Multiple diffraction spikes - Skywatcher Skyliner 200P

[skylarn]

Hi,

I'm still quite new to astronomy but recently upgraded from a 5" dob to an 8" (SW Skyliner 200P).

On my first viewings with the new scope I've noticed that (unlike with the smaller scope) diffraction spikes are visible on bright stars. However, the spike patterns I'm getting, particulary when I use a 20mm eyepiece, are not of the expected cross-hair type pattern, rather a kind of 'multiple' spike effect.

This is less obvious when I use my 10mm or 32mm eyepieces, but with the 20mm one (a Skywatcher UWA) it is very noticeable. Please see the below pic I took afocally using a digital camera through this eyepiece.

A few things to note:

1. I read that the multiple spikes can occur when the spider vanes are a bit loose and not lined up, or too tight and slightly bent. I slackened the vanes and tightened them again but this made no difference at all (I even did it while viewing through the eyepiece and nothing changed).

2. I also read that it can be caused by a deficient coating of the secondary mirror. Looking closely at my secondary through the focuser, the coating looks perfectly oval and there are no gaps at the sides. I've also shone a torch on the primary and can't see any marks or abberations.

3. Being a novice I haven't got around to collimating the scope yet. However, based on the typical collimation patterns I've seen, and having done a sort of star test, I'd say that at worst the it needs fine tuning, as the factory collimation seems okay.

4. The 20mm eyepiece I took that photo through does have a bit of dust in it, but I don't know if this makes any difference. I still get at least one diagonal spike with the other eyepieces, it just seems worse when I use that particular EP.

Based on the above, does anyone know what the problem might be (if there is one!)? As this is a brand new scope I just want to be sure I haven't bought a dud and there isn't something optically wrong with it.

Thanks in advance for any help.

[ronin]

That looks like diffraction spikes from the sceondary and something else added in.

The big one being the something else.

Problem is I cannot suggest anything for it.

OK could throw it and buy a refractor. :grin:

Need to identify where the basic cause is, which means scope or eyepiece.

Need to get back out, aim at a star and get the problem then while viewing slowly rotate the eyepiece and determine if it moves with the eyepiece or nothing happens.

After that. or before, check the mirror: is it held too tight and there is pinching, you can find that the main is set up and griped too tight. It can get set up such that the screws make contact, then tightened a bit more to hold it secure - that is too much. After that is the mirror actually held flat, collimation is really more then getting bits lined up.

Any chance of reflected light withing the scope - does it need flocking.

Multiple images, your initial thought, would I think be caused by a second surface mirror but they would give multiples of the normal diffraction spikes.

[Daniel-K]

First id buy a cheap Cheshire and go from there although these scopes do hold collimation its probably been knocked out in transit

[RobH]

It's not collimation. That won't give multiple spikes.

If you don't get it with your other eyepieces, only with one of them, then it's unlikely to be the scope but far more likely to be associated with that particular eyepiece.

As suggested, try rotating the eyepiece and see if the spikes rotate with it. If so, then the eyepiece is definitely the culprit.

Another thought.....you haven't got a telephone wire or something in the field of view have you? Sounds daft and I'm sure you've checked, but something like that will produce diffraction spikes.

[Peter Drew]

My first approach would be to check  the primary mirror to see if it is held too tightly, you should be able to rotate it with finger pressure. Also make sure that the optics are cooled to ambient temperature. Good luck. 

[John]

The dimmer of the 4 spikes are the normal diffraction spikes caused by the secondary supports I reckon. The brighter ones are caused by something else (not collimation). This may sound an odd question but was the star near any trees, roof lines or overhead cables / telephone lines ?.

I've seen just the affects that you show in your photo when the above sort of obstacles are near (not necessarily right across) the line of sight to the star being viewed.

[Alien 13]

As already suggested check for phone/washing lines etc i had a similar problem ith a frac caused by a spider thread hanging from a line about 2 foot away.

Alan

[itmo]

so. rotate everything , including your head to see what the spikes rotate with. 

[itmo]

also spider vanes should be checked that they are not even slightly "sideways" and they should "twang" when you twang them. 

[swamp thing]

I'd try the suggestions made by rob and Peter first. If no different try using the small hole in the dust cover. Pace the hole where there are no spider vanes. This should rule out the spider.

[skylarn]

Thanks for all the replies! Last night I finally got a chance to use the telescope again. Before I set up, I checked the primary mirror bolts to see if they weren't too tight, as suggested. They didn't seem very tight, but I made them just finger tight. After testing the 10mm and 32mm eyepieces again, I couldn't see any of those extra diffraction spikes. With the 20mm, however, it was just the same as before. Then I tried rotating the eyepiece while viewing and...yes, seems it was just the eyepiece!

Thankful that at least my scope doesn't appear to be dodgy, I decided to dismantle this particular eyepiece (I know this is generally not a good idea, but...) and clean the lenses, which did seem to have picked up quite a bit of dust. After putting it back together I tested it again and...voila, no more 'multiple' spikes!.. Not sure how dust can cause those spikes, but perhaps there was something loose or another issue?

Anyway, just glad that it's okay now. Cheers for your help.

[tingting44]

you have not left the focus mask on the scope have you.........thought id eliminate that one.....

[Dave_D]

I get that effect sometimes. It appears (in my case) to be linked with a neighbours bathroom light. It always appears vertical to the object i'm imaging which convinces me it's from a fixed point.