Diffraction spikes from refractors?

[nightvision]

I am curious about diffraction spikes/effects that refractors sometimes show (seen several examples in images posted today). Attached image shows the effect. Anyone know the probable cause?

[GaryG]

Hi Tony,

You won't get diffraction spikes in a refractor - the effect in the image you posted looks to me more like pinced optics. By that i mean the cell that holds the lenses in place normally has grub screws to hold/align the lenses, when the cell gets cold it shrinks fractionally and pushes the grub screws into the lenses.

I have seen this on my 90mm refractor but only when it got to a sub zero temperature.

If you're very brave you could try loosening the screws by a very small fraction of a turn but i think a better option would be to use a dew heater which will lift the temp of the cell and lenses so hopefully remove the effect you are seeing & any dew!. That's my plan for my 90mm scope.

Gary

[JamesF]

Or is it a diffraction effect caused by the spacers between the components of the objective lens?

James

[nightvision]

Thanks for the responses, I thought of pinched optics also but wasn't sure. I've not seen the effect on my scopes but it is apparent on several images posted here. I was considering buying a particular scope however another forum member posted the above image captured with that specific model so I'll hold off for now.

[FLO]

Or is it a diffraction effect caused by the spacers between the components of the objective lens?

I am sure you are right, it will almost certainly be caused by three spacer 'tabs' between the elements. If you look into the objective from the front you will probably see three metal tabs around the edge of the objective, they can diffract light the same way secondary mirror supports do on a Newtonian. And like a Newtonian, if you find them obtrusive, you can cut a small card/plastic ring that masks them.

HTH,

Steve

[ollypenrice]

Yup, that'll be the clips. I don't see any sign of pinching, which generally gives triangular stars. You can make a small front aperture mask to confirm this. Just stop down the objective a little.

Olly

Oops, too slow.

Edited April 4, 2013 by ollypenrice

[John]

I've seen the effects of the spacers in images but I can't say I've ever seen them visually. I've used a lot of refractors and most used foil spacers between the lens elements, including my current Vixen ED102 and Skywatcher ED120.

Is this an issue that really only affects imagers ?

[Gina]

I looked into my ED80 the other day and saw the objective lens spacers - 3 little square bits around the edge. I haven't notice any effect in my images though.

[Chris]

On 4/4/2013 at 21:25, John said:

I've seen the effects of the spacers in images but I can't say I've ever seen them visually. I've used a lot of refractors and most used foil spacers between the lens elements, including my current Vixen ED102 and Skywatcher ED120.

Is this an issue that really only affects imagers ?

Sorry for the thread bump, but I'm starting to think you can see them visually! I chanced upon this thread whilst researching why I seem to be seeing some sort of diff spikes on Jupiter with my Vixen 80mm achromat. They are a bit fainter than with my Newtonian during a side by side test, but they seem to be there, among other things. 

[Davey-T]

Get them on bright stars in my WO110FLT images but never noticed them visually.

Dave

 

[John]

I've only ever seen them when examining intra and extra focal star images and then only as very slight "nicks" at the edge of the diffraction ring pattern.

If you view a star that happens to have a flat surface running somewhere roughly between your scope and the target (eg: phone wire etc) then you do get diffraction spikes of sorts which can be rather disconcerting !

 

 

Edited May 11, 2017 by John

[Moonshane]

I have seen them in my 102mm f7 frac and when doing a star test. As John says like three little bites out of the rings. I very slightly (maybe 2-3 degree rotation of the grub only) loosened one of the associated grubs and they disappeared. Not really checked since but looks well enough to me.

[Merlin66]

I'd say definitely caused by the collimating grub screws  - I've seen it on an older ED80.

Visually you can check by pushing a star out of focus to show a bright circle, you should then the the "notches" in the edge. You can, by placing your finger in front of the edge of the objective  determine  that the notches match up with the radial position of the grub screws (rather than the spacers).

Get the correct Allen key/ instrument screwdriver (depending on the telescope maker) and gently back off each grub screw on the front element by no more than 1/8 of a turn (or less). DO NOT tighten these screws - this could lead to permanent damage to the lens.

Give the telescope time to recover.... and re-check with a star image.

If pain persists then get someone (supplier/ experienced user) to carefully carry out further re-adjustments - of the grub screws on the front and rear elements. Remember any dramatic movement of the grub screws can/ will affect the overall collimation of the objective - that's why they are there!!!

 

[Peter Drew]

Residual spherical abberation often found in short focus lower end achromats can also exhibit a spikey edge to star images.  

[ollypenrice]

Such artefacts don't normally bother me, but here's one that does.

Ignore the image quality, it's just a crop from a test stretch, but the artefact in question is the double spike only in red. In this example the one at about ten o'clock is faint. The one at five o'clock is more obvious. Sometimes they appear as really very bright and thin red lines either side of the star. They arise only in the TEC140 using a camera and filterwheel which does not produce them in the Takahashis. Indeed this example is from a new Atik 460 with different filters, so the effect must be produced by the scope. The lines are always and only in red. They don't afflict all stars by any means, or I'd be a lot more disgruntled. They few that they do affect I fix in Ps.

Any ideas?

Olly

[gorann]

The spikes that I get with my ES 127ED may not fit the theories presented here. There are 5 major dark ones unequally spaced and numerous light ones. When I look into the front I do not see any spacers at all, and there are only three (pairs) equally spaced adjustment screws.

Actually, I have not been bothered by these spikes since they are not protruding long into the images like reflectors spikes. I kind of like them.

But what is causing them?

[Peter Drew]

Perhaps 3 collimaton screws would produce 6 diffraction spikes in the same way as a 3 vane spider does? 

[Merlin66]

I agree....

I'd put a bet on the collimating grub screws.

 

[ollypenrice]

19 minutes ago, gorann said:

The spikes that I get with my ES 127ED may not fit the theories presented here. There are 5 major dark ones unequally spaced and numerous light ones. When I look into the front I do not see any spacers at all, and there are only three (pairs) equally spaced adjustment screws.

Actually, I have not been bothered by these spikes since they are not protruding long into the images like reflectors spikes. I kind of like them.

But what is causing them?

I don't know, Gorann, but our pair of Tak 106 FSQs produce just two of these dark shadows per bright star, 180 degrees apart. I call them the 'inverse lighthouse beams.' They don't bother me. On anouther forum Prof Greg Parker assured me they arose from slight optical pinching. I had always thought of pinching as producing triangular stars but Greg was confident.

Olly

[Merlin]

There can also be a subjective element for diffraction spikes, and that's the state of the observer's eyesight.

Some observers stretch wires across the front of their refractors to mimic the spikes produced by 'scopes with central obstructions. They regard the spikes as being aesthetically pleasing, but not to Yours Truly.

[gorann]

4 minutes ago, Merlin66 said:

I agree....

I'd put a bet on the collimating grub screws.

 

So everyone, then the question is are the spikes distracting and should I (and others) fiddle with the screw and risk messing things up? I have not thought much about them and this particular image was taken in December at -7°C. Maybe cold makes it worse.

Edited May 13, 2017 by gorann

[Ricochet]

That looks to me like something I had with my Newtonian where the dark spikes were not indicative of something causing diffraction in those areas but something blocking it. Try stopping down the aperture and see if the ring of diffraction (with the dark spikes in it) around bright stars is removed. 

[astrobagel]

I know this is an old thread, but on searching I didn't find anything to resolve this so I'm posting some info here provided to me by Scientific Explorer support when I raise a ticket for this for my ED80, in case it's useful for others. Basically, there are some shipping screws on ED80s which need to be loosened. Here is what they said ...

"To prevent the optics in this telescope from moving during
shipping, set screws have been installed at the factory. If
pinched optics are noticed during astrophotography
sessions, it is recommended that you loosen the shipping
set screws using the included hex wrench.
The pinch, which is visible on astrophotographs, looks like
wedge shapes projecting from bright stars.
To remove the pinch, you need to access nine set screws
that cause the pinch. The screws are hidden by the dew
shield.
To remove the dew shield:
1) If the telessopce is an Essential Series white tube or
an FCD100 white tube, the dew shield unscrews from
the slip collar. The threading is standard thread -- left
to loosen.
2) If the telescope is a carbon fiber tube, the dew shield
is held in place with screws. Remove the screws.
Once the dew shield is off, you will see six groupings of
three screws located 60° apart. Three sets are covered in
a rubberized sealant. DO NOT remove the rubberized
sealant on these screws. Only loosen the screws that are
not covered.

To prevent the optics from misaligning during situations of
excessive vibration, reinstall all set screws.
The images here are with an ED80, but the procedure is
the same for all the telescopes."

I know this is specific to ED80s, but would have helped me a bunch.

Pinched OpticsShipping set screws adjustment.docx