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Extra diffraction spike on a newtonian?


ONIKKINEN
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I have noticed that sometimes i get an extra diffraction spike for no apparent reason with my VX8. It has happened with 2 different focusers and in quite different shooting conditions now so not sure what to think about it. In neither of the cases it has happened has anything obstructed the light path to the primary mirror (like a focuser draw tube). There is nothing inside the tube that could cause an extra diffraction spike. Could it be pinched optics in either the primary/secondary mirror or one or more of the lens elements in the coma corrector? From what i can tell the primary is seated properly in its cell and not tightly held down. I can shim in credit card between the mirror clips and the primary mirror so its not tightly held down. Although i have not done this in very cold weather. But heres the thing, i had the extra spike at -5 last spring but i did not have it at similar temperatures this autumn (the pic with no extra spike). But then again in the most recent shot i had the extra spike again but now at -17.

Below are some examples of this. In the picture without the extra spike there are some halo/diffraction artifacts which i think are to be expected from a central obstructed newtonian with possible mirror clip effects in the mix. In the VX8 the focuser is between the secondary spider vanes so it could be from that. But the focuser just does not obstruct the light path so shouldnt, and also the same focuser in similar conditions produced an extra spike once and no spike another time.

1709238411_Leo-drizzle-sirilcopy.jpg.93f7dabc8f9ef4b2b058d81bd9752371.jpg2021-12-10T23_13_48.thumb.png.8a4406e8a0bb3a534c806fee989a8242.png2021-12-10T23_11_59.thumb.png.cfe1c52d621b34ef05de9642c412fe9a.png

 

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In the first picture collimation looks off as the spike at 2oclock looks distorted  not sharp , is the focuser 90degrees to the ota , I recently changed my focuser to a baader steeltack and found out the focuser wasn’t sitting true and shimmed to suit , I only realised because I couldn’t get the secondary centred properly using my concentre eyepiece , you could sit primary on a few blobs of low Modulous clear silicone so mirror not shifting , easy to remove .

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Not saying its related, but 8 years ago I had a similar issue of an addition spike with my 200P.  Tried everything in terms of adjustments, and must have collimated the scope 30+ times.  In the end it was proven to be unsilvered flats on the minor axis of he secondary mirror - it was documented in this long thread here

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7 hours ago, bottletopburly said:

In the first picture collimation looks off as the spike at 2oclock looks distorted  not sharp , is the focuser 90degrees to the ota , I recently changed my focuser to a baader steeltack and found out the focuser wasn’t sitting true and shimmed to suit , I only realised because I couldn’t get the secondary centred properly using my concentre eyepiece , you could sit primary on a few blobs of low Modulous clear silicone so mirror not shifting , easy to remove .

The double diffraction spikes were caused by bent spider veins in the first pic. I installed a new, much sturdier secondary spider and went through the trouble of properly centering that as well as i can to remove said effects. Its much better on the second picture and almost gone in the third one as i am improving it every time i go out and shoot. My collimation routine is quite thorough, i believe. The secondary mirror to focuser centering is done with both the secondary adjustment screws and the focuser adjustment screws built in to the Baader diamond steeltrack, so no need to shim the focuser base. I use a TS concenter to check the secondary centering and a laser to roughly check secondary to primary alignment. Then finally the concenter goes back in to confirm that the cheap laser didn't make things worse. These are also not edge stars, but close-ish to the center of the field where collimation issues (which do still exist as i have a large sensor and a small secondary mirror) are not nearly this apparent.

4 hours ago, alacant said:

+1

And remove the mirror clips.

 

I am hesitant to do this as i believe the mirror and the mirror cell are excellent. I dont get nearly as much mirror clip effects with mine as with mirrors from Skywatcher newtonians tend to. Also, i find it difficult to believe that 3 mirror clips at 120 degrees to each other would produce 2 perfectly symmetrical spikes at 180 degrees, it just doesn't seem right.

1 hour ago, malc-c said:

Not saying its related, but 8 years ago I had a similar issue of an addition spike with my 200P.  Tried everything in terms of adjustments, and must have collimated the scope 30+ times.  In the end it was proven to be unsilvered flats on the minor axis of he secondary mirror - it was documented in this long thread here

This thread was a wild ride... Miracle you went trough all that trouble!

59 minutes ago, Captain Magenta said:

Almost every Newtonian secondary mirror I have ever seen has a short section of straight edge in the silvering where the mirror has been grasped by a holder during the silvering process. That will produce a spike.

M

Sounds like something @malc-c went through, this is the number 1 contender for the issue i think.

28 minutes ago, AKB said:

I get extra diffraction spikes  if the elevation is low enough for the observatory wall to obstruct some of the FOV.  I guess that a garden fence, or building, can do the same?

Tony

All of these were shot at wide open spaces to 42-50-30 degrees of elevation where nothing can be in the light path.

 

After reading the linked other thread i thought i would mention a few other things.

Since the first picture i have acquired a new camera, a new secondary spider, a new focuser, flocked the tube properly and blackened the secondary mirror edge (only the edge visible from the camera side). And it still happens, but only sometimes. The sometimes part is what confuses me, i have several datasets between these 2 occurrences of the extra spikes where no such spike is present. Im thinking of a test to do when this happens again, and when it does i can notice it immediately in the NINA debayered and autostretched preview as it is quite obvious in bright-ish stars. The test would be to use 2 aperture masks, one to stop down the telescope and cover the primary clips completely and one to increase the central obstruction to a point where none of the secondary can be in the light path and there would only be a round obstruction.

Sounds reasonable? Also, ill have a look at my secondary. Although im not quite sure what the flat part thingy refers to.

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Hopefully this picture shows clearly enough that at the focus position the drawtube does not extend far enough in to cause any obstruction to the primary mirror. Its close, but there is a gap (for parallel lightwaves ofc). I should also note that this still does not explain why it happens sometimes and not other times because the focus position is always the same. Well not always, depending on the temperature there is a few millimeters of thermal expansion on the aluminum tube itself shifting the focus position of the focuser a bit. The temperature range my telescope is used at covers somewhere around +10 for the autumn/spring nights to colder than -20 for winter so the focus position is far from set in stone.

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In this picture i noticed that just a tiny bit of the secondary mirror peeks from behind the spider, but this is not the case on the opposite side, or on the up-down axis in this orientation. This could be the cause.

I also thought that it would be nice to know which axis the extra spike is in, so i will do another test when this happens. Basically just put a piece of cardboard/something in one axis and see if its the same axis as the extra spike. Should narrow down something,. Does it help to narrow it down? That i dont know, but cant hurt. So i will do some arts and crafts and make 3 aperture masks, a primary outer edge obstructing mask, secondary obstruction increasing mask and an artificial extra spider vane to figure out whether its the focuser axis or the one 90 degrees from it.

1 hour ago, alacant said:

 Oh, and a replacement secondary.

Would be a convenient excuse to upgrade the 63mm secondary to a 70mm one to decrease vignetting for APS-C sensorsize which is quite significant at the edges. Maybe if it bothers me more and more, and if i stop being broke from other astronomy purchases 😂.

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8 hours ago, Captain Magenta said:

Almost every Newtonian secondary mirror I have ever seen has a short section of straight edge in the silvering where the mirror has been grasped by a holder during the silvering process. That will produce a spike.

M

Well the replacement secondary  (larger than the stock 200P) came from Orion Optics and was fully coated (unlike both offerings form OVL (Skywatcher).  And seeing that the OP has an Orion Optics VX8 would assume that his secondary is fully coated.  If it is then this would suggest to me that having ruled out an unsilvered mirror, then collimation is the issue, including bent or mis-aligned spider veins.  If the scope is fairly new it might be worth the OP contacting the retailer (or if purchased direct - Orion Optics) for advice and what options there are to return the scope for inspection and repair if found faulty.

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26 minutes ago, wimvb said:

How high are your washing lines?

 

Clothes are drying right beside me in my clothes drying room at the moment. Also the same room works as a kitchen, living room, bedroom, telescope showroom, workshop, existential dread and weather based mental anguish room etc (live in a small flat 🤣). None of the images have been taken in a place where power lines or anything of the sort could be above 10 degrees from the horizon towards the targets at the time. I also googled this and came across many such threads over the internet where extra diffraction artifacts have made appearances. But cases like mine where they only happen sometimes are rare it seems.

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1 hour ago, malc-c said:

Well the replacement secondary  (larger than the stock 200P) came from Orion Optics and was fully coated (unlike both offerings form OVL (Skywatcher).  And seeing that the OP has an Orion Optics VX8 would assume that his secondary is fully coated.  If it is then this would suggest to me that having ruled out an unsilvered mirror, then collimation is the issue, including bent or mis-aligned spider veins.  If the scope is fairly new it might be worth the OP contacting the retailer (or if purchased direct - Orion Optics) for advice and what options there are to return the scope for inspection and repair if found faulty.

I too have an OO VX8, unfortunately I don’t have immediate access to it and I didn’t check it when I was last near it. I shall check it as soon as we meet again.

M

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Any sign of astigmatism when the scope is cooling by any chance? I'm assuming the mirror is properly cooled when it happens??

You can check this by rolling the focuser either side of focus, back and forth. Possible to see it with a camera but more visible with a high power simple eyepiece, or with a Ronchi eyepiece. Look for the out of focus star pattern flipping 90° as you move through focus.

Tim

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Also, you need to eliminate the coma corrector while you test if possible.

When you get an image with an extra spike, try moving the star around to the 4 corners of the image as well as the centre and see if the extra spike remains all over the field.

One more thing, it has been many years since I've used a Baader Steel track,  I don't suppose there is a bright or shiny edge or surface internally on it anywhere? I have very vague memories of blacking down the end of a focuser drawtube in the past, but can't remember which focuser it was now. 

Tim

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53 minutes ago, Tim said:

Any sign of astigmatism when the scope is cooling by any chance? I'm assuming the mirror is properly cooled when it happens??

You can check this by rolling the focuser either side of focus, back and forth. Possible to see it with a camera but more visible with a high power simple eyepiece, or with a Ronchi eyepiece. Look for the out of focus star pattern flipping 90° as you move through focus.

Tim

I just went and checked how long the imaging session was during the first occurrence and it was over 3 hours of imaging so at least 4 hours of the scope being out of its bag sitting on a mount cooling, so definitely completely ambient by the time i finished shooting. I also checked the first and last subs and both (and all in between) had the extra spike. If memory serves me correct i had the heater in my car on at just barely above 0 while driving to the shooting location because i knew cooling is an issue (it was around -5 outside). On the second case the scope was still cooling down when i started imaging. I actually had to discard about an hour of subs from comet Leonard because i did not have time to cool the scope properly before. I started noticing the extra spike appear only once the tube currents died down and HFR decreased to usable levels. Doesn't mean the extra spike wasn't there when the scope was cooling, but perhaps just that i could not see it because of fat shimmery stars.

I will admit i probably couldn't spot astigmatism visually as this is the best scope i have looked through, so how would i know if there are issues? Although people quote astigmatism as a nasty aberration that ruins views, which goes against my experiences of just easily devouring the Moon at no problems whatsoever in clarity at 300x on a decent night. I have glanced at out of focus star shapes but i cant really tell anything from them, and i dont think i have done this at high powers.

48 minutes ago, Tim said:

Also, you need to eliminate the coma corrector while you test if possible.

When you get an image with an extra spike, try moving the star around to the 4 corners of the image as well as the centre and see if the extra spike remains all over the field.

One more thing, it has been many years since I've used a Baader Steel track,  I don't suppose there is a bright or shiny edge or surface internally on it anywhere? I have very vague memories of blacking down the end of a focuser drawtube in the past, but can't remember which focuser it was now. 

Tim

The coma corrector is possibly part of the issue for all i know so good suggestion. What would this 4 corners kind of test prove or rule out, astigmatism of one or more of the mirrors? Easy to do so ill try to remember to do that next time. I went through the subs where this happened and couldn't come to a conclusion as only very bright stars have this visible well. These bright stars also just have happened to be near the center of the field so hard to tell right now.

The Baader focuser is nice and matte black inside, not that it matters much since the comacorrector fills almost all of the drawtube with the way i have it mounted slightly sunk in. Also worth noticing that the effect was present in the old focuser aswell.

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  • 2 weeks later...

Doing a bit of imaging at the moment and the extra spike is missing as it usually is. Nothing has changed since the last time in terms of optics, i have in fact not even collimated since then. Well i did shuffle a few spacers around but not important IMO.

Very similar conditions too, around -14, but this time the entire setup was cooled to ambient well before starting as i had my gear at work in my car ready to go if the forecast was true. Could be temperature related? Not enough info yet.

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