Focus and Diffraction Spikes

[AlistairW]

Hello,

I have a 4" refractor. Would being slightly out of focus cause some of the stars in my images to have diffraction spikes ?

Thanks

Alistair

[D4N]

No diffraction spikes are caused by obstructions in the light path.

Look down you scope both ways and see if you can see anything protruding.

It could be a screw, a lens clip or an OAG protruding too far.

/Dan

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[Herra Kuulapaa]

Actually you can tell from the diffraction pattern that your scope is in focus, also some refractors. Spikes are really small in them compared to reflectors though, at least usually they are..

[AlistairW]

Interesting - will have a look tonight - thanks

[michael.h.f.wilkinson]

Sometimes the clips (often 3) holding the lenses in place protrude into the light path. That can cause slight spikes. I also spotted a weird spiky pattern when  I am slightly out of focus with my 2" 90 deg Amici prism in my 80mm frac. That stems from the prism, not the objective

[John]

Air spaced lenses often use thin foil spacers to hold the lens elements at the correct distance apart. These can cause artifacts when imaging I believe.

[steppenwolf]

Could you post an image for us to check out, Alistair?

[AlistairW]

Here is a really zoomed in image. I have hollowed stars too. That's why I am wondering if it is focus, pre-processing or something else ...

Thanks

[steppenwolf]

Thanks, Alistair. Certainly, your image would appear to be out of focus but I suspect that this is secondary to the diffraction artefacts. The diffraction spikes appear to be caused by four intrusions in the light path - can you see and lens spacers and if so how many are there?

[AlistairW]

Interesting as there is nothing special with my setup - just a WO GT81 with a reducer and flattener. Lens spacers are a new thing to me - but i can understand their need - and although i have not checked - i would be surprised. I am interested in why you suggest 4 - but one thing that has occurred to me is that there are some grub screws i have issued in to the OTA quite far - maybe it is that - but i am not sure. I have a Astronomik clip filter in the train, but can't believe it is that.

[steppenwolf]

I am interested in why you suggest 4

From what I can see, there are 8 major spikes - each diffraction point normally produces two spikes at 180 degrees from one another, hence 4 sources.

[Sp@ce_d]

I had "an experience"  similar to this with a GT81. Afraid it's probably the spacers. Mine went back.

http://stargazerslounge.com/topic/226807-gt81-now-its-the-star-shapes-is-this-the-final-straw/?hl=%2Bgt81#entry2446898

[Herra Kuulapaa]

One source of the spikes are the aperture blades when you image with camera lenses. More you dim the lens the "better" diffraction spikes you get.

Just for reading:

http://www.bhphotovideo.com/explora/photography/tips-and-solutions/create-compelling-star-effects-sun-stars-starbursts-photos

[AlistairW]

Sp@ce_d - that's very interesting. FLO describe that it is the spacers protruding in to the light path. I have not had a chance to test this myself, but it would appear that if I use an aperture mask then I should be able to eliminate this. I would like to try this just to eliminate this as a possibility. I went back to some older images I had taken, and when I zoom I do see spikes on all of them, but it's only really apparent when I massive crop. Not sure if this is an expected artifact or flaw. Anyway I might be jumping ahead before I know for sure. Next question how do you make an aperture mask, or are commercial ones avaiable, - I don't fancy my attempts with cardboard in scissors .... ?

Thanks

Alistair.

[Zakalwe]

I've seen plenty of reports concerning WO refractors and diffraction spikes.

An aperture mask might cut them down, see here: http://www.cloudynights.com/topic/494098-williams-optics-star71-diffraction-issue-fix-thread/

[AlistairW]

Beginning to look like a known issue. Now I need to figure out how to make a decent aperture mask. This is going change things like F ratio isn't it ? - Just thinking that any of the software packages that ask for aperture size, will have to be changed I guess ?

[steppenwolf]

Just thinking that any of the software packages that ask for aperture size, will have to be changed I guess ?

Yes they would but be careful that the software doesn't then show the wrong focal length - it depends on whether the software asks for focal ratio and aperture or another combination - focal length is normally the most critical component and that, of course, will remain unchanged.

[ollypenrice]

It's easy to make an aperture mask and it only needs to be very thin to take out the clips.

As you can see, this was made for a camera lens but the principle is the same. (I wanted to avoid using the diaphragm because it created multiple spikes which I didn't like at all.)

The compass-cutters come from graphics outlets. In your case I think cellophane or thin plastic might be best, flocked or painted matt black. If you really want to be fussy barbecue or stove paints use pigments which are non reflective even outside the visual spectrum.

Olly

[blinky]

I quite like the diffraction spikes!

[AlistairW]

Thanks Olly - that's the sort of thing I was thinking of. When you say "very thin" are you only talking a few mm. My refractor is 81mm so I was considering dropping it to 75mm .... I found some large washers online and was thinking of using this as a starting point.

http://www.applegate.co.uk/b2b-products-services/70ee2fd0bbf011e39b4d005056822514.html

Spray it up black and put a mounting mechanism on it ....

Just for my sanity though, I don't think this is a defect in the scope, it just an unfortunate design thing. Or am I being too forgiving ?

Thanks

Alistair.

[AlistairW]

Even thought the main objective is 81mm, the dew shield is 104mm. So I presume any aperture mask need to fit over the extended dew shield. So in effect I would be reducing the 104mm diameter down to 100mm say. Does that make sence ?

[D4N]

If you really want to be fussy barbecue or stove paints use pigments which are non reflective even outside the visual spectrum.

Olly

That's a top tip, normal black paint can sometimes be highly reflective in IR, it would be like sticking a mirror to your dew shield if you image in IR.

I think the mask has to be smaller than the aperture of your objective or it will have no effect at all. Stopping down the dew shield to 81mm would have no effect as the light will still pass the lens clips.

/Dan

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[Alien 13]

Any mask needs to be as close to the lens as you can get its easy with camera lenses as they have a filter thread and cheap sacrificial filters are available off the bay.

A good material to work with is the black inner of a dvd case it can be scrubbed with a scouring pad to make it  matt or painted.

Alan

[AlistairW]

Oh dear what a complication. If I need to get it near to the lens then I have to either move the dew shield back, - which I don't think is an option as the tube rings stop it. I don't fancy putting anything inside the dew shield in case it was to dislodge and fall against the objective. So the only remaining option is to have a mask on the extended dew shield that goes from an external diameter of 104mm down to 75mm (for example).  Now I come to think of it it does make sense that the final diameter is less than the objective. Do other do this, or have I been unlucky with this lens clip issue ?

[FLO]

I had "an experience"  similar to this with a GT81. Afraid it's probably the spacers. Mine went back.

http://stargazerslounge.com/topic/226807-gt81-now-its-the-star-shapes-is-this-the-final-straw/?hl=%2Bgt81#entry2446898

Sp@ce_d - that's very interesting. FLO describe that it is the spacers protruding in to the light path. I have not had a chance to test this myself, but it would appear that if I use an aperture mask then I should be able to eliminate this. I would like to try this just to eliminate this as a possibility. I went back to some older images I had taken, and when I zoom I do see spikes on all of them, but it's only really apparent when I massive crop. Not sure if this is an expected artifact or flaw. Anyway I might be jumping ahead before I know for sure. Next question how do you make an aperture mask, or are commercial ones avaiable, - I don't fancy my attempts with cardboard in scissors .... ?

Unfortunately we didn't update Sp@ce_d regarding his returned GT81 (we simply refunded his payment and he purchased something else) but I wish we had because we were wrong to suspect lens spacers were the cause of his diffraction spikes. When we checked the telescope we found it didn't use spacing tabs in it's construction and we were unable to reproduce the problem. They must have been caused by something else in the light-path. 

The reputation WO scopes have here at SGL for diffraction spikes from spacer-tabs can be traced back to one magazine review. It was a FLT132 with strong spikes caused by metal spacers that intruded into the light path. Several manufacturers use metal spacers in their design, including some of the most prestigious. The method isn't unique to WO and they are not a characteristic of their telescopes. 

If AlistairW's GT81 is the same as Sp@ce_d's then it won't have lens spacer tabs so the cause of his diffraction spikes will lie elsewhere. 

HTH, 

Steve