Author Topic: IDAS LPS reflections image salvage  (Read 1875 times)

Offline mvln

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IDAS LPS reflections image salvage
« on: 2015 January 07 11:07:49 »
Hi everyone,

I have imaged M76 with my new 16"RC. I used a 2" IDAS LPS in front of the reducer. It turns out the IDAS has serious reflections in Green , Red and H Alpha. I've tried DBE to get rid of the reflections but to no avail. How could I salvage this image ? The image is the red image, dark / flat fielded...
see: http://supernovae.be/sites/all/RintegrationCROP.fit

Thank you for your suggestions !

kind regards,

Maarten

* I've put the IDAS LPS behind the reducer now, reflections are gone, but the focal length has changed...

Offline cfranks

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Re: IDAS LPS reflections image salvage
« Reply #1 on: 2015 January 07 14:38:44 »
Maarten,

Just my opinion, but it looks like the reflection is from the reducer back onto the filter then back to the camera.  Focus is always changed by adding additional glass into the light path, by about 1/3 the thickness of the glass.  Your optics seem a little un-collimated also.  What brand of RC do you have?
Regards
Charles

Offline mvln

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Re: IDAS LPS reflections image salvage
« Reply #2 on: 2015 January 09 06:37:29 »
Hi,

The scope is a 16" RC by GSO. By moving the filter behind the reducer, about 1 cm of extra spacing was added, decreading the focal length of the system. What would make you conclude the system is miscollimated ?

kind regards,

Offline cfranks

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Re: IDAS LPS reflections image salvage
« Reply #3 on: 2015 January 09 15:24:21 »
Hi Maarten,

The light and dark 'rings' of the reflections on your image are offset from the centre of the image suggesting that the optical axis of the telescope is slightly offset from the mechanical axis.  I was going to point you to a CloudyNights post on the subject but I see that is your post anyway!  ;) 
Assuming you have an adapter between the reducer and your camera; that has to be reduced by the mechanical mounting thickness of the new filter (your 1cm?) plus 1/3 of the filter glass thickness to get back to your required back focus.  I apologise if I am saying stuff you already know but I am going through the same add/subtract process with a different 'scope and it has been very confusing for an old bloke!
Regards
Charles

Offline mvln

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Re: IDAS LPS reflections image salvage
« Reply #4 on: 2015 January 11 02:25:08 »
Hi Charles,

No problem ///
The actual location of the rings cannot be used as a measure for collimation. When I look at the G an H images, the rings are at different locations. Nevertheless, the images LRGBH were taken in sequence so you would expect the rings to end up at the same place.
My geuss is that the IDAS will reflect light in a other angle with different wavelengths.

kind regards,