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weird airy disc pattern - what can cause this?


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#1 Konihlav

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Posted 23 September 2013 - 02:12 PM

Hi all,

 

does anyone have a clue what can cause "this" extra/weird diffraction - see red arrow in detail here:

http://www.astro.cz/...strike.png.html

full scale - http://www.astro.cz/...ageViewsIndex=1

 

can it be related to airy disc pattern? looks to me like on both images it's at about 5 o'clock possition:

http://www.astro.cz/...ageViewsIndex=1

full scale - http://www.astro.cz/...ageViewsIndex=1

 

both taken at the same time (in the morning, mirror was cooled down/stabilized after 5+ hours).

 

what is more interesting is that I have the same "weird"/extra diffraction on all my LRGB images even those taken with differnet CCD camera (the image above is done with Atik460EXM - small chip - while the one below is done with G3-11000 using KAI-11000 full frame chip) on different nights/sessions... e.g.:

http://astrofotky.cz.../1375952959.jpg

 

once, I wanted to do a roddier analysis of the mirror, but as my first attempt I didn't gather quality data (missing video camera). Next time I hope I get more relevant data. But anyway, in the meantime, I'd like to hear few comments/ideas.

 

thank you all for publishing your opinion

Pavel

 

 

 

BTW it is done with 10" N-ASA (mirror is glued, no holding brackets visible at all), there's nothing in the light path.

 



#2 GeorgeCarey

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Posted 23 September 2013 - 04:01 PM

Hello Pavel,

I had a similar problem with my Netonian scope when I replaced the secondary mirror with a larger one.

This webpage describes what I encountered http://geoastro.co.uk/february2011.htm

 

In the end it turned out to be a faulty secondary. A replacement cured the problem.

Does your defect stay at 5 o'clock inside and outside focus, or does it shift over to 11 o'clock?

 

George


ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#3 Konihlav

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Posted 23 September 2013 - 04:29 PM

Hi George,

 

I have just checked the data I took few months back when I tried the WinRoddier analysis (computed Strehl of 0.00something instead of 0.8 or 0.9something due to badly acquired data).

 

Here's what I found:

1. the "defect" doesnt stay at the same o'clock position - when I change focus postion (inward to outward or vice versa) it moves to the opposite side of circle (e.g. from 5 to 11 o'clock)

 

2. it was probably big coincidence that both cameras had same length threads so that screwed in resulted in same camera angle orientation within the focuser tube

 

3. because - I found that one day I rotated the camera (fixed on 3" Wynne reducer/corrector) about 90 degrees different way and the "defect" moved from 5 o'clock to 8 o'clock position...

 

(i) I was suspecting problem with either primary mirror or secondary mirror - maybe due to my unablity (not enough skill) of collimation :) and because I have always bad luck with new equipment so I am used to find problems where they do not exist

 

(ii) at the moment I realized I do have OAG prism in there. But can't understand that also tiny chip of 16mm diagonal (Atik 460) can show this defect (while I would expect big 36x24mm chip to show it which would be OK with me). but why the small chip shows that too is a mystery for me at the moment...



#4 GeorgeCarey

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Posted 23 September 2013 - 06:41 PM

If the defect rotated when you moved the camera then that rules out a defect in the camera.
Is it possible to rotate the Wynn corrector and keep the camera fixed?
Thanks for mentioning the Roddier software. I had never heard of it and it sounds just like the sort of Geeky stuff that I like!

ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#5 Konihlav

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Posted 24 September 2013 - 09:18 AM

Hi George,

 

defect in any camera is out of way as I have two cameras and both show it.

yes, it is possible to rotate both:

1. either only the Wynne corrector (by using different thickness spacers - I have a set of 0.66mm 1mm and 1.33mm to fine tune the backfocus distance) - therefore every time I change the spacer (already tested none, 0.66mm one and 1mm which make resultant focal length of some 906.something, 904.1mm or 903.4mm respectively).

when I put different spacer I have effectively rotated connection Wynne+CCD

2. rotate Wynne inside of the focuser tube (for composition orientation) results in different camera position (angle) for astroimaging composition. I have found two examples from two nights where I have this changed in my setup, once I was shooting vdb152 Luminance (I stupid uploaded NGC7129 from different night, but the orientation of camera was more or less (by 5-10 degrees) the same I am sure about it):

http://www.astro.cz/...-09-06.jpg.html

Pos Angle +272° 05', FL 903.4 mm (using 1mm spacer results in 903.4mm BFD)

here star pattern:

http://www.astro.cz/...-FL-W5.jpg.html

another night vdb152 RGB colors (ignore the alignment/rotation problem in red channel) and star pattern:

http://www.astro.cz/...ckstar.jpg.html

 

OTHER night and OTHER rotation of camera, shooting LBN603 in July:

http://www.astro.cz/...-07-15.jpg.html

Pos Angle +345° 58', FL 904.1 mm (using 0.66mm spacer)

and voila - different orientation of the defect

http://www.astro.cz/...inance.tif.html

 

so what does it say???

 

when I get the chance to go out and shoot the stars, I remove the OAG prism with one of the small hex keys and see if the pattern disappears or not though I doubt it is OAG now :-( I was very much hoping the problem won't be in mirrors. Any other possibility???

 

What do you say George, others???


Edited by Konihlav, 24 September 2013 - 09:22 AM.


#6 Wolfgang

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Posted 24 September 2013 - 09:24 AM

Hello Pavel

 

What ou are seeing is the straylight from the edge of the secondary, I wouldnt consider hat as an optical defect.

You could put in an oversized secondary with the downside of unnecessary obstruction, or cover the edges of the secondory.

In my opinion it also has its benefits as it takes energy out of the bright stars and keeps them from beeing saturated given quite nice star colors.

What you have on the out of focus image at 5 o clock is clearly a tube current.

 

Wolfgang

 



#7 GeorgeCarey

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Posted 24 September 2013 - 09:30 AM

How about indoor testing with an artificial star?

 

I suppose the most important part of the system is the primary mirror. You could rotate that 90 degrees and keep everything else the same.

If that seems OK then rotate the corrector. If that is OK then it must be the secondary.


ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#8 Konihlav

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Posted 24 September 2013 - 10:37 AM

Dear Wolfgang and George,

 

thank you very much for your insight, I really appreciate it.

 

Wolfgang - that was one of the questions I sent to ASA prior my N-10" purchase :-) I asked on edges of secondary if they are blackened or not (because in the past, few years back, I read on yahoo group of someone having issues shooting flat field frames and also from my friends that build their own Newtonians that it is a good idea to black it).

Do you think oversized like more oversized than the 100mm dia that is in my 10"-ASA ?

Could I try just to put some obstruction on the secondary (on top of it, like cut bigger circle aperture blocking light around secondary - increase central obstruction) ?

 

the tube current idea is also very nice (confident) - I try to turn off the primary mirror ventilator next time to see if there's any change...

 

George - problem is I have limited space in my office, I have no big laboratory to make these experiments. Also both my hands are left-hands so I have rather went with ASA then any DIY so any rotation of primary mirror is out of question as I do not want to screw things... but it is certainly a good idea for identifying which mirror is on blame (provided all else is OK). Anyway I very much hope both mirrors are fine, only the secondary one has "white" edges. I try to make some central obstruction and see if it has effect or not.

 

thanks anyway!

 

BTW the winroddier home page is here: http://www.astrosurf...ier/projet.html

 

in the meantime I purchased a planetary (video) camera in order to capture the airy patterns a better way...


Edited by Konihlav, 24 September 2013 - 10:41 AM.


#9 GerhardB

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Posted 24 September 2013 - 12:23 PM

Hi Pavel,

 

The pattern could be generated by an obstruction. Have you ever done an analysis with a strongly defocussed star? If you see more than the shadow of the secondary and the spiders, then this additional item could be the source of the pattern. Looking at the image of your setup, the pickup prism of the off-axis guider could very well be the origin of an additional obstruction.

 

Best regards,

Gerhard



#10 GeorgeCarey

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Posted 24 September 2013 - 05:36 PM

Reflection off the edges of the secondary could be a cause. I painted mine with matt black paint as seen here:

 

IMG_0110_cr.jpg


ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#11 Konihlav

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Posted 24 September 2013 - 09:24 PM

yes, I can confirm that my secondary mirror does have the whole side painted WHITE (or white/transparent) instead of black as George shows on the image above.

 

Therefore, next time out (could take a month or two in our damn bad weather conditions) I will:

- check tube current

- introduce central obstruction to eliminate white edge problem of secondary mirror

- remove OAG prism (though I doubt this is the case, but for sure I need to systematically do all stuff proposed)

- capture some video of defocused stars in all color channels in order to do roddier analysis...

 

I let you know when I get more data...



#12 GeorgeCarey

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Posted 25 September 2013 - 01:56 PM

I have got the Roddier sofware running. Now to get some images to put through it.

 

This is a test run:

 

Roddiergrab.jpg


ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#13 Konihlav

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Posted 07 October 2013 - 07:47 PM

so far, I can rule out (for 100%) that it is NOT a tube current - last night I tested imaging with mirror fan ON and OFF etc. and no difference. I still have 2 things to check left. I come back to inform you on my next findings...



#14 GeorgeCarey

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Posted 08 October 2013 - 09:50 AM

I found that I only got tube current effects when the telescope was at low altitude.
If you do a test with the scope aiming almost straight up at the vertex it should rule out tube currents.
George

ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk


#15 Konihlav

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Posted 23 December 2013 - 12:26 PM

I have found the problem - it is the primary mirror, the outer edge... I will describe it in detail in yet another blog article when I get some time to do so...



#16 Konihlav

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Posted 15 January 2014 - 11:01 AM

Hi, since I have no time to write an article about it (at least until I resolve it or get some solution or response from ASA/OrionUK), I would like to publish here my data and my findings:

because I ruled out already everything except of mirror(s) quality I focused on mirror testing again - BTW first test showed Strehl 0.02 :-( due to bad data acquisition.

I asked my friend to build some aperture masks for my 10" ASA-N that you can see here:

http://www.astro.cz/...G_8823.JPG.html

Since it's not easy for me, a both left-handed guy, to unmount the primary mirror from the telescope and rotate it (I am user, not a tinkerer) I decided to use the masks as a front aperture instead of putting the primary mask just above the primary mirror, i.e. inside the tube. By this (using front aperture mask instead of inside of tube) I had to test on a star that is perfectly centered in the FOV of the ccd camera so as the mask has full effect.

First try - secondary mask - ZERO effect, nothing changed.

Second try - primary mask - wow, the bad effect dissapeared!!!

here is the bad thing I talk about:

http://www.astro.cz/...strike.png.html

this is common to cheap China made mirrors and Newtonians where you have to put a primary mirror mask just above the primary mirror inside of the tube and below mirror holders, but my ASA 10 uses glued mirror so no mirror holders present on the outer edge.

IT IS EVIDENT that the problem is on outer edge/part of the primary mirror.

hrere is the reference BAD image from the night of testing (was done in the evening without proper mirror temperation so that's why also the FINE image doesn't look so good as it would after temperation - another thing is that that night I had mounted (hard mounted) H-alpha 3nm filter inside that I could not replace by Lum without dissmounting things so I had to use this setup as it was),

BAD/reference image without primary mirror mask:

http://www.astro.cz/...sc/BAD.jpg.html

FINE image with primary mirror mask:

http://www.astro.cz/...c/FINE.jpg.html

 

so the question is on Orion UK mirror quality and guarantee from ASA - I will ask ASA about this... as I reviewed my first light image (first night out with the scope) from 14.5.2013 that shows the same problem around bright star (bottom left star):

http://www.astro.cz/...est-W6final.jpg



#17 Konihlav

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Posted 15 January 2014 - 11:10 AM

BTW the primary mirror mask I asked my friend to create for me was sized at 235mm, i.e. masking 15mm (1,5 cm!) of the mirror outer radius. This causes a light loss of 13% making F/3.6 scope to become F/4.07 scope. Obviously, maybe the mask could be 240mm or even 245 who knows...

I would like to be able to measure Strehl of my mirror better then 0,1 or 0,2 anything over 0,7 0,8 is acceptable for me.

thanks for any comments anyone might have!!!



#18 Konihlav

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Posted 11 September 2014 - 08:09 AM

the issue was resolved by ASA (few months back from now) by replacing the mirror within warranty :) now I already got yet another 1st light... everything looks perfect...



#19 GeorgeCarey

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Posted 11 September 2014 - 09:51 AM

Glad to hear the problem is resolved.


ASA DDM60Pro, homemade 8" and 10" reflectors, QSI 683 camera, Astrodon 5nm filters, Pulsar dome observatory. Website: http://www.geoastro.co.uk





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