so even if there is some IR bouncing around off the 'black' surfaces, would we expect this much to get through?  I know the IR filtering may not be perfect, but I would have guessed it would be about as effective (or not) on all the filters.  So I'm surprised if IR leakage is affecting only one filter.

Adrian  

[kirkster501]

Sorry to ask an obvious question, but is this the first time you've taken flats with this setup - is there any history of the same equipment having worked OK before?  Have you tried again since and got the same result?  Just to eliminate the possibility it was an environmental one-off.

I had very weird flats once that turned out to be caused by dew on the filter (or on the camera window - can't remember which) , though admittedly they didn't show sharp shadows like that.

I thought that most LRGB filters these days had UV and IR cut-off filtering built-in, so even if there is some IR bouncing around off the 'black' surfaces, would we expect this much to get through?  I know the IR filtering may not be perfect, but I would have guessed it would be about as effective (or not) on all the filters.  So I'm surprised if IR leakage is affecting only one filter.

Adrian  

It is indeed the first time I have used a mono CCD on this rig.  

A random, spurious problem is a definite possibility Adrian.  I will try it again when I next get a clear night.  Tonight maybe if I am back home in time.  There is a *bit* of a secondary shadow on some of the other filters as well.  But it is hardly noticeable, I'd say it was 1% as much as the Red channel.

[ollypenrice]

While there's a good fit with the link I posted I agree that an environmental one-off is a possibility not to be ignored. The sudden passage of a red head torch, for instance, or a nearby red light source. Have you checked the individual red flat subs?

Olly

[kirkster501]

While there's a good fit with the link I posted I agree that an environmental one-off is a possibility not to be ignored. The sudden passage of a red head torch, for instance, or a nearby red light source. Have you checked the individual red flat subs?

Olly

Yes, all the individual red flats are the identical.  Its a strange one 'innit? !

[sharkmelley]

There are possible explanations for why you should see an image of the front opening and spider  in your flats but I don't fully understand why that would only occur for red light.  I wouldn't have thought EL panels generate much Infra Red. I would be interested to see the green and blue flats with an identical amount of stretching.

Mark