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Revolutionary new way of focusing! No less!

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#76 Snow dog

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Posted 02 September 2008 - 08:45 PM

I downloaded the InkScape opensource software Dennis had mentioned and worked my way through how to use it. I finally came up with this design.
I have calls out to various laser cutters in town for quotes. I will most likely go with a black opaque acrylic rather than aluminum unless I can find a place that has the anodized. I don't want to be placing spray painted surfaces next to my optics. My size is for the 12" SCT so the slats are 10mm wide and spaced 10mm. The central obstruction is 10cm and the entire diameter for the full disc is 35cm.

I really like the way it turned out and can't wait to try it out!

I named it the Pavel Mask.. I thought it was a fitting name. :)


I like your design also. What's the angle that the right side slots are cut at? It looks like it's around 35 deg or so. Left side looks like 90. Thanks.

#77 snowdragonusa

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Posted 02 September 2008 - 08:52 PM

I like your design also. What's the angle that the right side slots are cut at? It looks like it's around 35 deg or so. Left side looks like 90. Thanks.


Yup, the left side is 90 degrees, the right top side is +20 degrees while the right bottom is -20 degrees.
They will cut tomorrow so I should get them by Friday hopefully.

:jump:

#78 John Wunderlin

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Posted 02 September 2008 - 09:33 PM

Looks great. I also tried printing the pattern on a blank transparency, but it seems to be important to have the clear openings. I think the transparency material wasn't optically clear enough.

#79 Nocturnal

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Posted 02 September 2008 - 09:55 PM

That's actually a great idea :) Some overhead sheets for printers are quite matte to make the ink stick. If yours were clear I'm not sure why it wouldn't work with a bright enough star.

#80 mikefulb

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Posted 02 September 2008 - 10:04 PM

Ok just tested my mask made from thick felt on my Takahashi Epsilon 180ED and it works just awesome. The scope is 500mm and f/2.8 so focusing is always a PITA. I only tried focusing on brighter stars but it was very trivial to get perfect focus. The out of focus images looked very similar to the animation someone posted earlier. I could move the focuser in increments of 3 Robofocus steps (I think that corresponds to 1/1200 of a turn of the focus shaft) and see distinct changes in the diffraction pattern.

I thought about using a clear transparency sheet and printing the pattern but I was worried the sheet would slightly affect the focus position and so when I removed it then I would still be slightly out of focus. Make sense?

#81 John Wunderlin

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Posted 02 September 2008 - 10:12 PM

That's actually a great idea :) Some overhead sheets for printers are quite matte to make the ink stick. If yours were clear I'm not sure why it wouldn't work with a bright enough star.


Well, it worked fairly well visually- I could see the triple spike pattern, but it was a little 'wiggly'. When I added the CCD, it was badly distorted and I couldn't really use it. However, I was just holding it in front of the scope by hand- I'll bet if you had a rigid frame it would have worked much better.

#82 snowdragonusa

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Posted 02 September 2008 - 10:31 PM

I thought about using a clear transparency sheet and printing the pattern but I was worried the sheet would slightly affect the focus position and so when I removed it then I would still be slightly out of focus. Make sense?


I had thought the same thing. Placing a filter in the light path can cause the focus to go slightly out and I figured a overhead transparency would be quite below par to a filter. With that said though. I bet you could print it out on a clear transparency, cut out the "clear" areas, and mount it into a rigid housing like mentioned above. I think that might fit the bill as long as it doesn't wobble too much in a breeze.

#83 Jerry3672

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Posted 02 September 2008 - 11:01 PM

Good job Mike. Great night here in NC Too!!

How did you hold the mask in place?

You could be on to the reason why the clear transparency sheet hasn't worked for others.

#84 mikefulb

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Posted 02 September 2008 - 11:09 PM

Good job Mike. Great night here in NC Too!!

How did you hold the mask in place?

You could be on to the reason why the clear transparency sheet hasn't worked for others.


Yes it is spectacular weather here tonight. I'm at home so I'm doing H-alpha imaging from the suburbs. I hope to get to a dark site tomorrow however.

I just stuck 2 push pins into the wood frame at the right place so I could 'hang' the frame on the scope. If it were windy it might be a problem but I always carry blue painters tape for such occasions! :)

#85 Tomal

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Posted 03 September 2008 - 04:06 PM

I just made a mask from some cardboard, it works like a charm, very easy to focus with :)

#86 Nocturnal

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Posted 03 September 2008 - 04:20 PM

I just drew one with Inkscape and printed on regular paper. I hope to try it tonight. After cutting out the gaps of course :)

#87 groz

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Posted 03 September 2008 - 07:11 PM

I just drew one with Inkscape and printed on regular paper. I hope to try it tonight. After cutting out the gaps of course :)


I did one last night for an 80mm, the quick and dirty way. I grabbed the graphic out of an earlier post in this thread, scaled it up to 80mm diameter, and printed it. took the printout, and an exacto, sat out on the back deck for 30 minutes making fine cuts. The idea was, dont worry about a 'perfect' mask, just 'good enough' to take a look and test the concept.

After I had the mask cut, I put the ST-80 (normally used as a guider, but fits the small tripod) on a tripod, pointed at polaris, with a Canon 300D attached. I used the motofocus to make very small focus adjustments (about 1/20 of a second, at the low speed). Not the most precise measurents, but, I know that those kind of button pushes on the motofocus will take around a thousand of them to get a full rotation of the focus knob.

I've done this test in the past with software to measure FWHM and try find focus with this setup. Results vary from sporadic to inconclusive. I've never seen FWHM measurements give a conclusive result with this ST-80 and a dslr, have tried numerous methods.

I have 20 odd jpeg fine shots taken as I walked the camera thru focus in those small increments. There is absolutely no question which one is best focused, and there is absolutely no doubt it is moving between shots.

If I dont get to it tonite, over the weekend, I'm going to take the time to do a better job of cutting, using a strait edge to make the cuts, and do a coffee can lid that fits onto the 127mm mak perfectly. Based on what I saw last night, focussing issues for the DSLR on these telescopes is going to be a thing of the past. I've got an SCT coming next week, and if this works on the mak as well as it did on the little refractor, the mask will be waiting for the SCT when it arrives.

#88 snowdragonusa

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Posted 03 September 2008 - 07:22 PM

If anyone needs a scaled drawing of the one I made up let me know. I can try it out with your particular focal length and see if it scales properly. Download and install InkScape and you will be able to printout the vector based image and not have to worry about those nasty stair step edges caused from the jpg I posted earlier.

Cheers!

#89 isramirez

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Posted 03 September 2008 - 07:37 PM

If anyone needs a scaled drawing of the one I made up let me know.


What about for a maksutov with 180mm diameter and 2700mm focal length? it would be possible gentleman?

#90 snowdragonusa

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Posted 03 September 2008 - 08:04 PM

It is not an 'exact' scale but it is pretty close. Your scope would be 2700/150 or 200 so the slits would be 7-9mm slits 7-9mm apart. My scaled version would put it right about 5mm slits for your 180mm apature. Shot me your email in a PM and I'll email you the scaled version. Remember to download and install InkScape so you can view and print it out. :)

#91 groz

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Posted 03 September 2008 - 08:41 PM

If anyone needs a scaled drawing of the one I made up let me know. I can try it out with your particular focal length and see if it scales properly.


Some common sizes that I'm sure a lot of folks will be interested in.

2032mm fl, 200mm diameter (8 inch sct f/10)
1280mm fl, 200mm diameter (8 inch sct with .63 reducer)
4064mm fl, 200mm diameter (8 inch sct, 2x barlow, for planetary stuff)
585mm fl, 80mm diameter (Williams zs80)
1540mm fl, 127mm diameter (orion apex 127)
400mm fl, 80mm diameter (orion st80, common guidescope)

Ok, that covers most of what we have here :)

But this does bring up the interesting question. Will I still get useable diffraction spikes in all of the 8 inch configurations, from a single mask, or, does it really need the different spacings on different focal lenghts ?

#92 John Wunderlin

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Posted 03 September 2008 - 09:12 PM

Will I still get useable diffraction spikes in all of the 8 inch configurations, from a single mask, or, does it really need the different spacings on different focal lenghts ?


I tested our 12" model on both f/4.7 and f/10 10 inch scopes. Although the spike pattern is slightly different, both provided the diffraction spike pattern. Here are 2 rough images I shot for comparison:

f/4.7 scope

f/10 scope

Both provided usable patterns. Note that both of these images were in scopes that weren't currently tracking, so these are very fast exposures and are not as clear as they could be. At the time I just wanted to get the basic tests done.

#93 Jerry3672

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Posted 03 September 2008 - 10:02 PM

Big thanks to those in Pavels group for the original concept. If anyone here speaks Russian it sure would be nice to have someone to tell them thanks. I have sent e-mails to Pavel, but I'm afraid my russian translation is rough. I did get his web site translated. he offers a lot of information to the amateur astronomy community in Russia.

#94 groz

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Posted 03 September 2008 - 10:04 PM

Both provided usable patterns. Note that both of these images were in scopes that weren't currently tracking, so these are very fast exposures and are not as clear as they could be.


What would you call a 'very fast' exposure? I was using 1/2 second last night in the ST80, and got the bit below. In hindsight, I probably should have let it go a few seconds, allow the star to blow out a bit, and let the diffraction spikes come up better.

Attached Thumbnails

  • 2622547-test1.jpg


#95 snowdragonusa

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Posted 03 September 2008 - 10:12 PM

Groz, I just tried out a paper cutout for my 80mm scaled down from the original 35cm disc. The pattern was really distinct. I focused on my own first and then placed the mask over to see how close I got. While thought I was right on the diffraction pattern showed I was still off. Sure enough, after refocusing with the mask the image of Jupiter was spot on!

I think for 'optimal' performance the strict equation should be followed but to try it out and build a working model, the scaling should work just fine.

#96 Jerry3672

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Posted 03 September 2008 - 10:20 PM

Looks like a lot of this conversation deals with the math to get the correct cutout. The math formula is based off of a mag 0 star. It takes into account the focal ratio, but the bottom line is the light gathering power of the scope is what determines the cutout and not just the focal ratio.

The cut out size is not just a linear graph. so just scaling the 8 inch down for a 80mm scope will need a small tweak. The easy cheat method is just to cut the angled side just a bit bigger to balance the diffraction spikes. so if you have a 4.75 mm slot and 4.75 spacing on the horizontal side, then you should cut 5.25 mm slot and 4.25 spacing on the angled side. This will help balance the spikes.

To answer groz- yes! the same mask will work for all 8 inch scopes. The tweak in cut sizes is only if you want dead perfect diffraction spikes.

#97 cvedeler

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Posted 03 September 2008 - 10:49 PM

Holy precision focusing Batman!

I think I've just fallen in love. I have never focused so precisely so quickly with my astrophotography in 20 years of doing this hobby.

I went from putting my DSLR on my scope to PERFECT focus in about a minute. This is a test shot of M2 I just took. This is a 30 second unguided exposure, 100% crop. I've got stars down to almost a pixel of my Canon DSLr at 1200mm focal length!

Attached Thumbnails

  • 2622613-M2---30-sec.jpg


#98 isramirez

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Posted 03 September 2008 - 11:02 PM

It is not an 'exact' scale but it is pretty close. Your scope would be 2700/150 or 200 so the slits would be 7-9mm slits 7-9mm apart. My scaled version would put it right about 5mm slits for your 180mm apature. Shot me your email in a PM and I'll email you the scaled version. Remember to download and install InkScape so you can view and print it out. :)


Got the mail ...i'm praying the rain season finish soon here... thank you so much Pavel :)

#99 Dennis Sakva

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Posted 04 September 2008 - 01:36 AM

A few considerations.
You need to have cuts straight and clean or else additional diffraction effects will wash out the main pattern.
While the mask is quite forgiving to cuts sizes it is still recommended to keep close to those values. The mask is optimized to work on either first or third order spectrums (depending on multiplier). By deviating too much you may have a situation when the first orders spectrum is too tight, while third and consequential orders are too wide for precize focusing.
You may also try experimenting with exposure length. The longer the exposure the higher orders appear.

#100 nimthor

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Posted 04 September 2008 - 04:43 AM

Hi all!

This is the gratest thread ever!

Few questions ofthings I don't understand.

In the formula f/s=150-200
Firstly I guess this means 150 or 200 or does it mean 150 to 200, so that you can choose any number in between? What does these numbers mean and how do you decide what to use?

The angle is around 40deg to each other for the sloped side, any science behind this angle?

Thanks!
Charles


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