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Vingetting test?

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#1 tomhole

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Posted 19 May 2003 - 08:11 PM

I did an experiment tonight on Jupiter and need to know if this is a good measure of whether something is vingetting or not.

I have the Denk standards, 1 1/4" OCS, 2x Televue barlow, 2.5x and 5x powermates. I was using 25mm Sirius plossls in a 10" f/4.7 dob. Target was Jupiter. I have a tracking platform.

I put Jupiter in the FOV and let it drift to the extremes to see if it got dimmer. It did not. Not with any of the OCS/barlow/powermates. I tried it several times with all the combos. I haven't looked at star fields yet, it's not quite dark enough.

Is this a good measure of whether a combo is vingetting? If not, I'll be out looking at stars/clusters to see if I notice anything on them.

Clear skies,

Tom

BTW, with all the discussion of focusing and OCS's, I thought I'd pass on that the Denkmeiers came to focus with the 1 1/4" OCS, 2x barlow and both powermates. Not close to the extremes on any of them. Makes for a nice evening of dial a power with 1 set of ep's. Should be even easier when I get the Siebert 1.3x OCA.

#2 Guest_**DONOTDELETE**_*

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Posted 19 May 2003 - 09:19 PM

Hi Tom,
If vignetting is occuring the entire field may be dimmer
with no obvious edge only dimming.Most reports of vignetting
with this combination involve longer focal length ep's 30mm,32mm,35mm,your milage may vary.When you get your 1.3OCA
the image may be brighter due to better light throughput
and a lack of vignetting.Try a longer fl eyepiece if you get the chance.Have fun and try not to grin too much.
Scotty :grin:

#3 tomhole

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Posted 19 May 2003 - 09:57 PM

Scotty,

I only have the 25mm plossls as a pair. I am going to try to double up some ep's, but haven't figured out which ones yet. The 24 Pans look enticing with their widest possible TFOV in a 1.25" ep. We'll see if the wallet can afford them.

Clear skies,

Tom

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Posted 20 May 2003 - 10:19 AM

I think Jupiter is a little bright to really be able to tell. Best that I've found is to look at light polluted sky background (if magnification/light pollution combo is right) or at dim stars.

There's really two types of vignetting you need to consider:

1) the binoviewer/powermater/whatever the first element is at the eyepiece end cuts into the light cone from the primary. That is a problem you're much more likely to encounter at lower magnifications and wider fields of view. With 1.8x or 2x or 3x lenses in front or short FL eyepieces, you're already only interested in the more innermost parts of the image, which generally is more heavily contained in the innermost part of the light cone (see last paragraph) so even if this vignetting occurs, you likely won't notice it as much.

2) The binoviewer itself can't fully illuminate the eyepiece. This is a more pronounced separate issue if you have a high magnification lens in front of the viewer which will flatten out the light cone and change the geometry of the light through the viewer compared to what it would be for your scope otherwise.

Also, vignetting may or may not occur as edge darkening. The closer to the focal point of the primary mirror lens the culprit that causes vignetting lives, the more it will be exhibited as edge darkening. The further away from primary focus the light cone is impacted, the more the effect will be "spread out" over a wider field of view. Not to mention the fact that the light cone will be wider further away ... See drawing below.

Anyways, as everything causing issues in a binoviewer will be up to, say, 6 inches in front of primary focus, edge darkening is going to be more pronounced in longer FL scopes, where in short FL scopes you may encounter more overall vignetting affecting wider fields of view.

Generally, the steeper the light cone and the further away from primary focus, the worse it will be.

Attached Thumbnails

  • 2017-Simple light cone - vig.jpg



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