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  #21  
Old 05-09-2008, 03:13 PM
Ian Robinson
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Update…
Managed to do a quick test last night.
Flipped the filter over so that the filter glass was now about 3mm further back from the rear lens element.
Attached pic shows the effects of the filter in its 2 different positions.

Left hand pic – filter closer to lens
Right hand pic – filter further from lens

The vignetting/reflection effect seems to be less defined and demarcated in the right hand pic – looks more like a vignetting effect that could be dealt with by flats.

I’m too lazy for flats though!!!
That's a very big difference in the observed effect.

How would flats fix it ?
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  #22  
Old 05-09-2008, 03:25 PM
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How would flats fix it ?
As it's a constant, reproducable variation in the optical system, subtractng the flat field should hopefully sort it (...hopefully!!!)
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  #23  
Old 05-09-2008, 04:05 PM
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...although on reflection (no pun intended), I wonder if the internal reflections would show up on the shorter exposures used to make a flat!?

Only one way to find out I suppose...

Last edited by dugnsuz; 05-09-2008 at 04:21 PM.
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  #24  
Old 05-09-2008, 04:18 PM
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As it's a constant, reproducable variation in the optical system, subtractng the flat field should hopefully sort it (...hopefully!!!)
Spotted this http://vignetting.republika.pl/help.html - might be worth a try .... perhaps ...
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  #25  
Old 05-09-2008, 04:28 PM
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Spotted this http://vignetting.republika.pl/help.html - might be worth a try .... perhaps ...
Cheers Ian - bookmarked for a later date
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  #26  
Old 05-09-2008, 06:07 PM
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Try a CWB.
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  #27  
Old 05-09-2008, 06:14 PM
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Cant hurt to TRY it... although the reflection is definitely a physical problem, so I doubt that a software setting will make much difference
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  #28  
Old 05-09-2008, 06:46 PM
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Try a CWB.
What image would I use to calibrate the CWB Robin?
Doug
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  #29  
Old 05-09-2008, 07:53 PM
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Don't you already use one for your modified 40D? You should be.

The simple way is to put in the filter and take an image of white sheet of paper under direct sunlight. Set that image as the CWB. The camera will do the maths between that image and the image your taking. The real way is to get a gray sheet from a photo shop and do the same.

I'd like to say it works like a flat but I don't know enough about the workings inside of a canon camera to say that and the guys at canon are very reluctant to tell you about it.
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  #30  
Old 05-09-2008, 08:21 PM
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Quote:
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Don't you already use one for your modified 40D? You should be.

The simple way is to put in the filter and take an image of white sheet of paper under direct sunlight. Set that image as the CWB. The camera will do the maths between that image and the image your taking. The real way is to get a gray sheet from a photo shop and do the same.

I'd like to say it works like a flat but I don't know enough about the workings inside of a canon camera to say that and the guys at canon are very reluctant to tell you about it.
Got it!

Yeah Robin I use CWBs regularly for daytime stuff - I'll try that.

Cheers
Doug
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  #31  
Old 05-09-2008, 08:56 PM
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I use CWB for astro shoots as well with a modified camera else your white balance will be set to the cameras default unmodified setting. However with astro shoots remember that most stacking software can be set to ignore the cameras CWB so make sure it is using the cameras white balance.

On a side note, I notice there seems to be no option in maxim to ignore the cameras WB. Here is a shot processed in maxim after someone reset my CWB to a normal daytime image. This had me stumped for ages.
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  #32  
Old 05-09-2008, 09:06 PM
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Took this really rough flat tonight to see if the vignetting was just that or an internal reflection.
To my surprise, I got the image below (stretched to highlight)...

Looks like I may be able to sort this out with decent flats perhaps?

Doug ()

ps...now I need a decent step by step for taking good T-Shirt Flats
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  #33  
Old 05-09-2008, 09:27 PM
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Well a flat is similar to a CWB in that it's graphics maths are applied to an image.

Build a light box.

I outlined how in this thread.
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  #34  
Old 05-09-2008, 09:41 PM
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Quote:
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Well a flat is similar to a CWB in that it's graphics maths are applied to an image.

Build a light box.

I outlined how in this thread.
Your light box looks just what I need to standardize the flat taking process.

So, you get a number of bright, white images with the histogram roughly 1/3 from the left - what next?
Use them as-is in calibration software? (ImagesPlus in my case)

Sorry to be a dumbass - I've childishly avoided flats
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  #35  
Old 05-09-2008, 09:53 PM
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I screw around with exposure time to get the histogram away from the left hand side and then runoff a 1/2 dozen images. The scope has already been focused on a star. Made a big difference to post processing for me when I was using the 8" dob.

Last edited by Tandum; 06-09-2008 at 11:53 AM.
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  #36  
Old 06-09-2008, 08:18 PM
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Not able to fix the internal reflection issue with flats or CWB so the filter is now up for sale.
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  #37  
Old 06-09-2008, 09:15 PM
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That's unfortunate. What lens was giving the trouble?
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  #38  
Old 06-09-2008, 09:28 PM
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That's unfortunate. What lens was giving the trouble?
Canon 70-200mm f4L - my main imaging lens unfortunately!
As you can see, the rear element is pretty reflective!!!
Cheers for all your advice Robin
Doug
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  #39  
Old 06-09-2008, 09:43 PM
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No problem. I managed to get all my stuff setup tonight and I have a new counterweight arm camera mount and a junk 90-300 lens to try out later on. I'm wondering if I'll have the same trouble with this front filter.
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  #40  
Old 06-09-2008, 09:52 PM
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...I'm wondering if I'll have the same trouble with this front filter.
Hope not!
Good luck
Doug
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