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What causes these weird shadows - first light help!

Skipper Billy

By Skipper Billy
in Imaging - Discussion

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Skipper Billy

Skipper Billy

Posted
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Hello all.

First light last night - Explore Scientific 208 f3.9 newt - AZ EG6 GT mount - Atik 460 camera - Atik EFW2 filter wheel - Baader LRGB filters

Seeing poor - barely dark - just wanted to run some Focusmax V curves. First time I have used this outfit.

The bright stars have a set of 3 shadows 120º apart - what could be causing that ???

Pinched optics ??

The OTA is flocked.

The attached image is a single 300 second Lum sub - stretched in PI.

Single__Luminance_2017-08-20_23-00-35__-10C_Bin1x1_300s.jpg

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AKB

AKB

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12 minutes ago, Skipper Billy said:

What can be done about it ??

An aperture mask slightly smaller than the mirror clips and sitting just above the mirror should do it.  There's a post somewhere around about someone doing this on a SW Quattro.  Will look for the link.

 

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Skipper Billy

Skipper Billy

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Sorry - caught return before I typed the message fully !!

GREAT RESULT !!!!! 

When I took the primary mirror cell apart and removed the clips I found that the mirror had already been glued in at the factory - inside each of the three primary mirror clips and a trio of substantial bobs under the mirror in the middle of the back. It's VERY secure - I am 99% certain that I couldn't remove the mirror from the cell even if I wanted to. Looks like Sikaflex rubbery sealant type stuff.

Whilst it was in bits I took the opportunity to blacken the edges of the mirror and anything else that was remotely shiny.

Just need to put it all back together now and start collimation from scratch !

There are some photos of before and after on my website here if anyone is interested or is having the same issue.  https://sites.google.com/view/astro-imaging/equipment

Thanks everybody for the speedy and high quality guidance.

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Zakalwe

Zakalwe

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Good luck :thumbsup:

In a way its good that Skywatcher make their kit at the price point they are. Sure, they could sell better cells and they could blacken the edges of the mirror bevel. But then the cost would be a lot higher. As long as the mirror and the other major bits are good quality, then it's easy for us to do a little "fettling"* to get it just so.

 

 

*What a great word! I think that I will aim to use the word "fettle" more often!

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AKB

AKB

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5 hours ago, Skipper Billy said:

I took the opportunity to blacken the edges of the mirror

Can I ask what you used for that?  It looks like you covered the whole sides of the mirror and not just the bevel.

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JonathanA

JonathanA

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Definitely the mirror clips. I have recently had exactly the same problem with my ES comethunter (there is a thread on here somewhere about it). Strangely, the 3 shadows only became really evident once I had (finally) got the scope properly collimated and removed some astigmatism from the system. Unfortunately, my mirror isn't glued in place and the mirror supports seem quite narrow so I don't think I have the option of removing the clips. I would be slightly cautious of removing the clips entirely though unless you are completely confident the glue will hold the primary in place even when the scope is at near horizontal. I've heard stories of disasters of this kind.

Enjoy the new scope, it looks like a corker!

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ollypenrice

ollypenrice

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15 hours ago, JonathanA said:

Definitely the mirror clips. I have recently had exactly the same problem with my ES comethunter (there is a thread on here somewhere about it). Strangely, the 3 shadows only became really evident once I had (finally) got the scope properly collimated and removed some astigmatism from the system. Unfortunately, my mirror isn't glued in place and the mirror supports seem quite narrow so I don't think I have the option of removing the clips. I would be slightly cautious of removing the clips entirely though unless you are completely confident the glue will hold the primary in place even when the scope is at near horizontal. I've heard stories of disasters of this kind.

Enjoy the new scope, it looks like a corker!

The upshot of the other thread was that it was not the mirror clips but the absence of the mirror clips which was the problem! The mirror clips were the solution and just needed extending right round the mirror, so to speak.

Olly

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JonathanA

JonathanA

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1 hour ago, ollypenrice said:

The upshot of the other thread was that it was not the mirror clips but the absence of the mirror clips which was the problem! The mirror clips were the solution and just needed extending right round the mirror, so to speak.

Olly

I take your point, Olly. As I understand it most of the diffraction is from micro-irregularities in the edge of the mirror and the clips (irregular edge obscured) just leave a shadow in the diffraction pattern. I should have been more careful in how I phrased it. Aesthetically, I think I would probably accept the diffraction though, provided the distracting shadows weren't there. Presumably, this is why some people completely remove the clips and glue the mirror rather than make a mask to extend the clips. Not sure which way to go myself. I see perils in both approaches!

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Skipper Billy

Skipper Billy

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Quick update on progress made.

I removed the mirror clips and found that the mirror was already glued in - last night there was a very brief window of opportunity and this is one 120 second sub - the 3 x 120º dark shadows are gone !!!

I have now added a blind ring to effectively extend the mirror clips all round the edge of the mirror and to hide the bevelled edge. I will try this out again on the same target and report back but the removal of the clips is a huge impovement!! Hopefully the ring will help to control the flare.

The blind ring and cut down mirror clips are detailed here - https://sites.google.com/view/astro-imaging/equipment

 

 

no clips - Copy.jpg

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iapa

iapa

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Interested to know impact of collimating, as one step is to check the clips are all visible

if there’s a dark ring instead, not sure how this would work, unless marks where made on the ‘blind ring’?

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Skipper Billy

Skipper Billy

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This is the same target - single 300 second sub - with mirror clips replaced and blind ring in place - much better despite truly appalling seeing and transparency. Single 300 second sub.

Just to sort out the camera tilt now and we are in business - many thanks to everyone that has offered help along the way.

early.jpg

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AKB

AKB

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1 hour ago, Skipper Billy said:

This is the same target - single 300 second sub - with mirror clips replaced and blind ring in place - much better despite truly appalling seeing and transparency.

I see that it also gets rid of satellite trails... magic!

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