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Acceptable Star Shape from New Scope?

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#1 nhmorgan79

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Posted 19 January 2021 - 02:22 PM

So just made the transition from using my 70-200mm nikon lens to a Meade 70mm astrograph quad. Last night I got a few hours to test it out. However, looking at the results this morning, there are uneven and pronounced cross shapes on the brighter stars. It is evenly occurring across the whole frame My question is, is the star shape in the attached image considered acceptable, or is this scope out of adjustment in a significant way? Thanks for any input; I appreciate it. stardiff.jpg


Edited by nhmorgan79, 19 January 2021 - 02:25 PM.

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#2 sg6

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Posted 19 January 2021 - 02:35 PM

What is the scope, and may as well ask what is the camera?

 

Simple guess is that the small lens spacers are encroaching into the light path. Unfortunately seems to be getting fairly common these days.

 

The other is the microprisms on the sensor. However I read your post as a DSLR and that seems less common on them - not sure why. The 1600 dedicated seems well prone to microprism problems.



#3 Midnight Dan

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Posted 19 January 2021 - 02:35 PM

Those are diffraction spikes caused by the clips that hold the objective lens in place.  Better scopes should not have them.  Whether they are objectionable or not is up to you.

 

Personally, since that scope is sold as an astrograph, I would not be happy with them.  If it's a new scope, I would send it back and ask for a replacement.  If they say it's normal, I'd get my money back and go with a different brand.

 

Some people have had some success reducing the effect by loosening the screws that hold the lens cell in.  But that's tricky and a bit risky and for the price of these scopes you should not have to mess with that.

 

-Dan



#4 nhmorgan79

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Posted 19 January 2021 - 02:41 PM

Nikon D850 unmodified for the camera. The scope for this picture is a Meade 6000 series 70mm astrograph. Haven't had this problem with lenses, just with this new scope.



#5 ngc7319_20

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Posted 19 January 2021 - 02:44 PM

Yeah this looks like some combination of optical turn-edge, lens spacers / clips, and maybe a little pinch (something too tight).  I would try to mask off the outer couple mm of the lens (mask off the turn-edge) and try again.  Get a circle cutter from the craft store and some black construction paper.  This is fairly conservative approach.

 

Can you post a picture of the front of the lens?  I'm a bit puzzled if the lens has 4 spacers instead of the usual 3.

 

Great image!!!


Edited by ngc7319_20, 19 January 2021 - 02:44 PM.


#6 nhmorgan79

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Posted 19 January 2021 - 03:09 PM

Yeah this looks like some combination of optical turn-edge, lens spacers / clips, and maybe a little pinch (something too tight).  I would try to mask off the outer couple mm of the lens (mask off the turn-edge) and try again.  Get a circle cutter from the craft store and some black construction paper.  This is fairly conservative approach.

 

Can you post a picture of the front of the lens?  I'm a bit puzzled if the lens has 4 spacers instead of the usual 3.

 

Great image!!!

Excuse the dust on the front, I did clear it off before shooting, but for some reason Meade thought felt on the front cap was a good idea.image05060.jpeg



#7 georgian82

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Posted 19 January 2021 - 03:30 PM

I have this same scope and I went through the same situation as you. I mostly fixed it by removing the lens hood and backing away the four front screws by a tiny bit (1/8 turn max)

 

But if you decide to do this, make sure you mark the original position of the screw so that you can go back to the same spot if needed. And of course, do this at your own risk! wink.gif

 

Cheers

 

Meade screws 2
Meade screws 3

 


Edited by georgian82, 19 January 2021 - 03:36 PM.

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#8 ngc7319_20

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Posted 19 January 2021 - 03:52 PM

I'm not seeing any spacer tabs in the light path in the photo.  So this is probably a case of pinching / centering screws too tight, as @georgian82 suggests.

 

It seems to me, backing off the centering screws would reduce the "spokes", but you will still have the halo...   Guess I'd still try to mask the lens down a couple mm and see what happens.  Make a mask from construction paper or cardboard or margarine lid and attach on the outer lens cell with painter's blue tape.   Should be a fairly benign intervention.

 

The dust wont hurt anything.

 

post-351608-0-44323100-1611086939 x.jpeg




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