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Refractor Diffraction Spikes

astrophotography CMOS collimation equipment imaging optics refractor
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#1 Brisby2

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Posted 02 March 2020 - 11:06 AM

Hello all,

 

I've recently acquired an Explore Scientific ED80 Triplet Refractor and noticed upon first light that it suffers from "iron cross" type diffraction spikes. I'm almost certain that it's the collimation screws that are pinching the objective, but I'd like to know any ideas out there for treating or maybe even eliminating them?

 

Here's a zoomed in pic of one of my images to show you what I mean:

 

DiffSpikes.png

 


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#2 fmeschia

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Posted 02 March 2020 - 11:24 AM

Does that happen uniformly across the field? if not, then it’s probably a side effect of the optical design: two apertures or field stops combine together to produce a non-circular overall aperture, with two cusps that produce those spikes.


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#3 Jim Waters

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Posted 02 March 2020 - 11:27 AM

Try backing off on the collimation screws 1/8 turn and see if the images are better.


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#4 sg6

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Posted 02 March 2020 - 11:41 AM

Most likely the collimation screws or whatever holds the whole lot in place.

Owing to the nature of glass it can deform under pressure, and not necessarily a lot of pressure.

 

As previously find the screws and slacken one off a fraction.

Now you may then want to fractionally tighten it up again - basically trying to feel how much inwards it could go before slight resistance, then back off a fraction.

 

They are the sort of thing that needs to not be loose and not be gripping. So getting it right could take a little work.

 

If there are 3 it should be a case of back off 1 and the other 2 should allow the lens to relax while holding it stable.

If 4 then it could be a case of which pair are gripping too much.

 

Alternative is lens spacers getting into the light path. You need to look down the front of the lens cell to see if anything is protruding. If there is then the lens need removing and the spaces reset then the lot reassembled - not for the faint hearted.

 

So try the screws first, let it rest a couple of hours and see what has transpired. If you look and see spacers intruding, then not sure as the obvious is back to ES. But that is time and money.


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#5 Midnight Dan

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Posted 02 March 2020 - 11:55 AM

I purchased one of those a while ago and found the exact same problem.  I sent it back and got a Skywatcher.  A lot more money, but beautiful round stars.  

 

My guess is that it's the clips or screws holding the lens in place, intruding into the light path.  I couldn't see anything looking into it, but I really wasn't happy with it so back it went.  Not sure you can totally get rid of them.

 

-Dan



#6 Mohman

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Posted 02 March 2020 - 12:17 PM

I have a similar issue with my ES ED102. You can try masking it off if you believe its something intruding on the light path. For testing purposes I used a cereal box and a craft circle cutter from Amazon. You will probably not be able to cut a clean enough circle for this with scissors. Cut a circle the size of the ring on the front of the objective and then remove the center, say 75mm. Use painters tape to hold it in place and take a test shot of a bright star. This will increase the F ratio of your scope slightly. It seemed to improve mine quite a bit, so I'm planning to make a more permanent/durable mask out of Kydex. I'd tried backing of the locking screws a bit but that didn't help.

Good luck.

#7 Brisby2

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Posted 02 March 2020 - 02:07 PM

Does that happen uniformly across the field? if not, then it’s probably a side effect of the optical design: two apertures or field stops combine together to produce a non-circular overall aperture, with two cusps that produce those spikes.

The spikes are across the entire field, yes.



#8 fmeschia

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Posted 02 March 2020 - 02:09 PM

Then I agree with the suggestions to check for pinched optics.



#9 Brisby2

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Posted 02 March 2020 - 02:10 PM

Most likely the collimation screws or whatever holds the whole lot in place.

Owing to the nature of glass it can deform under pressure, and not necessarily a lot of pressure.

 

As previously find the screws and slacken one off a fraction.

Now you may then want to fractionally tighten it up again - basically trying to feel how much inwards it could go before slight resistance, then back off a fraction.

 

They are the sort of thing that needs to not be loose and not be gripping. So getting it right could take a little work.

 

If there are 3 it should be a case of back off 1 and the other 2 should allow the lens to relax while holding it stable.

If 4 then it could be a case of which pair are gripping too much.

 

Alternative is lens spacers getting into the light path. You need to look down the front of the lens cell to see if anything is protruding. If there is then the lens need removing and the spaces reset then the lot reassembled - not for the faint hearted.

 

So try the screws first, let it rest a couple of hours and see what has transpired. If you look and see spacers intruding, then not sure as the obvious is back to ES. But that is time and money.

Would backing off a single screw not cause it to be thrown out of collimation? I can probably try it, but I'm a bit hesitant.




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