Vignette Problems

[niteman1946]

Hi Chris,

Thanks for your continuing input.  A few points about the focal reducer:
a)  There's lots of chatter about the correct distance from focal plane to reducer.  The range appears to vary from  95mm to 105.  Too bad Meade doesn't publish something.
b)  Vincent Giordano at OPT Corp states the value should be 105mm.
c)  Vincent also states the measurement should be taken from the reducer shoulder, just above the external threads, on the end facing the camera.  This should produce the "theoretical" 6.3 reduction.  Note that this shoulder is about 9mm forward of the back element.
My current setup measures 101mm measured per the above.  So if 105mm is the magic number for 6.3, then simple math says I should be getting around 6.45  (slightly different than what I have been stating - sorry).

Just thinking out loud, here is my understanding of the normal and expected vignetting that occurs with a reducer.  The outer edges of the light cone are dimmer than the center.  A reducer reduces the diameter of the light cone and allows for more of this diminished edge light to hit the sensor, showing up as vignetting.

I genuinely don't believe that this is the problem in my case.  In my case, when I shoot through the 1 1/4" Luminance filter (and arguably all the others), I get the garish vignetting.  This requires me to crop about 30% (or more) from the outer perimeter before the image will process out.

However, when I shoot through the open port, of the filter wheel, I get virtually none of the vignetting.  I believe the limited size of the filter is substantially truncating usable image light from contacting the sensor.  I don't think this should be happening.

Here is some imaging I shot last night.  It's a more credible example of how much better the "No Filter" image looks.

As promised, next up are shots without the reducer.

Thanks,

Mark

[chrisjbaileyuk]

Mark

When I used a 0.63 FR on my 8" LX200 I got a figure of 87mm, I believe from Meade but to be honest I was never entirely happy with the way it performed to perhaps the 87mm figure was a poor steer. Best way to tell is to do a plate solve of an actual image.

The no filter images do suggest the 1" 1/4 filters to be playing a part but its an expensive trial to see if 36mm replacements would solve it entirely.

Chris

[niteman1946]

I’m re-posting the image of my component setup here.
http://astrob.in/30273/

I have continued to compare the the Luminance filtered images with the No Filter images (i.e. shot through an open port in the EFW2 filter wheel).  This time the goal was to shoot comparisons with the Meade f/6.3 reducer out of the optical path.

I did this in two different ways: 
a)    Optics of Meade reducer removed, but reducer housing left in place.  I refer to this setup as “Long”.  It has the same length of stackup as the previous f/6.3 tests .
b)   Complete removal of Meade f/6.3 reducer.  I refer to this setup as “Short”.  This shortens the overall stack-up by about 31mm.

Results for Luminance filter  “Short”:
   http://astrob.in/54012/
   http://astrob.in/54013
Results for Luminance filter “Long”
   http://astrob.in/54018/
   http://astrob.in/54021/
Results for No Filter “Short”:
   http://astrob.in/54026/
   http://astrob.in/54027/
Results for No Filter “Long”
   http://astrob.in/54028/
   http://astrob.in/54029/

Here is my take away from these results:
1.    With the reducer optics gone, Luminance filter no longer shows the vignetting problem. 
2.   I could see only a slight difference in the line profile and raw images taken in either the “Long” or “Short” condition.  Somewhat disappointing, since I was hoping to gain an advantage by shortening the setup.

Consequently:
A.   The combination of the Meade reducer, along with the Luminance filter produces the unwanted vignetting.  However, the Meade reducer along with the No Filter does not.
B.   With the Meade reducer removed, there is no un-manageable vignetting while using the Luminance filter.  This is true regardless of “Long” or “Short” position.

So does Atik’s statement about no relevant vignetting down to f5 hold true?  Does it only apply to the native focal reduction?  Am I way off on the actual reduction?

Chris, can you point me to where I could do a plate solve on the images to get an accurate reading of reduction?

Mark

[chrisjbaileyuk]

Mark

If you can dropbox a raw fits star field image I would happily run in through SkyX Plate Solve and give you the field of view that results from it.

I suspect you will find that the 0.63 FR is actually reducing a lot more than that and that is shrinking the image circle down. At least you have found the major culprit!

Chris

[MikeM]

Mark, I'm going to preface this by stating I don't know much about Meade sct's so forgive me if this is an elementary question. But is it possible the problem lies
at the opening of the rear of the sct? I ask because I read about the "Peterson's Eye-Opener" which I guess increases the diameter of the rear opening of Meade
sct's. If you already have that then you can disregard this post and I'll go back to continue reading about how you go about resolving this issue.

[niteman1946]

Mark

If you can dropbox a raw fits star field image I would happily run in through SkyX Plate Solve and give you the field of view that results from it.

I suspect you will find that the 0.63 FR is actually reducing a lot more than that and that is shrinking the image circle down. At least you have found the major culprit!

Hi Chris,

Here are two images taken with the 12"LX200.  Both are of IC5146.
The "F10" version was shot without reducer.  The "F6.3" version was shot with the reducer.

I've used PixInsight for plate solving and reduction calculation just now for the first time.  I get some outragious values:
F10:  4391mm
F6.3:  2766mm
 
The only thing that makes me wonder is that their ratio does equal 0.63.

Thanks, and let me know what you determine with your software.

Mark

https://www.dropbox.com/sh/lypvmqu6x0dttrn/AE13mZANNQ

[niteman1946]

Mark, I'm going to preface this by stating I don't know much about Meade sct's so forgive me if this is an elementary question. But is it possible the problem lies
at the opening of the rear of the sct? I ask because I read about the "Peterson's Eye-Opener" which I guess increases the diameter of the rear opening of Meade
sct's. If you already have that then you can disregard this post and I'll go back to continue reading about how you go about resolving this issue.

Hi Mike, and thanks for the response.

I have the Starlight Feathertouch focuser mounted to the large threads (3 1/2"?) on the back of the scope.  I do not think that the focuser is contributing to the problem due to truncating the light cone.

I do suspect that the additional length of the focuser may be adding to an overall drop-off in image brightness.  But that's a different story.

Mark

[chrisjbaileyuk]

Mark

I get focal lengths of 3204.9 and 2007.7 so pretty much 0.63 and near enough what you would expect.

HTH

Chris

[niteman1946]

Mark

I get focal lengths of 3204.9 and 2007.7 so pretty much 0.63 and near enough what you would expect.

HTH

Chris

Thanks Chris,

After a little hand holding over on the PI site, I was able to come up with the same focal lengths as you.

Consequently,
F10 Image:  3205mm and therefore 10.51 focal ratio
F6.3 Image:  2008mm and therefdore 6.59 focal ratio

So, at least on the surface, my focal ratio should satisfy Atik's stated minimum of f5.

Mark