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Diffraction spikes on an ED80

edjrgibbs

By edjrgibbs
in Getting Started With Imaging

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edjrgibbs

edjrgibbs

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I'm confused... I recently got a Skywatcher ED80 and had a chance to use it last night. After several hours of sorting issues with my mount and OAG i got some short exposures on M45. Equipment was ED80, Reducer/flattener, Atik 414, ZWO electronic filter wheel, Baader LRGB filters and ZWO OAG. Why are there diffraction spikes of different colours?

 

Atik 414 - L15m, R3m, G3m, B3m - ED80 v4.tif

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IanL

IanL

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Might get more response if you uploaded a copy that wasn't half a gigabyte in size, i.e. convert it to a jpg or png file. My guess, without looking, is that the OAG prism and possibly stalk are intruding in to the light path that is falling on the sensor and you need to move it further out.

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Zakalwe

Zakalwe

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Wow...how the heck did you manage to make this file half a gig in size???? :BangHead:

 

I had a quick look....firstly very bright stars like those in M45 are a very tough test of any optics. You're going to get some sot of aberrations. it looks like the imaging train was altered during runs too...that might explain the green streaks. It's like the diffraction patterns are rotated between each filter. You can see this in the reflection aberration that's in the centre of the image...theres one for the green filter, one for the red and one for the lum filter.

The patterns could be many things- perhaps the lens spacers are intruding into the light cone. A circular mask that reduces the objective apparent diameter by a couple of millimetres might sort that.
The other thing might be slightly pinched optics caused by a too-tight lens cell, though I'd be checking simpler things first.

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vlaiv

vlaiv

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This is really confusing, since only red channel displays signs of OAG stalk / prism being the problem - if you get single spike in stars on one side of the image (either side or corner)  - this usually means OAG protruding into light path.

It can be seen that red filter has this in top right / right part of the image - other stars have no red spike problem.

Problem with other spikes being different color - means that something that is causing spike - and that would be usually straight edge in optical train - is rotating between frames, and also is not the same for each channel - Lum has spikes all over the place, while red and blue have what seem to be single spike, green looks like it has two spikes rotated by small amount.

It really depends on times when each channel was shot - was there meridian flip in session and also if you had a multiple night session, but did not align / rotate frame properly between sessions - this can explain two prong spike of green channel.

It can be either related to filter wheel, or something else if you were shooting on multiple nights with different FOV rotation between sessions.

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edjrgibbs

edjrgibbs

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2 minutes ago, Zakalwe said:

Wow...how the heck did you manage to make this file half a gig in size???? :BangHead:

 

I had a quick look....firstly very bright stars like those in M45 are a very tough test of any optics. You're going to get some sot of aberrations. it looks like the imaging train was altered during runs too...that might explain the green streaks. It's like the diffraction patterns are rotated between each filter. You can see this in the reflection aberration that's in the centre of the image...theres one for the green filter, one for the red and one for the lum filter.

The patterns could be many things- perhaps the lens spacers are intruding into the light cone. A circular mask that reduces the objective apparent diameter by a couple of millimetres might sort that.
The other thing might be slightly pinched optics caused by a too-tight lens cell, though I'd be checking simpler things first.

Ooops. To be honest, I have no idea... Trying again. A more manageable JPEG.

 

Atik 414 - L15m, R3m, G3m, B3m - ED80 v4.jpg

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edjrgibbs

edjrgibbs

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2 minutes ago, vlaiv said:

This is really confusing, since only red channel displays signs of OAG stalk / prism being the problem - if you get single spike in stars on one side of the image (either side or corner)  - this usually means OAG protruding into light path.

It can be seen that red filter has this in top right / right part of the image - other stars have no red spike problem.

Problem with other spikes being different color - means that something that is causing spike - and that would be usually straight edge in optical train - is rotating between frames, and also is not the same for each channel - Lum has spikes all over the place, while red and blue have what seem to be single spike, green looks like it has two spikes rotated by small amount.

It really depends on times when each channel was shot - was there meridian flip in session and also if you had a multiple night session, but did not align / rotate frame properly between sessions - this can explain two prong spike of green channel.

It can be either related to filter wheel, or something else if you were shooting on multiple nights with different FOV rotation between sessions.

Thanks for the response. All done last night. I took around 10 L then realised i'd not connected the filter wheel so connected and took the R, G and B. All in all around 30 mins. No meridian flip as far as I can remember.

 

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vlaiv

vlaiv

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1 minute ago, edjrgibbs said:

Thinking about it, there looks to be a small chip in the optic on the OAG. Could that be causing some light scatter/reflection of some sort?

Yes it could indeed, try shooting without it to see if it will make a difference. Next on suspects list would be filter wheel - try removing that next and see what happens

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edjrgibbs

edjrgibbs

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1 minute ago, carastro said:

It seems to be more predominant in the Green channel, is it possible a hair or something similar has got on the Green filter?

Carole 

Not sure. I'll check this evening to make sure they're all clean and clear. 

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edjrgibbs

edjrgibbs

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3 minutes ago, Zakalwe said:

This is probably a daft question, but are you sure that you have the right side of the filters towards the objective?  it's possible to get them the wrong way round if you haven't mounted the filterwheel correctly.

I did start to question my own sanity on this last night. I think I did. In the pic (https://www.firstlightoptics.com/rgb-filters-filter-sets/baader-lrgbc-ccd-filter-set.html) sitting in the filter wheel with the glass closest to the CCD i.e. the indented filter cell pointing towards the ccd. Does that even make sense?

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vlaiv

vlaiv

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Cell side with filter thread should be pointing towards sensor, but I guess filter wheel is already designed that way.

Same way as filters are screwed in eyepiece - filter thread side goes towards eyepiece.

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edjrgibbs

edjrgibbs

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1 minute ago, vlaiv said:

Cell side with filter thread should be pointing towards sensor, but I guess filter wheel is already designed that way.

Same way as filters are screwed in eyepiece - filter thread side goes towards eyepiece.

Yep, all good. They were the right way around.

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