Annoying refraction spikes.

[JohnH]

I am hoping that an expert can help me to reduce the number of refraction spikes I am getting in my images, with four vanes holding the secondary mirror I would expect a nice clean cross shape:



My telescope is Sharpstar 15028HNT, a Newtonian with a hyperbolic primary mirror. Looking into the tube there are seems to be a number of obvious protuberances into the light path:



Here is an example in closeup:



The tube is actually very compact. This means that the attachments to the tube are concentrated in a small area:



My thoughts of possible solutions would be:

1) Dabs of matt black paint.

2) Pads of fuzzy black material over the screws to smooth out the shapes.

Any help would be much appreciated.

Regards,

John

[Carole]

Hi John,

I am no expert on this, but I wouldn't have thought that these small screws would not have affected the diffraction spikes. 

Is this without the dew heater attached, as it would be good to see how that is fitted.

Hopefully some-one more technical than me will be able to give you better advice.

Carole

[JohnH]

Hi Carole,

Thanks. For the moment at least I am abandoning the dew heater. The stack was without the dew heater.

I have another option, I made a dew shield from a roll of black craft foam which I joined with black tape. There is a small step where it is joined. I do not think that it is very likely but this might be a problem. It looks as though tonight will be clear (full moon though) and I will leave the dew shield off to see gif that makes a difference.

Alternatively, I just shouldn't be allowed anywhere near a reflecting telescope!

Regards,

John

[RobertM]

Personally I would remove the imager/filter wheel and corrector and use a torch to search for anything shiny impinging in the optical path.  I would then use ultra flat Matt black camouflage paint to blacken those screws for a start and anything else obvious.  Black 2 or Black 3 would be better but might be a bit of a wait.

This is what I've done with success.

Robert

[Carole]

Sorry I left out the not in " I wouldn't have thought that these small screws would not have affected the diffraction spikes". 

[Carole]

Yes sometimes it can just be a hair in the optical train. 

Carole

[Mac]

You could try this.

Get a single artificial star and take a bright exposure so that its clean and sharp.

Then get a length of something that you can introduce in to the optical train on the top. (This is your extra rib)
( thin string, cotton is too thin, a pencil is too thick, knitting needal perfect if you can get a matt black one)

Then take a single sharp image so that you can see your spikes.
Introduce you EXTRA ribnext to one of your supports. This is for referance so that you can see on the image which spike is caused by which rib.

Then move the EXTRA rib next to the next rib and take another image, do this for all 4 ribs, this will allow you to identify the exact rib that is causing the exact spike.

Now you know the orientation of the Ribs Vs the spikes, look down the scope and align the Rib so that it is over one of your screws
Take the photo and see if the NEW spike is aligned with that screw, repeat for all the screws,

That way you can see if the true spike aligns with the extra spikes you have and if they are being caused by the screws protruding.

It might be that the screws need to be painted matt black and they are just reflections from the screw heads.
Black shiney heads will still reflect and alter the optical paths.

So looking at your scope you should see something like this.

L-R
First image just the ribs.
2nd extra rib placed next to top (N) rib
3rd Extra rib next to (E) Rib
4th Next to (S) Rib ect.
The bottom image shows the extra rib aligning over one of the screws

If that all makes sense.
If not i'll try and butcher an image so its clearer.

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[The Thing]

Hi John,

Small screws e.g. the finder bracket, focuser mount, ring at front of scope do cause diffraction elements. There is a tool for modelling and correcting the effects in StarTools. Also something I have come across since getting a 3d printer is people printing a mask to go round the primary mirror to cover the retaining clips - presumably to stop effects from them. You should make sure all the shiny screws are blackened, is you use a 'feathery'  paint such as Black 3.0 it will also soften the diffraction effect. Fuzzy flocking would also work but would protrude into the optical path more than paint and might cause secondary (heh heh ) effects. In my SCT OTA the seam in the flocking where the join overlaps I put in it causes a spike...

Good luck solving it!

Duncan

[JohnH]

I have a feeling this is going to be a long slog!

In the meantime I have come across a review in Sky and Telescope which, whilst generally favourable, does mention a profusion of diffraction spikes. The review also suggests that I was tweaking the back focus from the reducer in the wrong direction.

Fingers crossed,

John

[Carole]

I guess you could try putting a blocking Ring in front of the aperture, that makes the aperture smaller and hides the bolts. 
Not sure I am explaining this well.  But simply as an experiement to temporarily block out the bolts, and see whether it improves the star spikes, then at least that might prove or disprove them as being the problem.

Caroie

[The Thing]

I am hoping that an expert can help me to reduce the number of refraction spikes I am getting in my images, with four vanes holding the secondary mirror I would expect a nice clean cross shape:

Found this http://planetediy.fr/index.php/2019/03/10/masque-de-primaire-pour-aigrettes-disgracieuses/

The image of the problem looks rather like your issue. Simple solution. I'm going to print one for my F4 TS1506 Newt.

[Carole]

Just read that article, needed a bit of a translation assistance as French not quite good enough.  I think you might be on to something Duncan. 

I think John has had the scope apart recently so hair and spider threads hopefully have been cleaned out if any.
Why don't they make telescopes without such problems. 

Carole

[Carole]

Just asked a friend about it who apparently has the same scope and he thinks it might simply be incorrect colimation.

Carole

[JohnH]

Just read that article, needed a bit of a translation assistance as French not quite good enough.  I think you might be on to something Duncan. 

I think John has had the scope apart recently so hair and spider threads hopefully have been cleaned out if any.
Why don't they make telescopes without such problems. 

Carole

No hairs, no spiders and (having used computer translation to read that French article) "no egrets".

I am very willing to believe that it is incompetent collimation, I am still plugging away at that. My biggest problem, I think, is centring the secondary in the focuser - I have set it at the recommended 5 mm but it looks non-circular no matter how much I try to rotate it. I have a Concenter Eyepiece to assist as I suffer from astigmatism without my glasses.

I have had another go and what I have noticed this time is that a flat field is much more even with greatly reduced vignetting. The flat field is not perfect yet - still slightly offset but, potentially, a real improvement. If we ever get another clear sky I will set up and get some defocussed stars.

Regards,

John

[Mac]

Are you an electronics tinkerer?
If so, just run a clear led underpowered until it just glows, should be small enough to get you going as an artificial star.

The other way which i've used is a home made fibre optic made from glass blowing glass, just hang a weight heat it up and let it draw out.

The other one you could do which might be cheaper is to actually get a fibre patch chord.

Mac.