diffraction spikes - "double" reflection on bright stars, Newtonian

[Konihlav]

Hi folks,

 

I am a new user of N-series astrograph, more precisely, I am a newbie user of Newtonian system at all (so far I've been imaging with refractors and cassegrains only) though I find myself already to be an experienced astro imager.

 

Do you know what is the exact root of this issue? I can see double reflection on diffraction pattern around bright stars. In my pre-zero light first night of testing new AG in the field, I made few tests to see how the ASA really works. I have touched collimation only slightly, only of the primary mirror. So far I did not touch secondary nor tested axial error of the focuser (I simply trust ASA ). I do own, already, all CatsEye tools from Jim Fly and all tools from Howie Glatter so I know it's not perfectly collimated (infinity always shows a residual error, howie shows "almost OK").

 

I very much hope it's caused by improper collimation and not by some other mechanical problem of my astrograph (I have both hands "left hands" so I can't fix anything myself without a visit to specialized laboratory - someone with mechanical skills).

 

here is the full sized image, ignore the vignetting of the window, that will be fixed today on my camera:

 

http://www.astro.cz/galerie/v/uzivatele/Konihlav/misc/CCD+Image+55LUMLUM.jpg.html?g2_imageViewsIndex=1

 

please, let me know what you think, I really need exact and precise answer and proposed solution. No time left to "try and see". I'd like to make a real first light this year.

 

Pavel

 

 

 

[GeorgeCarey]

Hi Pavel,

Are you talking about the apparent 'splitting' of the spikes? This is due to the secondary support vanes on opposite sides of the secondary not being perfectly in line. Each vane produces a spike that runs through the bright star. When sharply focused the two spikes will be superimposed. Try an out of focus shot and you will see the spikes separate even more.

I don't have an ASA scope so I do not know if their is any adjustment that you can make to line the vanes up precisely.

[Konihlav]

Yes George, it's unfortunately really apparent I will have to contact ASA about it.

Here's a quick test of uncalibrated 1x3min RGB bin2x2:

http://www.astro.cz/galerie/v/uzivatele/Konihlav/misc/usa-001-RGBtest1x6min_nocalib.jpg.html?g2_imageViewsIndex=1

[GeorgeCarey]

Can you post a picture of the front end of the scope? It should be possible to check if there is misalignment.

You will need to get the camera on the central axis of the tube.

When I was adjusting my homemade Newtonian I used this image to check the vanes:

 

Edited May 16, 2013 by GeorgeCarey

[gmalits]

Hello Pavel,

 

Collimation ist very critical on this fast newtonians, the newer 10N have massive double spidervanes at the secondary holder. i think you own one of these, and you mentioned a residual error on the catseye. so give collimation a chance....

[Konihlav]

thank you guys for now, I asked ASA on their opinion...

 

gmalits - I have checked the collimation now (if it lasts after driving 220km) and it is "almost" OK checked with tuBlug Howie Laser. Using infinity is not easy as I can't do both tasks - look into the infinity and use the hex allen keys to turn the collimation screws... and the infinity doesn't fit the 2" clamp very well - there's (big enough) wobble making a use of precise infinity useless.

 

what I may be able to check next time is the backfocal distance (though we precisely calculated I need camera adapter of some 21.79mm which ASA produced for me because I got OAG from ASA too). I may be able to increase BFD next time by a 1mm and see. I may also try to focus in mid/half position from center of the image to the image edge (this pre-zero light (test) ) was done with precise focus, but in the middle of the field... I may also try to capture diffraction rings of a defocused star (though the secondary offset should make the circles not being round at this fast F/3.6 speed).

 

at the moment I am still "convinced" the problem is in spider vanes being not in-line.

 

I keep you informed on my next findings.

Pavel

[BobGillette]

Pavel,

 

What am I missing here? The diff spikes on the bright star in your initial post look perfectly normal to me. Small spikes on less-bright stars indicate not bad collimation.

 

I use Jim Fly's tools on my N12, and usually achieve error between 0 and 2 arc-seconds. (It's very helpful, BTW, to have the primary mirror rotated so the adjustment screws align with the points of the target triangle.)

 

Best,

 

Bob

[Konihlav]

Bob,

 

the problem is not that dimm stars have small diffraction spikes and bright have large spikes, that's obviously pretty normal. The problem is that the diffraction spike diverges from single line (near the bright star center) into two lines with increased distance proportional to distance from the focused bright star center. That shows the double spider vans to be not parallel (in-line) with each other, I am pretty sure about it.

 

Anyway, here are new data I collected this weekend (6th clear sky night this year!!!), I learned many new things:

 

First of all I spent almost 2 hours collimating the telescope using both CatsEye toolset and Howie toolset.
Then in the night I tested:
- focus in center
- focus in corner
- rotate camera
- ... bla bla
- increased backfocus using a rubber band that I inserted between camera adapter and my CCD camera (resulting focal length seems to be closer to what it should be)
- with this new config I well focused and voila, great star field images (except of the damn stupid double which should not be present!)

here is a sample of ordinary green output:
http://www.astro.cz/galerie/v/uzivatele/Konihlav/misc/NewBFD_Focused_Green.jpg.html

BTW Plná velikost MEANS full size and další MEANS next

here is a sample of superb luminance output:
http://www.astro.cz/galerie/v/uzivatele/Konihlav/misc/NewBFD_Focused_Luminance.jpg.html?g2_imageViewsIndex=1

here is flat field in CCDInspector:
http://www.astro.cz/galerie/v/uzivatele/Konihlav/misc/NewBFD-FlatAnalysis.jpg.html?g2_imageViewsIndex=1

vignetting in MAxDL:
http://www.astro.cz/galerie/v/uzivatele/Konihlav/misc/NewBFD-FlatMaxIm.jpg.html?g2_imageViewsIndex=1

AND FINALLY the star field image:
http://www.astro.cz/galerie/v/uzivatele/Konihlav/misc/CCD+Image+50-highscreenstretch.jpg.html?g2_imageViewsIndex=1

 

so now, EVERYTHING is perfect EXCEPT of the star spikes.

 

I am in contact with ASA guys already, will let you know when I (we, they) resolve this issue.

 

Pavel

[BobGillette]

Pavel,

 

Your collimation, with CCDI error measurements between 2.9" and 4.6", is good, but not quite as good as Jim Fly's tools can make it. You should aim for 2" or less.

 

Maybe it's my eyes, but I still see nothing amiss with your diff spikes. I zoomed in and put a straight-edge on the screen. Looks to me like they line up.

 

Myself, I try to avoid huge bright stars in imaging. They're distracting.

 

Bob

[Konihlav]

end of the story:

http://blog.astrofotky.cz/pavelpech/?p=958

 

[GeorgeCarey]

Glad to hear that you have the problem fixed.

I am building a new scope and have bought the Telescope Express spider. The vanes look perfect but I will only know when I make the scope!

[Konihlav]

George, looking forward to hear from you back your personal experience (most valuable thing). One friend of mine uses the one from TS and doesn't have it perfectly in-line and other friend just broke the carbon spider when he tightened the screws so he replaced with custom made single piece spider that partially causes square shaped stars if the ratio between thickness of the vanes versus scope front aperture is not in favor...