Why am I getting diffraction spikes?

[JonathanA]

Hi,

Just had first light with my new Explore scientific comethunter - a Maksutov Newtonian. First image  and I am getting strong diffraction spikes on bright stars, also some flaring around them. How is this even possible with no spider vanes? Really puzzled and wondering whether something is badly wrong.

[vlaiv]

Any flat surface in optical train will produce spikes - spikes are created when light passes by some straight edge - you can think of it like this: due to uncertainty principle photons can't have both well defined momentum and position. When they pass close enough to an edge they become very well defined in position, so momentum must be undetermined - since photons travel at speed of light all of the time, speed is set, but direction is not (momentum depends on velocity) so photons spread perpendicular to edge (this is why there is airy disk - photons spread evenly perpendicular to aperture). In your case judging by the image - you have aperture in optical train that is not perfect circle - this happened to me when I was doing DIY aperture mask - I did not cut out perfect circle but rather connected straight lines that followed circle (like when you cut with scissors - you create almost circle but it is produced out of straight cuts). So it can be something like - if mirror has turned down edge and has been masked of, and mask is not perfectly circular. Or it can mean that secondary is not cut to perfect circle, or any kind of reducer, corrector might not have perfect circle.

Mind you, this is one possible explanation. There might be other causes for this as well.

Here is an example image that I took with achromatic refractor and DIY aperture mask. I was trying to stop down aperture in order to control chromatic aberration, but because it was not perfect circle I ended up with bunch of spikes around bright stars.

[mikeDnight]

I'm no imager but that's reminiscent of photographs  taken through one of the large reflectors in the U.S. from the last century. I can't remember which one, but the pics are in Burnhams Handbooks. It suffered gross spherical aberration but the effect was pretty! Other than SA, is there a contaminant on the lens of the scope? I've seen similar effects on eyepieces that have oil from eyelashes on their eye lens. Also, there is a particularly strong spike that looks like a telephone wire is crossing the field. A similar effect occurs when using an errecting diagonal.

Mike

[Dave In Vermont]

I won't touch the spikes you're getting - the two other have the best reasons I could offer. But as for the 'flaring' I'll only say this - could you be confusing this with the inherent effects of the nebulosity of your subjects here? Nebulosity is prone to propagating any & all 'spread' of the light in an image.

A very nice shot anywho! Thanks for sharing it!

Dave

[Peter Drew]

The first thing I would do is to have a look visually. If it produced the same effect on bright stars I would suspect spherical aberration either inherent or introduced by additional optical components in the light path. Incorrect spacing of focal reducers can also produce similar effects. It's not a collimation issue IMO.  

[Sargs]

When imaging, how far in is your focuser? Wondering if maybe it's possible the focuser is racked in so much that the inner edge of the drawtube is clipping the light path. Someone with more experience that me might be able to tell us what that would actually look like! 

 

 

[JonathanA]

Thanks everyone, very useful thoughts. I'm going to have a check now.  Whe the scope arrived the secondary was quite tilted out of position so to get it central I adjusted. This took me to the limit of possible adjustment so I'm thinking perhaps the edge of the secondary is at a strange angle and causing the effect.  I'll also check for screws etc.

[JonathanA]

4 hours ago, Sargs said:

When imaging, how far in is your focuser? Wondering if maybe it's possible the focuser is racked in so much that the inner edge of the drawtube is clipping the light path. Someone with more experience that me might be able to tell us what that would actually look like! 

 

 

Well I've just been out there on a fantastic night for imaging and still getting nasty diffraction spikes. I tried masking off a part of the secondary mount that was protruding into the light path with no effect. I have noticed that the focuser tube is protruding by about 2 cm into the light path when in focus position for my DSLR so I am now wondering whether sargs is correct and that this might be the problem. It would account for the single large linear diffraction pattern. I have no idea what I can do about this short of sawing off a couple of centimetres from the tube. Any ideas would be very welcome - beginning to wonder whether I have chosen a lemon.

[ollypenrice]

The long, almost horizontal, spikes do strike me as being likely results from an incursion into the light path but who knows?

Olly

PS, Clothes lines, overhead cables? All that stuff!!!

[JonathanA]

6 minutes ago, ollypenrice said:

The long, almost horizontal, spikes do strike me as being likely results from an incursion into the light path but who knows?

Olly

PS, Clothes lines, overhead cables? All that stuff!!!

If only it was as you say but there are no exterior obstructions.  Looking at the angle of the spike my understanding is that the this will be at 90 degrees to any edge that is obstructung the light path. This would mean the obstruction is vertical - fits the theory that the focuser tube is protruding into the light path.

[ollypenrice]

21 minutes ago, JonathanA said:

If only it was as you say but there are no exterior obstructions.  Looking at the angle of the spike my understanding is that the this will be at 90 degrees to any edge that is obstructung the light path. This would mean the obstruction is vertical - fits the theory that the focuser tube is protruding into the light path.

Prime suspect in my view as well.

Olly

[Sargs]

On 25/11/2016 at 22:04, JonathanA said:

Well I've just been out there on a fantastic night for imaging and still getting nasty diffraction spikes. I tried masking off a part of the secondary mount that was protruding into the light path with no effect. I have noticed that the focuser tube is protruding by about 2 cm into the light path when in focus position for my DSLR so I am now wondering whether sargs is correct and that this might be the problem. It would account for the single large linear diffraction pattern. I have no idea what I can do about this short of sawing off a couple of centimetres from the tube. Any ideas would be very welcome - beginning to wonder whether I have chosen a lemon.

 

Do you have a Barlow? If so, try setting your focuser to a position where it is in focus for the DSLR, then take off the DSLR, fit the Barlow and add an eyepiece. With some luck, you may be close enough to focus to get a sharp image in the eyepiece (possibly with a bit of judicious sliding the eyepiece out of the Barlow a bit without touching the focuser). If that works and you can see diffraction spikes that you wouldn't normally see without the barlow, then it's the focuser drawtube. You can do the same trick with simple spacing tubes, but you'd need quite long ones. Sky-Watcher Newtonians that are for visual and imaging come with fairly short drawtubes and really big extensions that you have to add when using eyepieces- or at least my 200P did.

One way to test it with the camera is to add a cardboard aperture mask over the corrector plate. Cut a circle in it that stops down your scope's aperture by an inch or so- just enough to leave the drawtube edge in shadow. Only problem is you need to be fairly precise otherwise your aperture mask will cause its own diffraction problems.

If it is the drawtube, one non-hacksaw option would be to get a dedicated low-profile focuser, but chances are you'd need to do some drilling if the previous focuser's screw-holes didn't match.

I grew up with Newtonians, so personally I think that's a really nice picture. Focus is great and the only time I've managed to come close to that level of nebulousity out of the Pleiades was when my scope fogged over last Friday!

 

[JonathanA]

On 28/11/2016 at 14:59, Sargs said:

 

Do you have a Barlow? If so, try setting your focuser to a position where it is in focus for the DSLR, then take off the DSLR, fit the Barlow and add an eyepiece. With some luck, you may be close enough to focus to get a sharp image in the eyepiece (possibly with a bit of judicious sliding the eyepiece out of the Barlow a bit without touching the focuser). If that works and you can see diffraction spikes that you wouldn't normally see without the barlow, then it's the focuser drawtube. You can do the same trick with simple spacing tubes, but you'd need quite long ones. Sky-Watcher Newtonians that are for visual and imaging come with fairly short drawtubes and really big extensions that you have to add when using eyepieces- or at least my 200P did.

One way to test it with the camera is to add a cardboard aperture mask over the corrector plate. Cut a circle in it that stops down your scope's aperture by an inch or so- just enough to leave the drawtube edge in shadow. Only problem is you need to be fairly precise otherwise your aperture mask will cause its own diffraction problems.

If it is the drawtube, one non-hacksaw option would be to get a dedicated low-profile focuser, but chances are you'd need to do some drilling if the previous focuser's screw-holes didn't match.

I grew up with Newtonians, so personally I think that's a really nice picture. Focus is great and the only time I've managed to come close to that level of nebulousity out of the Pleiades was when my scope fogged over last Friday!

 

Yes, I went through the business of making an aperture mask and it did seem to be the focuser that was causing the errant spike. Not normally a problem with the comethunter though according to manufacturers and others so a bit of a puzzle - perhaps the focuser wasn't square to the tube giving the draw tube an extended profile in the OTA but I'm just guessing with that. Anyway thanks to the nice people at FLO I'm exchanging it for another one. I'm also tring to source a low profile Nikon T ring to give me an extra few mm of withdrawal if you know what I mean.

I'm ok with spidervane spikes - the 200P gives glorious ones on the Pleiades but the single spike just looks weird!