Jump to content

sgl_imaging_challenge_banner_31.thumb.jpg.b7a41d6a0fa4e315f57ea3e240acf140.jpg

Help needed with strange diffraction spikes.


Recommended Posts

Hello,

despite the full moon and the humid weather I could not resist to take some exposures yesterday, merely to test a couple of things, not with the intention to get something decent. I had all sorts of problems, like my ZWO ASI 120MM (mini) not being able to display any stars in PHD2, so I decided to go without guiding, and honestly, as my polar alignment was very good, I was happy with the results of my 1 minute shots.

But having taken a closer look at the subs, I noticed strange diffraction spikes of the bright stars (see attached example).; I'm using a Orion 8 Astrograph and a Canon 500D.

One of the things I modified, I attached a cooling system to the DSLR, bringing it's weight from 480g to 1080g, could that be the problem; I don't remember where, but I seem to have read somewhere that the Orion8 focuser could take up to 1300g. I know I'm close, but still.

Or is it my collimation that is not ok; I admit, I did not check it, as I collimated last week during the day and the whole setup has not been moved. It stays outside under a car cover.

Anybody any idea what's going on here?

Thanks in advance,

AstroRookie

strange_diffraction.png

Edited by AstroRookie
Link to post
Share on other sites

Hi

Sorry, only got a 'phone ATM but will reply anyway in case you have a clear night ahead...

The stars are trailed bottom left to top right. That may explain the unusual diffraction spikes. Did you try guiding the mount?

The stars are distorted toward the right hand side. Is the camera square to to focuser? Do you have a good coma corrector?

Cheers

 

 

Edited by alacant
Link to post
Share on other sites
33 minutes ago, alacant said:

Hi

Sorry, only got a 'phone ATM but will reply anyway in case you gave a clear night ahead...

The stars are trailed bottom left to top right. That may explain the unusual diffraction spikes. Did you try guiding the mount?

The stars are distorted toward the right hand side. Is the camera square to to focuser? Do you have a good coma corrector?

Cheers

 

 

  1. Did you try guiding the mount: the mount - EQ6R Pro - was tracking, but my guiding scope - ZWO ASI120MM - did not show any star in PHD2, as it was a 99% full moon, I thought this was the cause and as it were just "test-shots", I did not bother. But could this have caused this?
  2. Is the camera square to to focuser: I did pay attention when fixing it in the focuser, but I only noticed these spikes after my imaging session; so it could be possible that it shifted.
  3. Do you have a good coma corrector: to be honest I would not have a clue how the check that; this is what I bought last september: Baader 2" Newtonian Coma Corrector MPCC Mark III - from f/3.5

Thanks,

AstroRookie

Edited by AstroRookie
Link to post
Share on other sites

I don't think it is anything "standard".

Collimation might be a bit off - but it is not the cause of this issue. Similarly, tracking/guiding might be a bit poor and there is slight elongation of the stars - but again, it's not what is causing this.

Here is a close up:

image.png.fd71bf31645bdd899051311bf717801a.png

What is really confusing is the fact that that strong vertical spike is both diffuse (not sharp as horizontal, and first part of vertical spike) and clearly showing spectral separation.

What glass elements do you have in optical train? Do you by any chance have ONAG or maybe strangely large prism OAG or anything similar.

Second question would be - when you are in focus, could it be that either focuser tube or maybe end of coma corrector (glass element) is sticking inside the tube in light path?

image.png.d02657463e0732351a74db9f37da4c8f.png

Could this be far inside tube when you achieve focus with your setup?

Link to post
Share on other sites
4 hours ago, AstroRookie said:

how the check that

Is there a filter anywhere?

Was there a dew shield?

The artefacts are in line with the elongation.

There was a full moon.

Blacken the edge of the secondary.

Rotate the camera. Same?

 

 

 

Edited by alacant
Link to post
Share on other sites
4 minutes ago, AstroRookie said:
  1. Did you try guiding the mount: the mount - EQ6R Pro - was tracking, but my guiding scope - ZWO ASI120MM - did not show any star in PHD2, as it was a 99% full moon, I thought this was the cause and as it were just "test-shots", I did not bother. But could this have caused this?
  2. Is the camera square to to focuser: I did pay attention when fixing it in the focuser, but I only noticed these spikes after my imaging session; so it could be possible that it shifted.
  3. Do you have a good coma corrector: to be honest I would not have a clue how the check that; this is what I bought last september: Baader 2" Newtonian Coma Corrector MPCC Mark III - from f/3.5

Thanks,

AstroRookie

This also answers some of my questions.

You are using guide scope, so no OAG/ONAG - that is good, one less thing to worry about

MPCC III should not stick on the other side of focuser tube like some longer coma correctors - like this one:

tsgpu-koma-korrektor-m48-gewinde.jpg

Only other thing that I can think of would be focuser tube protruding far inside ota and blocking some of the mirror.

Can you check how much is it sticking out inside tube when you are in focus?

Link to post
Share on other sites

Vlaiv,

I'm no expert, but did spend a year trying to resolve an extra diffraction spike on my 200P a few years back.  The one thing that is really confusing is the angle of the spikes.  I've taken the OPs image, cropped and deasurated it, then inverted so it's easier to see.  Normally when something gets in the path you either have additional spikes, or one set gets blurred , but they still remain at 90 degrees to each other.  Here we can see that the horizontal ones are angled in relation to the vertical ones.  This to me could be that the secondary is off axis to the primary, secondary is rotated, which might cause interference with the light cone, or the spider is bent.

If this were me, I would remove all the correctors etc and go back to basics with collimation, having first checked the spider is centering the secondary in the OTA and that the spider vanes are at 90 degrees to each other and that they are flat and not twisted.  Then re-do the collimation, and perform a star test (use a really bright star like Vega).  Check focus and then take an exposure without any correctors, filters or OAG in place to see if the diffraction spikes are as they should be.  If they are then its a case of adding one bit of equipment at a time (filter, then OAG) and repeating the test to see if the issue comes back again.  If it does then you know what the cause is and that might dictated any course of action to resolve it.

image.thumb.png.2697af290471b4f299853d37d3fc9cb5.png

  • Like 1
Link to post
Share on other sites

I think that focuser tube protruding is the easiest thing to test - one just needs to see if it indeed goes deeper than baffles in OTA.

I found this image of Orion 8" Astrograph and it suggests that it might be focuser indeed.

image.png.ebf2d1e545f103b698970ac69725f7ba.png

With visual newtonians focuser is at 45° to spider support. Here it is almost parallel to them - which means that diffraction from it will be almost parallel to spikes - and indeed on image it is:

image.png.46595179764e552e1d3f6467e0a3744b.pngimage.png.c768388eb6929498b7651dca9885fb3f.png

It looks like star has regular spikes and then these larger spikes are in just one direction - they are not as sharp (means slightly curved surface) and at a slight angle to spider support.

I could be totally wrong with this focuser thing - but I believe it is easiest thing to inspect - check to see if spider looks as it should and how much is focuser tube sticking inside OTA when focused at infinity. Maybe also checking to see if it is highly reflective. If it is - it could be worth painting in black.

 

Link to post
Share on other sites

Hello,

finally some nice weather over the weekend. As the scope is permanently outside (under a cover) it has to be nice weather to do the check ups suggested here. I checked the following:

  • spider vanes: they ware orthogonal
  • spider vanes are bleck
  • spider vanes were not tilted
  • secondary mirror back and side is black
  • re-did the collimation as outlined by astro baby
    • mirror was not 100% centered, so adjusted that with the spider vanes
    • the laser beam for the secondary mirror collimation was slightly outside the donut: adjusted that (but I admit I did consider throwing it all in the ditch, and purchase a refractor ...)
    • primary mirror was way off; adjusted that
  • checked the focuser position - it was still fixed at the position that I took the here included shots with; it intrudes, but is more or less even with the baffles(?) inside the tube

Unfortunately no clear skies over night, so could not take any test picture. That wil have to wait.

I was wondering though, when I took the included picture, it was full moon and heavy dew; so could it be that it is moonlight reflected by dew on the spider vanes?

Thank you all for your advice.

AstroRookie

Link to post
Share on other sites
6 minutes ago, AstroRookie said:

I was wondering though, when I took the included picture, it was full moon and heavy dew; so could it be that it is moonlight reflected by dew on the spider vanes?

That one is going to be tough to reproduce :D

I have no idea what sort of effect that would produce.

Link to post
Share on other sites
  • 2 weeks later...

Hello,

did the same exercise, 60s subs of M52 last Tuesday, with these differences:

  • new moon
  • no cooling device attached to the camera

But same result, still the same weird diffraction spikes and as it was a new moon it was even more prominent. Due to the fact that there's a spectral separation visible, I suspect it's an optical element (mirror?, coma corrector?) that is having an issue, But how to find out? On the other hand why are the spikes not orthogonal, the spider vanes are. I have this scope only since  April this year, and it was ok, I'm getting rather desperate with this hobby :(

AstroRookie

Edited by AstroRookie
correct
Link to post
Share on other sites

Hey, chin up, we have all had times like these.

When I was trying to resolve the issue with my 200P a lot of folk thought it was collimation issues that was the cause.  So I stripped the OTA completely, and then refitted everything back as follows.

  • The focuser was removed and refitted.  A sheet of paper was placed inside the tube and the distance from the centre of the focuser to the edge of the OTA was transcribed to the paper so a parallel line to the edge of the OTA was made
  • Using the Hotech laser, the focuser was tightened down so that the dot of the laser hit the line on the paper.  My focuser had small thin wavy washers along with a normal washer that was placed between the OTA and the focuser. Once these were compressed all the way down it was just a matter of backing them off slightly until the laser was on the line, but still meant the focuser was firmly fitted
  • The secondary mirror was removed from the spider, and the spider was refitted, ensuring the central boss is equidistant on all veins.  You can use a digital caliper to do this, or simply use a strip of card and mark off the distance of each side and then if the boss needs moving, move it half the distance between the two lines.
  • With the boss centred, a length of threaded rod was placed through it and held firm by two nuts either side.  
  • With the Hoteck back in the focuser, the three collimation screws are adjusted until the rod intersects the laser beam.  This has effectively performed a basic collimation and squared up the focuser to the centre of the OTA
  • Carefully remove the rod and refit the secondary, ensuring that the mirror is fitted squarely so the major axis is in the same line that the rod was in when fitted.  Looking through the focuser with a cheshire collimator in place, the secondary should look circular.  Once your happy, tighten up the central screw to secure the mirror to the boss.
  • On the mirror cell, unscrew the collimation screws and then refit the primary mirror and the cell back into the tube
  • Refit the Hotech (or use your collimation aid of choice) and perform the final collimation.  You should find that the collimation is close already.
  • On the next clear night perform your start test to fine tune the collimation

Now if you are still getting the strange spikes you can rule out collimation as being the issue.

Hope the above helps.

Link to post
Share on other sites
23 hours ago, malc-c said:

Hey, chin up, we have all had times like these.

When I was trying to resolve the issue with my 200P a lot of folk thought it was collimation issues that was the cause.  So I stripped the OTA completely, and then refitted everything back as follows.

  • The focuser was removed and refitted.  A sheet of paper was placed inside the tube and the distance from the centre of the focuser to the edge of the OTA was transcribed to the paper so a parallel line to the edge of the OTA was made
  • Using the Hotech laser, the focuser was tightened down so that the dot of the laser hit the line on the paper.  My focuser had small thin wavy washers along with a normal washer that was placed between the OTA and the focuser. Once these were compressed all the way down it was just a matter of backing them off slightly until the laser was on the line, but still meant the focuser was firmly fitted
  • The secondary mirror was removed from the spider, and the spider was refitted, ensuring the central boss is equidistant on all veins.  You can use a digital caliper to do this, or simply use a strip of card and mark off the distance of each side and then if the boss needs moving, move it half the distance between the two lines.
  • With the boss centred, a length of threaded rod was placed through it and held firm by two nuts either side.  
  • With the Hoteck back in the focuser, the three collimation screws are adjusted until the rod intersects the laser beam.  This has effectively performed a basic collimation and squared up the focuser to the centre of the OTA
  • Carefully remove the rod and refit the secondary, ensuring that the mirror is fitted squarely so the major axis is in the same line that the rod was in when fitted.  Looking through the focuser with a cheshire collimator in place, the secondary should look circular.  Once your happy, tighten up the central screw to secure the mirror to the boss.
  • On the mirror cell, unscrew the collimation screws and then refit the primary mirror and the cell back into the tube
  • Refit the Hotech (or use your collimation aid of choice) and perform the final collimation.  You should find that the collimation is close already.
  • On the next clear night perform your start test to fine tune the collimation

Now if you are still getting the strange spikes you can rule out collimation as being the issue.

Hope the above helps.

Hello malc-c,

thank you for the elaborated answer, but I don't think I have the courage to go that way, being afraid that I will introduce more problems. I will however follow the advice you gave earlier, that is removing the corrector and see what that gives, if that is ok then we know it's the corrector. I didn't think of that last Tuesday because I suspected that having centered the central boss would have solved the issue. Let's wait for the next clear skies, and thanks again.

AstroRookie

Edited by AstroRookie
correct typos
Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    No registered users viewing this page.

  • Similar Content

    • By Desmond
      Hi,
      I recently spent my Xmas money on an L-Enhance filter as I have seen amazing results with this filter.
      I took it for a test run on the Y Cas Nebula last night (not the best night, but when is?)
      I managed to get around 43 mins of time.
      I normally shoot 1 min exposures with my 1000D modified camera but I thought I'd go for 90 seconds as the filter would reduce brightness? (any thoughts on exposure times here would be great)
      The results were not as great as I was hoping for - see below.
      O over stretched it just to see if I had captured the 'ghost'. He's there, but it turns out really grainy and not the nice subtle wispy nebula that I see from other folks.
      So my questions are:
      - is this a tricky nebula target?
      - What exposure times should I run? 
      - Is 43 mins way too little for this (or any) target?
      - any other advice???
      Setup is:
      Canon 1000D - modified
      L-Enhance filter
      Skywatcher 72ED Telescope
      Skywatcher AzGTI mount (AZ mode at the moment until I can get polar alignment working with SharpCap)
      Any thoughts, hints, tips greatly appreciated!
      Thanks in advance!
       
       

    • By Mark 2020
      Hi guys
      Help please. Last night I purchased an old celestron c8 sct, 2000mm fl, f10 on a fork mount. The mount has no power cable so is pretty useless, but i brought it to defork the ota for planetary imaging and hopefully some small faint dso's. It was a reasonable price. Ive got a heq5 pro mount that i will be mounting it onto. The guy said it needs collimation. I know i need to mount it on a vixen dovetail but the bolt holes dont line up anywhere. Its as if someone has rotated the corrector plate housing around 120 degrees. Would it make a huge difference if i removed the housing screws and put it back so that the bolts line up, keeping the mirror and plate in the same spot on the housing bracket,  or would i have to try put everything back separately in different positions and collimate everything from scratch. The picture at the minute doesn't quite get crisp so whatever has been done needs rectifying. Can this even be done by us regular folk? Any thoughts or ideas, instructions? I feel the plate could do with a clean and the primary mirror itself. I just want it as optimum as possible. And can a hyperstar be fitted to these older versions? Im literally starting from scratch with an sct setup. Also what type of camera would suit a scope with this focal length for faint dso's, i have a gp290c for the planetary side of imaging. Sorry for the bombardment of questions. 

    • By Yearofthegoat
      I was surprised not to see any stickies for 'how to...'  in the Getting Started with Imaging forum, but for anyone who is looking for imaging advice sticky threads - they can be found in the 'Imaging' forum here:
      https://stargazerslounge.com/forum/8-imaging/
       
      There seem to be the most in the Tips Tricks and Techniques section here:
      https://stargazerslounge.com/forum/34-imaging-tips-tricks-and-techniques/
       
      Maybe a sticky link could be added at the top of the Getting Started... forum? For the hard-of-searching types like me Might save folks adding posts when the answers are already there too.
       
       
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.