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Astrophotography and Sketching >> DSLR & Digital Camera Astro Imaging & Processing

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carlstronomy
super member


Reged: 06/03/12

Loc: Brisbane, QLD, Australia
Strange results with reducers new
      #5546771 - 11/30/12 08:12 AM Attachment (40 downloads)

Hi all, thanks for all who helped me work out my distance issue with my DSLR and 6.3 reducer. I have taken ownership of a nose piece today which has allowed me to take very nice focused images without running out of focus (JUST). As you can see by the attached image which is a 120 sec sub at ISO800 I am getting some sort of rounding around the edges (but only with the reducer). Anything over an exposure of 30 seconds is producing this. When I process the image I get circular very vissible layers getting darker from the center of the image working outwards. Not sure what causes this can anyone help with this? Is it just my light polution? and also the moon is almost full.

Thanks

Carl

Edited by carlstronomy (11/30/12 08:14 AM)


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waassaabee
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Reged: 11/26/07

Loc: Central California Coast
Re: Strange results with reducers new [Re: carlstronomy]
      #5546781 - 11/30/12 08:23 AM

My first guess is that you are using 1.25" adapter causing this vignette.

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carlstronomy
super member


Reged: 06/03/12

Loc: Brisbane, QLD, Australia
Re: Strange results with reducers new [Re: carlstronomy]
      #5546782 - 11/30/12 08:24 AM Attachment (16 downloads)

After some thought on this I think it is the moon shining back into the telescope, would I be correct in this assumption, I may as well post my first ever moon shot with my DSLR, any coments would be great. This image is also taken with the 6.3 reducer and the 1.25" nose piece (thought I should add that)

Thanks
Carl

Edited by carlstronomy (11/30/12 08:32 AM)


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carlstronomy
super member


Reged: 06/03/12

Loc: Brisbane, QLD, Australia
Re: Strange results with reducers new [Re: carlstronomy]
      #5546783 - 11/30/12 08:28 AM Attachment (23 downloads)

Gary, that was my first thought so I tried a shot with my 2" T adaptor. Altough I could not reach focus the same was still evident on the image. See image below

Carl


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D_talley
Carpal Tunnel
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Reged: 07/07/05

Loc: Richmond VA
Re: Strange results with reducers new [Re: waassaabee]
      #5546803 - 11/30/12 08:45 AM

You can use this link to make a Artificial flat if you have not taken any flats with this setup:

Artificial Flat

The flat will fix your problem.


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carlstronomy
super member


Reged: 06/03/12

Loc: Brisbane, QLD, Australia
Re: Strange results with reducers new [Re: D_talley]
      #5546811 - 11/30/12 08:51 AM

Dwight,

Thank you I have not played with flats yet, there is so much to learn to get good results. So as Gary also stated it is vignetting that has effected my images. Is this an error on my part or something that just happens?

Thanks


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Footbag
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Reged: 04/13/09

Loc: Scranton, PA
Re: Strange results with reducers new [Re: carlstronomy]
      #5546836 - 11/30/12 09:14 AM

Quote:

Dwight,

Thank you I have not played with flats yet, there is so much to learn to get good results. So as Gary also stated it is vignetting that has effected my images. Is this an error on my part or something that just happens?

Thanks




Vignetting just happens, but more so with a focal reducer. It's a good excuse to start taking flats. They will improve your images and allow you to process them further.


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Tonk
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Reged: 08/19/04

Loc: Leeds, UK, 54N
Re: Strange results with reducers [Re: Footbag]
      #5546962 - 11/30/12 10:45 AM

Those donuts show your collimation needs some attention.




Quote:

After some thought on this I think it is the moon shining back into the telescope, would I be correct in this assumption




No - what you have is plain ordinary vignetting. This is cause by lightpaths at the edge of the image being partially eclipsed by a tubular edge part of your optical train. If you like - if you could put your eye in the place of the camera sensor edge, the scene looking out through the scope has part of it obscured by the tube - probably focusing draw tube, or nose piece etc . I.e. the light cone gathered by the focal reducer has something cutting into it. Focal reducers widen the field of view so its important to allow for this - say with a wider 2 inch nose peice or whatever it is. You are unlikely to completely eliminate vignetting so adding flats to your imaging regime is a must.

Avoid artifical flats - do proper ones. Artificial flats serve when you have forgotten to take proper ones - the artificial method actually does not correct properly though mostly this can go unnoticed


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Escher
Pooh-Bah
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Reged: 08/30/07

Loc: Fenton, MI
Re: Strange results with reducers new [Re: Tonk]
      #5547271 - 11/30/12 01:52 PM

Startools will take care of that vignetting in short order. No flats needed.

Edit to add:

Also - if you stretch your shorter exposures, you will find the same vignetting... you just dont see it, but its there.

Edited by Escher (11/30/12 01:54 PM)


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Tonk
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Reged: 08/19/04

Loc: Leeds, UK, 54N
Re: Strange results with reducers new [Re: Escher]
      #5547431 - 11/30/12 03:50 PM

Quote:

Startools will take care of that vignetting in short order. No flats needed.




Yeah its a artifical method using subtraction and not the correct division correction. Vignetting is a variable transmission issue and NOT a gradiant issue. These artifical methods are fine to fix additive gradients from external light sources (moon, light pollution etc) but they do not correctly deal with vignetting. Real flats are always the best solution for vignetting combined with correct treatment from a calibration and stacking program such as Images Plus, Deep Sky Stacker, Nebulosity etc (i.e. applied as a flat)


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carlstronomy
super member


Reged: 06/03/12

Loc: Brisbane, QLD, Australia
Re: Strange results with reducers new [Re: Tonk]
      #5547505 - 11/30/12 04:46 PM

Tonk,
Thanks yes my collimation is out slightly I had to remove the corrector plate to tighten a lose nut the other day and as I am doing so much practicing at present and the weather is so up and down I have left it as is due to get up some shooting experience. Once I get a grip on my tracking and taking some good images I will certainly fix it up.

Carl


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carlstronomy
super member


Reged: 06/03/12

Loc: Brisbane, QLD, Australia
Re: Strange results with reducers new [Re: carlstronomy]
      #5547516 - 11/30/12 04:50 PM

Adam, Tonk, Escher thanks very much for your advice, now I think I should learn how to take flats I will look for some threads later today, also how many flats should I take I know I should take as many darks as light but not sure on flats. Nothing I have seen as yet states how many to take.

Carl


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Tonk
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Reged: 08/19/04

Loc: Leeds, UK, 54N
Re: Strange results with reducers new [Re: carlstronomy]
      #5547741 - 11/30/12 07:28 PM

Quote:

Nothing I have seen as yet states how many to take.




25 is a good compromise figure. Combining flats to create a master flat serves to reduce random noise. The reduction is an inverse square progression - so combining 25 reduces noise to 1/5th of a single flat. So 49 will drop this to 1/7th, 100 to 1/10th and you will see that it is a law of diminishing returns. 25 is good enough IMHO - takes < 1 minute to shoot this number if done in reasonable light.

Oh and you will have to take bias OR flat darks to complete the correct calibration (flat darks are same ISO/duration as the flats - but with lens cap on - 25 of these too)

The work flows for the 3 legal (i.e. correct) method to calibrate your images is explained at the bottom of this page

http://deepskystacker.free.fr/english/theory.htm


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Escher
Pooh-Bah
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Reged: 08/30/07

Loc: Fenton, MI
Re: Strange results with reducers new [Re: Tonk]
      #5548375 - 12/01/12 07:48 AM

Quote:


Yeah its a artifical method using subtraction and not the correct division correction. Vignetting is a variable transmission issue and NOT a gradiant issue. These artifical methods are fine to fix additive gradients from external light sources (moon, light pollution etc) but they do not correctly deal with vignetting. Real flats are always the best solution for vignetting combined with correct treatment from a calibration and stacking program such as Images Plus, Deep Sky Stacker, Nebulosity etc (i.e. applied as a flat)




Understood and agreed...

I should have said - In the case that you *don't* have flats,etc... startools will take care of it for you.


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Escher
Pooh-Bah
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Reged: 08/30/07

Loc: Fenton, MI
Re: Strange results with reducers new [Re: Escher]
      #5548387 - 12/01/12 08:04 AM Attachment (8 downloads)

Pretty brutal vignetting on that one - I'm sure if I had original data I could do better...

Couldnt get all the vingetting out on this one:


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carlstronomy
super member


Reged: 06/03/12

Loc: Brisbane, QLD, Australia
Re: Strange results with reducers new [Re: Escher]
      #5549994 - 12/02/12 09:22 AM

Escher, thanks for having a try but I will put this one down to experience and on my next outing I will take some flats (white T shirt method). I tried Startools with no success as no software would recognise my Canon RAW files including Photoshop so I had to load them into other software and save and then DSS would not use them so I gave up as not worth the effort.

Seems the simple rule is forget the 1 minute worth of flats throw away 2 hours of data - so dont forget.

Thanks

Carl


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Escher
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Reged: 08/30/07

Loc: Fenton, MI
Re: Strange results with reducers new [Re: carlstronomy]
      #5550023 - 12/02/12 09:40 AM

Carl - Startools doesnt stack, itonly operates after stacking... I use Nebulosity for stacking and startools for enhancing / repairs...

I was just curious how much details could be pulled from a small .lpg file...


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James74
super member


Reged: 01/29/12

Loc: Sacramento, CA
Re: Strange results with reducers new [Re: Escher]
      #5551294 - 12/02/12 11:43 PM

Yup, I see the same vignetting with my Celestron f6.3 reducer/corrector on my C8. Regular old flats will take care of that.
I've never made artificial flats. Good old-fashioned flats are easy enough to make.


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