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Dew Shield Skewing Collimation

collimation SCT
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#1 Lasko

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Posted 16 January 2021 - 07:22 PM

Here is something weird I discovered a few days ago.  I set up the 8SE with the dew shield on because the dew point was high enough to worry about.  During alignment I noticed the collimation was off with the donuts bent a bit to one side.  This seemed odd because I had just set this new scope up a few weeks ago and meticulously adjusted it until I thought it was perfect.

 

Thinking I might have to tweak it again I removed the dew shield and magically it was back to fine.  This is the stock Celestron velcro lens shade and I installed it lined up as evenly as I could with the front ring on the scope.  I also discovered that tipping the front of the shade up a bit got rid of most of the distortion.

 

Maybe the shade is just a bit out of round and intruding on the light path?  The only thing I could think to try, other than adjusting it until it looks better, is to re-fasten the velcro so that the "front" end is bit wider to cut down on the chances of it interfering.

 

Is this a common trick to do so that the shade doesn't wreak havoc on the image?  I don't think I will need it today since the T/D is more than 15°F, but I might experiment if I have time.

 

 



#2 c2m2t

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Posted 16 January 2021 - 07:59 PM

Hi Lasko!

I don't think the blockage by the dew shield is changing the collimation...it is simply blocking some light entering the front of the scope. What you are seeing as a distorted out of focus star is the affect of the dew shield blocking light. It should ideally not affect focus. Of course it will be better to have no blockage from the dew shield, so trying to open up the leading edge of the dew shield should help. I have a 9.25"and 12" SCT and trying to keep the lead edge of the dew shield round is difficult at the best of times!!

 

Cheers, Chris. 


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#3 alphatripleplus

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Posted 16 January 2021 - 10:54 PM

+1 on the shield just partially obstructing one side of the corrector. Collimation of the secondary in the middle of the corrector would have nothing to do with the dew shield.


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#4 Lasko

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Posted 17 January 2021 - 01:17 AM

Success in fixing that issue today.  I did use the dew shield and adjusted the velcro to make the outlet side about 10mm wider than the scope side.  When I did it before I tried to line it up straight.  I installed it the same way by lining up the back edge with the edge of the front ring on the OTA and there was no hint of distortion this time.  Like you said, it must have been blocking just a bit of the edge and made it look like the collimation was off.


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#5 junomike

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Posted 17 January 2021 - 08:33 AM

The 8" SCT was the only one I could watch collimation as I tweeked it (Dew Shield off).  With the others (larger) I needed a second person.



#6 alphatripleplus

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Posted 17 January 2021 - 09:24 AM

Success in fixing that issue today. 

Glad that it worked out.



#7 Lasko

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Posted 17 January 2021 - 10:18 AM

Here is the final result, the scope side is on the right.  The outer diameter is now a few mm wider than the inner.  With the velcro lined up straight it was probably slightly smaller, or even if the same not exactly round.

 

Attached Thumbnails

  • 20210117_070406.jpg


#8 c2m2t

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Posted 17 January 2021 - 11:23 AM

Hi Lasko!

I am guessing that you are not getting any interference from a dovetail. With my Celestron 9.25", I had to notch out the dew shield so that it would lay flat/follow the curvature of the OTA...was a bit of a nuisance but did work out. My 12" Meade was not an issue by virtue of the fork mounting.

 

Cheers, Chris.



#9 Lasko

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Posted 17 January 2021 - 01:20 PM

Hi Chris,

There is a cutout on the opposite side for the rail.  This makes the Celestron logo upside down when viewed from the non-mount side.    They didn't really think that through.  roflmao.gif 

 

As long as it stays lined up there is enough room to adjust it to where everything is even.  If the notch wasn't already there I would be cutting one myself because it clearly needs to be there.  


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