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Williams Optics Star71 Diffraction Issue Fix Thread

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#1 josh smith

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Posted 12 March 2015 - 01:50 AM

I have to admit, I've had enough of the weird diffraction spikes coming from the WO Star71.  I think part of it is that I'm undersampled and it seems to cause really weird looking small stars, part of it is that it must be having some sort of affect on the overall quality of the data, and part of it is I want my stars to look nice and round.  Lots of good ideas were thrown around and I agreed with the idea of the step down mask.  I think initially some were trying to put something right up against the element, but I didn't like that idea as I didn't want my grubby hands or my plastic piece resting on the glass, so I made a mask to go over the dew shield.  It slides on very easily, it is cheap, it is light, and I designed a bahtinov mask plug for it so it doesn't need to be removed for focusing if you are manually focusing.  The mask and bahtinov mask has been made with a 3d printer and can be easily replicated if others are interested.

 

Here's my results with an inside diameter mask of 64mm.  I wasn't happy with the stars until I got down to 64mm as the effect was still visible.  These are two stacks of 5x3' of the star Per near the California Nebula taken in the same hour.  I'm pleased with the results, but a little aggravated that it needs to be stepped down to f/5.45 to get diffraction free stars.  The affect does seem to be a little temperature dependent and is more dramatic when it is colder from what I've seen.  If anyone has any other ideas on how to beat this problem without slowing the scope down, I'm all ears!  

 

16791365185_25fd894757_h.jpg

 

16603448798_3dc42f465a_b.jpg

 

16791024995_fd29ac34b9_b.jpg



#2 rflinn68

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Posted 12 March 2015 - 02:30 AM

Looks like a big improvement Josh  :D   If you put it on the lens cell could you open it up a couple more mm's? It wouldnt actually be on the glass would it? Wouldnt it actually be on the lens cell? I do like this simplicity though and it looks like you'd have less worry with flats. I was thinking if it were right against the cell it would need to fit pretty snug so it wouldnt be moving around. Looks like a nice simple fix though the way you have it. Nice job!  :waytogo:



#3 josh smith

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Posted 12 March 2015 - 07:11 AM

I think you could put it on the cell itself and I have a good idea of how Id do that, but I too like the simplicity of the cap mask and not having to worry about accidentally touching the glass or something.

#4 kraegar

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Posted 12 March 2015 - 07:20 AM

Seeing this, if nothing else, makes me want to 3d print a Dew Shield extension for my WO Star 71.  Nice job with the fix for the stars!



#5 Footbag

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Posted 12 March 2015 - 08:26 AM

It's a big improvement, but 64mm is more then I was expecting.  Still, it seems effective.  F5.45 isn't so bad. 

 

Funny thing is, your test photo above shows an uglier spike then I'm used to seeing in your images.  More like something is in the light path, and less like a typical spike.  Phil had the ugliest I've seen.  It's interesting that there is so much variation.  I hope it's not something that's going to show up at higher temps. 



#6 josh smith

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Posted 12 March 2015 - 08:48 AM

It's a big improvement, but 64mm is more then I was expecting.  Still, it seems effective.  F5.45 isn't so bad. 

 

Funny thing is, your test photo above shows an uglier spike then I'm used to seeing in your images.  More like something is in the light path, and less like a typical spike.  Phil had the ugliest I've seen.  It's interesting that there is so much variation.  I hope it's not something that's going to show up at higher temps. 

 

I mentioned above, the diffraction seems to be really temperature dependent and is much more prevalent at colder temperatures.  My first testing night when I tried it out at 5* F was horrendous.  Part of why you didn't see the spikes so bad on the other images is that it was 20-40 degrees warmer when I was shooting in Florida for all of the images I've published with the combo.  The other thing is that I've been processing the images tediously with this combo and tended to let the big stars bloat a little since I had to stretch them so hard which also hides the spikes some.  I actually saw a very slight increase in snr with the slower version using the step down mask and I suspect the sharper stars and cleaner optical path contributed to that, but don't know for sure so maybe it makes the slower setup a wash.



#7 keithlt

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Posted 12 March 2015 - 09:09 AM

nice experiment Josh. that printed material looks a bit shiny ,does that need to be addressed?


Edited by keithlt, 12 March 2015 - 09:09 AM.


#8 josh smith

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Posted 12 March 2015 - 09:31 AM

nice experiment Josh. that printed material looks a bit shiny ,does that need to be addressed?

 

The flash on my phone made the material look shiny, however it is a matte gray.  I could also print this out in matte black, although it won't be necessary after seeing my data from last night.  



#9 Puck Ja

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Posted 12 March 2015 - 10:50 AM

Does the brightest star in 1st image have some non-symmetric halo?  Are you concerned about that?  I wonder if it came from the reflection of your filter/optics or the mask.



#10 Jon Rista

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Posted 12 March 2015 - 11:03 AM

Puck, the offset halo is in both images, pre and post mask. I think it's just because the star is offset.



#11 Puck Ja

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Posted 12 March 2015 - 11:52 AM

Good point.  You got sharp eyes, Jon!  ;)  


Edited by Puck Ja, 12 March 2015 - 11:52 AM.


#12 yzhzhang

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Posted 12 March 2015 - 03:15 PM

Hi Josh,

Excellent! Can you please share where you have this and the Bahtinov mask made? Would like to have a set as well.

Thanks

#13 Dr Benway

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Posted 14 March 2015 - 11:47 AM

I am impressed with your results, Josh. I made a 64mm mask with set screws to sit on the lens cell retainer. I did test one at 66mm and still saw diffraction spikes, although more pleasant looking than the "iron cross". I believe I will scrap that idea and print one like you are using. It would be much easier to install.

 

John



#14 Jon Rista

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Posted 14 March 2015 - 11:51 AM

I think this right here shows the awesomeness of 3D printers. :D That we can just pop a design into a printer and have it produce a physical object of fairly exacting specifications is just sweet.

#15 josh smith

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Posted 14 March 2015 - 11:56 AM

I think this right here shows the awesomeness of 3D printers. :D That we can just pop a design into a printer and have it produce a physical object of fairly exacting specifications is just sweet.


Totally agree. I use them very frequently at work and for my Astro stuff. I've made my light box el panels, Bahtinov masks, focusers, dovetails, electricronics holders, mount accessories, etc... with the printers. It is great because it is cheap, easy to iterate designs, and you get exactly what you want.

#16 josh smith

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Posted 14 March 2015 - 11:59 AM

I am impressed with your results, Josh. I made a 64mm mask with set screws to sit on the lens cell retainer. I did test one at 66mm and still saw diffraction spikes, although more pleasant looking than the "iron cross". I believe I will scrap that idea and print one like you are using. It would be much easier to install.

John


Thanks John. I worked my way down to 64 and that was the sweet spot. I suspect it could be more open during the summer when it is warmer. I may make an adjustable version when we get back to more comfortable temps.

#17 kraegar

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Posted 14 March 2015 - 11:59 AM

I'm going to a 3d printer Festival next weekend, and actually plan to use this as an example of how useful they can be.



#18 gdallmann

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Posted 14 March 2015 - 02:04 PM

Hi Josh,

 

Looks like the same issue that I'm having (had?) with a William optics GT 81 i recently purchased.  This must be something inherent in how they make their lens cells. I had the same kinds of maddening spikes that made bright stars look like Maltese crosses.  I also found that temperature was a key to the problem.

 

I complained to the manufacturer and William Yang replied back that the issue resulted from the plastic in the lens cell contracting in the cold resulting in the screws holding the lens putting too much pressure on the outer lens element.  He recommended loosening the screws 1/6 of a turn to relieve the pressure.  Since there were 8 screws holding the thing in I didn't want to do that so he had me send it back to him were he did some of the adjustments and made sure it was still properly collimated. 

 

It didn't completely solve the problem but I'm finding that if I crank up my dew removal strap to maximum as well as keep the whole scope warm in the house before imaging, the problem is minimized. 

 

I'll try to add before and after cropped images below.  The top one is a crop of a portion around the Bubble Nebula before sending the scope back for adjustment and with the dew strap set on a relatively low setting.  It shows the same kinds of defects that your image had. 

The bottom one is a crop of a region around the Rosette nebula after the adjustments at William Optical and with the dew heater turned up to ~3/4 of max.  The problem isn't totally gone but much reduced.  I'm still in the process of figuring out where to set the dew heater power but I know that the effect gets worse with less heat.

 

Good luck & clear skies!

Garry

Attached Thumbnails

  • WO GT81 before.jpg
  • WO GT81 after.jpg


#19 mikeyL

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Posted 14 March 2015 - 05:55 PM

Well, it would seem that the problem is definitely temp related in the GT81 at least, and I am guessing then also most likely in the WO Star 71 as well.Thanks to all who continue to add excellent info to this thread, for those of us who are still trying to decide if buying the WO Star 71 makes sense or not.

 

ML



#20 josh smith

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Posted 14 March 2015 - 06:09 PM

Mike, I agree with you... Those of us who've experienced this before didn't have too much of a question of the cause, the symptoms, or the fixes...  A couple of us saw the same exact issue in the SV102ED and now it seems in the GT81.  It is a shame that it is designed this way, but I still thank it is a very good value for the scope.



#21 Dr Benway

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Posted 14 March 2015 - 06:32 PM

My WO 81GTF has always produced the best stars with no sign of spikes. This example of the Pleiades is a RGB 10X600 with a one shot color 8300 chip CCD camera. That iron cross pattern evident in Garry's image looks exactly like the problem we are seeing with the Star 71. This leads me to believe that it could possible be adjusted regardless of William Yang's emails...

 

John

 

 

Attached Thumbnails

  • pleaides_cropCN.jpg


#22 josh smith

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Posted 14 March 2015 - 06:35 PM

My WO 81GTF has always produced the best stars with no sign of spikes. This example of the Pleiades is a RGB 10X600 with a one shot color 8300 chip CCD camera. That iron cross pattern evident in Garry's image looks exactly like the problem we are seeing with the Star 71. This leads me to believe that it could possible be adjusted regardless of William Yang's emails...

 

John

 

 

Do we know for sure that the lens is mounted the same way in the GTF as the GT model...  Also, I suspect it could be fixed without the mask by doing something different to the mounting of the lens, but not sure I want to go there yet.  One possibility would be loosening the screws, the other and probably better would be to mount it differently altogether.



#23 Dr Benway

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Posted 14 March 2015 - 06:46 PM

My mistake. I assumed Garry had the same scope as mine. I just looked up the GT and it is a triplet; the GTF is a 5 element. 

 

John



#24 josh smith

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Posted 14 March 2015 - 06:48 PM

My mistake. I assumed Garry had the same scope as mine. I just looked up the GT and it is a triplet; the GTF is a 5 element. 

 

John

 

It could still be mounted the same way, just don't know.



#25 gdallmann

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Posted 15 March 2015 - 01:21 PM

Hi Josh & John,

 

Great shot of the Pleiades John! That's the kind of image I was hoping for out of this scope based on other images I've seen online. 

 

I don't know when you bought yours John, but I think that the lens cell must have changed at some point, with the new type causing this issue.  When William Yang sent me instructions to loosen the lens cell screws, the images showed a lens cell with 4 screws.  When I opened up mine I was surprised to see 8 screws instead of the 4 that I expected.  That spooked me and I followed up with Yang and he suggested I send it back for adjustment.  While that didn't completely work, I'm hoping that summer temps and aggressive dew heating will solve this thing otherwise I'm still going to send it back to him within the warranty period. 

 

It's been very aggravating and I've spent way too much precious observing time messing around with it.  I might also give your idea of stopping down the opening a try as well Josh, thanks.  Also, if there was another thread discussing this could someone provide a link since I couldn't find it.  Thanks

 

Garry




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