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Eggs and multiple diffraction spikes and quality control.


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#1 pesa

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Posted 23 March 2015 - 05:18 PM

Hi

 

I have an ASA 10N astrograph on an ASA DDM60Pro mount. I received it in this january. I use ASA's 3" wynn Corrector (x0.95) No guiding.

 

I've never been able to get (even Close to) perfect images with it.

I've tried with glatter laser from asa (not perfectly collimated collimator!) and TUBLUG.

Since laser isnt perfectly collimated (and I dont use ANY force when fasten it with screws) its hard to judge when laser hits middle of primarys triangle. Supposedly, thats not critical. Correct?

 

With Tublug I collimate primary to move triangle to tublug's center, as well as I can judge. Obviously my judgment isnt good enough cause eggs remains

 

I will try cats Eye later but descriptions how to use it seems to be somewhat dependent on secondary placement so that will be Another difficult excercise (I've read somewhere here on forum that ASA moves secondary down to avoid assymetric vignetting (as they should)

However it isnt clear to me if they just moved it down or moved it parrallel to seconday Surface, that makes a difference for how to use cats Eye and other Tools like sight tubes, I THINK at least.

 

As you notice I'm not very knowledgeable when it comes to collimation. I'm not very interested in it but now I have to. This is NOT what I wanted, I bought with a hefty premium for a newtonian precisely so I DIDNT have to figure all this out myself.

 

Makes me wonder about their quality Control. Also Orion optics can be a bit sloppy from Reading this forum.

The tubes where described by asa as having tested mirrors with accompanying protocol. I never got any protocol and I strongly suspect my primary is faulty.

 

Now my question is this, how much of these problems/thoughts do you share?

My experience is not completely negative tho, I had a focuser thrashed by not being careful when unscrewing Corrector but they sent a new one without hitting me with a charge. Kudos for that.

 

I have read all threads here I can find about eggs and diffraction patterns but I'd appreciete your feedback about my image as well:

is the eggs likely caused by bad collimation and is extra fiffraction spikes due to faulty primary?

Any help/thought is appreciated

Pelle

eggs
Album: collimation
1 images
0 comments

 



#2 Gerald

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Posted 25 March 2015 - 02:51 PM

Dear Pelle,

 

You are right - collimation is a tricky issue. First of all - as I have seen at the internet the tublug is 1 1/4" - I personally prefer 2" collimation tools - because one additional source of error can be avoided. Because I have problems with the laser to see it only sharp with glasses I prefer the Cats Eye Collimation tools 2". Only to at a first centering of the secondary to the main mirrors center I use a 2" collimated laser. Then with the Cats Eye tools it is a ping pong from secondary to main mirror and back. Most of the time several passages are necessary. I use the Cheshire to align the main mirror and the autocolli to align the secondary. At the end I have to see the star of david through the autocolli eyepiece as well as the center pupil in the middle of the hole of the mark.

At a permanent facility I recheck collimation on a star.

 

Hope that helps good luck

 

Gerald



#3 pesa

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Posted 26 March 2015 - 10:22 PM

Thanks for the answe Gerald,

my tublug is 2", much better than 1 1/4" ofc
You mention centering the secondary as seen with a cheshire but do ASA move secondary mirror (not holder ofc) straight down from focuser?
If you slide the secondary parrallel to its surface (so a laser doesnt hit it in its center) it shoulnt be exactle centered in the cheshire,right?

With the help of the swedish reseller I was able to get a decent collimaion by taking shots with camera in place and then adjusting one screw at a time. Quite effective.
Later we will try to fine tune it with catseye xlk.

One thing you didnt adress is the extra diffraction spikes you see in the image. Doesnt ASA check both mirrors before shipment?
As I mentioned I never got the test protocol that you mention in the brochure.

Pelle

#4 maxmirot

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Posted 30 May 2015 - 09:10 PM

Make sure your primary mirror is not pinched.  If the screws are too tight you will definitely see aberrations.

 

I had a mirror with turned edge that caused extra spikes. To test for this,  make a mask out of paper that goes right over the mirror's surface. Make it about 10mm wide to cover the edge. Take a image.  If the spikes are gone or much better then it is the mirror's edge. Sometimes the mirror is very good except the last 2-3mm.  Bad edges can be missed on some test systems.

 

Max



#5 pesa

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Posted 01 June 2015 - 03:05 PM

Thx Max for the reply!

 

>Bad edges can be missed on some test systems.

For the price we pay for ASA-astrograph they should be perfect. A test protocol is promised in the brochure but I never got one.

I have made a mask to reduce F-number but spikes looked the same. I also rotated the primary (not glued contrary to the brochure) with same result, no change in spikes..

 

pelle



#6 Dr. Werner Van Alstyne

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Posted 11 January 2017 - 12:02 PM

Dear Pelle,

What was your outcome and how was it resolved?

#7 pesa

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Posted 11 January 2017 - 02:14 PM

Dear Pelle,

What was your outcome and how was it resolved?

 

Hi Werner

The core problem was that the secondary was astigmatic. I ruled out primary as problem by rotating it 120 degrees and noting that pattern didn't change. Same with wynne.

This turns out to be very easy to spot; take images both inside and outside focus (very small delta will do). If you have astigmatism you will get ellipsoids with big axis' (plural) 180 degrees apart out- and in-side.

Combine that with sequence-autofocus not doing a god job with astigmatic system. This resulted in stars "in focus" (after autofocus) looking like scope needed collimation.

So I collimated.

Repeat ad nauseum. Akin to try stearing a car with the rear view mirror.

Finally I understood that secondary was astigmatic. ASA then exchanged the mirror (still slightly astigmatic but better)

I think the system is diffraction limited so the slight astigmatism has a positive effect, I always knows which side of focus I am in. / outside and \ outside.

hope this helps

 

pelle



#8 Dr. Werner Van Alstyne

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Posted 11 January 2017 - 11:23 PM

Thank you vey much Pelle. I am looking to purchase this year a ASA scope and DDM85 mount and I am now for the first time reviewing the various forms for issues that hopefully have been corrected since any scope has been first introduced. I don't want any bugs that are an issue with either the design or assembly. I understand if the issue is solely user based. What type of primary and secondary mirrors are in the 10N and 12N? If you know. I was looking at the 8 inch H model if offered in the near future with much larger aperture. Are any of ASA's three astrographs suitable for both imaging and visual? Are you truthfully satisfied with your purchase and would you have looked at another manufacturer such as Officina Stellare if you had to do over?

#9 pesa

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Posted 18 January 2017 - 01:23 PM

Hi Werner

>What type of primary and secondary mirrors are in the 10N and 12N?

  Its an ordinary newtonian design but in a very well engineered (IMO) design. The H8 is very intersting too but I see 2 disadvantages, its on the small side and you need a very big chip to justify its cost since its advantage is a big imaging circle. No point buying a H-series and attatch a camera with small chip to it.

 

Neither of those would be ideal visual instruments IMHO. They are designed for imaging.

I'm sure officina stellare makes good stuff but they dont have newtonians AFAIK. Also their mounts isnt interesting for me, I want encoders.

I would andwill buy more stuff from ASA. Sure, they should have more ppl for us amateurs - they are too focused on professionals IMO - but I dont know that other firms are any better. I've also never heard of any supplier with better engineered scopes/mounts. Again I have some reservations, their quality control could be more alert, but I know from friends that their most obvious contender on the scope side is MUCH sloppier.

Also if I have problem with hardware they have been very fair about it, even when I myself messed up.

 

One example of their fine engineering is the new focuser with absolute encoders, its a marvel! At F3.6 (in my case) focusing have to be very precise and hysteresis on an ordinary focuser adds to the problem. With absolute encoders hysteresis is not a problem. Have saved me a lot of time

 

hope this helps

pelle






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