? are these diffraction artifacts - visual observing

[Altair8389]

Hi SGL,

I recently purchased a 203mm Dobsonian (Bresser Messier) at F1200mm (F6 focal ratio). I had to remove the mirror as one of the clips appear to have slipped from its usual position. After collimation, I think it is back to correct collimation.

Whilst testing focusing and collimation on a bright star e.g. Vega or Deneb, I noticed in the eyepiece view, that most of the artifacts showed a flare from the star at about 60 or 120 degrees from each other (see photo of a sketch I did). I thought it was meant to be at 90 degrees  ? as this is the position of the secondary vanes. I thought there was possibly a faint spike at right angles, although I could not see all 4 of these flares at right angles).

I have not started imaging yet with this, as things are not tracked.

 

Is this normal ? Should I be worried ? Mirror compression by the clips or something.

I was using a 10mm Plossl (basic or low quality) eyepiece when observing. I think it was similar with a 25 mm eyepiece (less noticeable).

 

Many thanks.

Magnus

[vlaiv]

19 minutes ago, Altair8389 said:

Is this normal ? Should I be worried ? Mirror compression by the clips or something.

No, Yes, Don't think it is related to clips.

Are you sure your collimation is ok?

Did you remove secondary spider while tending to primary and if so - is it properly returned to its place?

[Altair8389]

Hi,

Yes I think collimation is fine.

No I did not remove the secondary spider, just made minor adjustments to it - the three adjustment screws only.

 

Thanks.

Magnus

[vlaiv]

Diffraction spikes are formed by straight edge in optical system.

Resulting spike is perpendicular to edge that created it.

Single vertical stalk will create horizontal spike. This spike will go both ways from star in horizontal direction - not just one.

Cross that we see at 90 degrees on bright stars with telescopes that have regular spider comes from supports - but not as one might think - vertical support will create horizontal spike and horizontal support will create vertical spike.

It is interesting that three spider supports at 120 degrees holding secondary will create 6 spikes oriented in hexagonal lattice.

In order to have two spikes that are roughly at 60 degrees to each other - you'd need sort of V shaped straight edge in optical system with sides of V being at 60 degrees. Either that or X shape - again where two angles would be 60 degrees and remaining two would be at 120.

Any sort of issue with primary like pinched optics or tilted primary would show primarily in star image.

What is your defocused star image like?

[Altair8389]

Hi, the spider supports are at 90 degrees to each other, and the flares I drew are 60 degrees between 2 flares and of course 120 degrees across the other pair of flares.

Is it because the secondary mirror is slightly displaced as in this description -see the manual. The secondary is displaced about 1/8 inch towards the primary mirror and away/towards the focuser - see the description at bottom of the page.

 

 

[Astronomist]

Have you actually measured the angle of the spider vanes? There may be a manufacturing flaw, although the quality of the JOC dobsonians is generally very good. as long as there are no issues when collimating I would think that this would be no big deal anyway.

clear skies

Harry

[vlaiv]

13 minutes ago, Astronomist said:

Have you actually measured the angle of the spider vanes?

I'd be really surprised to see them at other angle than 90 degrees.

25 minutes ago, Altair8389 said:

Is it because the secondary mirror is slightly displaced as in this description -see the manual. The secondary is displaced about 1/8 inch towards the primary mirror and away/towards the focuser - see the description at bottom of the page.

Secondary offset is normal and depends on speed of the scope (it is more important in faster scopes like F/5 and F/4) - and has nothing to do with what you are seeing.

[Altair8389]

Hi,

The spider vanes are all 90 degrees to each other.

The defocused stars show concentric circles although I cannot discern the all the concentric rings due to poor seeing.

I will have to take some astrophotos of Vega to see what is going on.

 

Thanks.

Magnus

[Captain Scarlet]

Could you also maybe show us a photo of your main mirror taken from the open tube end?

Cheers, Magnus (another one )

[vlaiv]

4 minutes ago, Altair8389 said:

I will have to take some astrophotos of Vega to see what is going on.

That would be helpful.

If you can (even phone at the eyepiece type of image) - take focused and defocused Vega images.

One will show spikes at the angle and other will show collimation.

[Altair8389]

Hi Vlaiv/Captain Scarlet

Yes, the next chance I will try some Vega images even phone at the eyepiece types.

Here is my photo of the mirror. Hard to take from such a distance and in darkness (big torch ran flat ! and secondary mirror obstructs good view).

Here it is, hope it is enough.

 

[vlaiv]

36 minutes ago, Altair8389 said:

Here it is, hope it is enough.

Everything looks pretty standard.

Only thing that I can think of is to check those mirror clips.

Is one protruding in light path (covering part of main mirror) - much more than other two?

That could be cause of spike at 60 degree angle - but that would be additional small spike on top of already existing ones that are at 90 degrees.

Even reflection of secondary spider looks ok from this angle.

[Altair8389]

Hi vlaiv,

 

Yes thank you. I think the mirror clips are about the same. I will take photos of stars and see if its local lens effects/dirt on lens or something else. The 120 degrees could point to mirror clips as they are 120 degrees. I hope it is not mirror clips. After I take a star picture… I will know more. Also clean up my eyepieces..

 

Magnus

[Captain Scarlet]

Very odd. Intrusive mirror clips would produce 3 spikes not two and should not overwhelm the 90 degree ones from the spider. The only thing I can think of is that a protubering (I just invented a word!) focus tube would produce a noticeable spike, and if there is a straight edge on the secondary mirror-silvering at just the correct angle, that might do it. Every non-super-premium secondary I’ve seen has such a straight edge, from where the mirror is held during coating.

Can you show us a photo of your primary clips as seen from the secondary?

I’m intrigued.

Magnus

[vlaiv]

2 minutes ago, Captain Scarlet said:

The only thing I can think of is that a protubering (I just invented a word!) focus tube would produce a noticeable spike

Don't think that is the case.

As far as I can see from the images - all edges on the focus tube are parallel to secondary spider vanes. That would just increase secondary spikes.

[Ricochet]

How did you collimate the telescope? Can you take a photo of the collimation through a collimation cap or cheshire?

[Altair8389]

Hi,

I will try and take a photo of the collimation taken through the Cheshire, not sure how to do this and get enough detail/contrast.

Magnus

[Altair8389]

Here is an attempt at the photo of the view down the Cheshire collimating tube.

[Altair8389]

Hi,

I just realised I cannot focus on the Cheshire collimator's cross hairs - they are out of focus no matter how I arrange the focuser and if I move the Cheshire out as much as possible or in as much as possible !

I have the short tube version of the Cheshire, what is wrong here ?

[Altair8389]

I am -6 dioptres myopic, so if I take my glasses off, I can see the cheshire cross hairs ok, but the rest of the image - spider vanes and mirror are blurred. I think the Cheshire cross hairs are in the centre and coincide with the donut dot in the centre of the primary mirror though.

 

Magnus

[Captain Scarlet]

16 hours ago, vlaiv said:

Don't think that is the case.

As far as I can see from the images - all edges on the focus tube are parallel to secondary spider vanes. That would just increase secondary spikes.

Yes you’re right. My thinking was that because the focus-tube intrusion is a solid edge (rather than a thin stripe) it would be brighter shorter and dirtier than the vane-spikes and therefore make them less noticeable. Having said that I don’t think it’s the culprit.

A couple of extra questions: do you wear glasses whilst viewing, which might be fine-scratched in one or more predominant direction  from habitual wiping? Really grasping at straws here.

It’ll be most interesting to see some star images.

M

[Altair8389]

Hi Captain Scarlet,

I usually use glasses and I think the flares are there with or without, but I am not 100 percent certain.

I will have another look and try and image with a camera !!!

 

Magnus

[Pixies]

On 27/11/2022 at 13:27, Altair8389 said:

Hi,

I just realised I cannot focus on the Cheshire collimator's cross hairs - they are out of focus no matter how I arrange the focuser and if I move the Cheshire out as much as possible or in as much as possible !

I have the short tube version of the Cheshire, what is wrong here ?

I have the same problem with Cheshires. Longer ones are easier, as the crosshairs are further away. I usually end up getting a teenage child to make final confirmations!

[JeremyS]

Do you have astigmatism in your observing eye? This can give unusual flares on bright objects.

[Altair8389]

Hi Jeremy,

I had more attempts at the dobsonian. I found that the “diffraction flares at 120 degrees separation” lessened or disappeared on using a better eyepiece. I saw the concentric rings when I defocused on a star; perfectly (?) concentric circles, and I saw the typical 4 spikes at 90 degrees to each other like you see in photos or artificially generated by software. So my original problem may be due to low quality eyepieces and using glasses (specs). My left eye has significant astigmatism; to get minimal artifacts, use your best eye without glasses! 

 

Good tip about the Cheshire! I may have to try thinking getting a longer Cheshire tube! Previously I was confusing the 4 spider vanes with the Cheshire alignment hairs!