Focus and Diffraction Spikes

[AlistairW]

Hi Steve,

Thanks for that much appreciated. I have contacted Martin and have emailed him through a .CR2 sub and associated compressed .TIFF. Hopefully he will get a chance to look at them tommorrow. I am going to have an inspection of the OTA right now and see if there is anything obvious. Excellent service from FLO again. 

Thanks

Alistair

[Zakalwe]

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. 

I wasn't aware of that article. There are a couple of thereads on CN where Star 71 users were getting "Maltese Cross" shaped spikes:

http://www.cloudynights.com/topic/491910-william-optics-star-71-first-light/

http://www.cloudynights.com/topic/494098-williams-optics-star71-diffraction-issue-fix-thread/

One about a GT-81

http://www.cloudynights.com/topic/511732-bloated-star-images-in-gtf-81/

[steppenwolf]

I am going to have an inspection of the OTA right now and see if there is anything obvious.

From what Steve (FLO) has said, it would seem that spacers may well not be the issue here - your inspection would clearly show the metal tabs if they were present. I would certainly agree that this is ( a ) not a problem related to just WO and ( b ) absolutely not a problem on my own WO FLT 98.

Assuming that spacers are not an issue here, it's time to start some tests starting with the minimum of gear, namely the OTA and camera. Capture some images including bright stars then slowly add any other components to the light path one at a time, capturing images and looking for the diffraction artefacts to appear.

We should also be clear that Alistair's telescope is a GT-81, not a GTF-81.

[AlistairW]

I have had a good look and can't see any spacers, but also no other obstruction either - but I will be doing as suggested and taking images with a basic rig and add components back in - hopefully it will become apparent.

Thanks for all the comments - Alistair

[Sp@ce_d]

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

How odd. The only thing I can think of, out of all the kit I tried on that scope, that I haven't been using on a fairly regular (as regular as one can be in our climate) basis, over the past year. Is the flattener. I was waiting for the proper matched one to come into stock & I used an original GT81 flattener that I had used with no probs on the ZS66. However, I'm pretty sure I'd removed that in testing anyway. I'd have taken it down to basics with just a CCD on the end, The rest of the kit CCD's, filters, adapters etc has all been in use, no problems. Very strange.

[AlistairW]

Ok so here are some futher test pictures:

Unfortunately I do not have a simple extention to eliminate the Flattener \ Reducer so I could not test that combo. What do I try next ? It spikes seem so symmetrical.

First one: William Optics GT81+Flattener & Reducer. Canon 450d+Baader mod. (I have removed my Astonomik CLS clip filter)

Second one: William Optics GT81+Flattener & Reducer. Canon 600d (no mod). (I have removed my Astonomik CLS clip filter)

Thanks

Alistair

[D4N]

Have you got a diagonal you use for visual that you could use in place of the flattener?

/Dan

Sent from my iPad using Tapatalk

[AlistairW]

I do have a diaganal ... will see what I can do - thanks for the suggestion

Here is another with it totally out of focus

Third one: William Optics GT81+Flattener & Reducer. Canon 600d (no mod). (I have removed my Astonomik CLS clip filter)

[AlistairW]

I wasn't able to figure anything out to eliminate the flattener\reducer, but I think I have eliminated the Camera and filter. I will get an extender sometime, but still confused, as there are so many spikes, but I can not see any obstruction, not even a spider web .....

[AlistairW]

So what I am thinking of trying next is an aperture mask to make the objective 75mm. I feel my logic is a bit flawed though as to stop diffraction spikes happening I thought you should have the aperture as open as possible (at least I think that is true from a photography point of view). However, I am trying to see if there is any internal reflection off the baffels happening. I will give it a go anyway as I have heard of similar things being done on refractors to try and eliminate these diffraction spikes. Got a few ideas how to make a mask. 

[D4N]

The diffraction occurs because the light from the star is acting as a wave when it hits an obstruction in the optical path, by reducing the aperture the light will no longer hit the obstruction (if it is at the edge of the light cone) and no diffraction will occur. Of course the aperture mask needs to be smooth or it too will also cause diffraction.

When using a DSLR lens you get multiple spikes from the iris as it is not circular, these can also be removed by using an aperture mask that is smaller than the iris.

/Dan

Sent from my iPad using Tapatalk

[AlistairW]

Thanks Dan, that's what I thought. I think it will be an interesting test at the very least. I have also read in threads that Star Tools has a "Repair" tool that can possibly remove some of these atrifacts. Though I do not have Star Tools so cannot really commnet.

I have managed to get hold of a 0.5mm thick shim with an aperture of 75mm, just need to find a way of capping it on to the scope. I have seen some tapered cork bungs that will fit over the dew shield. So I am thinking of combining the two to make my aperture mask, and paint it up black etc ...

[almcl]

Thanks I have also read in threads that Star Tools has a "Repair" tool that can possibly remove some of these atrifacts. Though I do not have Star Tools so cannot really commnet.

If you want to try StarTools, you can download the prog for free from the website, all the features work except save, so you could see if it does what you need?

[D4N]

paint it up black etc ...

Sounds good, just remember that sometimes black paint can be highly reflective in IR.

/Dan

Sent from my iPad using Tapatalk

[AlistairW]

thanks for reminding me - BBQ paint I think was suggested earlier in the post.

[Sp@ce_d]

thanks for reminding me - BBQ paint I think was suggested earlier in the post.

Yes.. related is this interesting read - http://www.sbig.com/about-us/blog/flat-fields-the-ugly-truth/

[AlistairW]

Very interesting Sp@ce_d - thanks for that ....

So I was wandering around the local hardware store at lunch time looking for inspiration. In the plumbing section I have come across a 110mm pipe to 80 mm connector. Now given the diameter of the GT81 dew shield is 104mm, this should simply slide on, with a nice bit of felt on the inside. I should be able to saw off the screw bit, and sand it down. Then I have a machined 75mm shim (0.5mm thick), so it would be a matter I think of just gluing this on. Anyway I need to have a think about it.

My next question though is this current idea is all with the dew shield extended, and a cap being fitted over the end. However as light enters via the shim it is going to be refracted in to the wider dew shield. So do modifications like this really need to have the dew shiled retracted to make any differance ? - It's going to be very difficult to rectract the dew shield due to the postion of the OTA rings, so this is where I have seen insets going in to the dew shield. The only thing that worries me about putting a mask in the dew shield, is if it were to slip it would hit the objective. Any comments ?

[D4N]

That looks like a good solution, Best just to try and see what the result is like.

My bahnitov mask has fallen into the dew shield before, it is light plastic though so no chance of it causing any damage to the objective.

/Dan

Sent from my iPad using Tapatalk

[AlistairW]

Slight change in plan. The 75mm to 95mm shim I bought seems a good internal fit. I have put some felt on the back and it a nice snug fit inside. Unfortunately means dew shield is retracted. Certainly not going to fall out or move. No sign of cloud free sky to check it out though. Makes sense it is as near the objective as possible. Doesn't look like much of a change of aperture, but only clear skies will tell.

[Waldemar]

The number of spikes make me think it may be some reflecting surface on the edge of a glass component or even on the inside of your focuser.

If that is the case, an aperture mask will not help...

[AlistairW]

Slow process testing this ...

So the excellent FLO lent me a T extension so that I could remove the FR and Flattener. This is the result :

WO-GT81 NO FR/FLATTNER, NO FILTER on a astro modded 450d Canon.

The above is obviously zoomed in.

The next test was with my inserted 75mm aperture mask

WO-GT81 NO FR/FLATTNER, NO FILTER on a astro modded 450d Canon. 75mm aperture mask inserted in dew shield.

I think the aperture mask has minimised the difraction spiking. But not sure if the key point here is that the spikes "are" a function of the telescope ?

My next test will be to use an aperture mask to drop the diameter to 70mm. Just need to make one of these up now.

Thanks

Alistair

[Waldemar]

Alistair, keep in mind that a 70mm aperture mask on your 81 mm scope leaves you with quite some lightloss

Your scope goes to F/6.9 instead of the F/5.9 you bought it for...

Personally I think  if this is a scope issue, it should be solved by the manufacturer, not by you.

As soon as you ruled out any other possibilities without using any masks, so it is obvious the root cause of the problem is the scope, warranty should kick in.

Maybe you know somebody with a CCD camera, who can give it a shot.

Just my 2 €cts...

Waldemar

[Waldemar]

To go a step further: this means your exposure times will be 27% longer to get the same results... will you like that? I don't think so.

We all are fond of finding solutions for technical problems, me too because it seems to be part of the hobby.

But in fact we do not do our selves a favor by doing this kind of problem solving.

The manufacturers will think: 'O they will solve that themselves...' and get lazy about quality issues.

You paid good money for that scope, you may expect a good scope without issues!

Maybe I am a bit too assertive about this, but that is how I feel.

Waldemar

[FLO]

Personally I think  if this is a scope issue, it should be solved by the manufacturer, not by you.

We all are fond of finding solutions for technical problems, me too because it seems to be part of the hobby.

But in fact we do not do our selves a favor by doing this kind of problem solving.

Don't worry, Alistair knows we will support him 100%. It would take us only a day or two to arrange a DHL collection and replacement  

I think he is experimenting with an aperture mask because he 'wants' to. Not because he needs to. 

HTH, 

Steve 

[Waldemar]

Thank you Steve, I know there are a lot of serious retailers and producers around, but I did have some bad experiences as well...

This was by all means not meant to your address. It was just a general remark. Sorry if I offended you.

It is very nice to hear that you will back him up all the way! 

Best regards

Waldemar