Gus_Smedstad
Pooh-Bah
Reged: 07/02/06
Posts: 1108
Loc: Boston 42° 16' N 71° 08' W
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I think it's fairly obvious that the final focal length is what matters. Which makes me think that I will need a different mask if I'm using a Powermate to reach 2000mm!
Fortunately for you, the 555mm vs. 444mm difference is well within the margin for error he posted. 3 x FL / (150-200) = 8-11 mm at 555mm, and 7-9mm at 444mm, so 8-9mm will suit either.
- Gus
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Gus Smedstad image gallery
Orion Atlas 8 EQ-G
Celestron Onyx 80EDF (guiding / widefields).
Orion Starshoot Autoguider
QHY8
Philips SPC900NC
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jgraham
Postmaster
Reged: 12/02/04
Posts: 6694
Loc: Dayton, Ohio
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Adam; it took me a while to appreciate the improvement in the light passing through the Bahtinov mask (I like that name) as compared to a Hartman. I also ran into the problem of the classic Hartman loosing sensitivity near the focus and I've been using one of the "+" designs that provides information all the way through the region of best focus like the Bahtinov mask, but it only works well on bright reference stars.
One question about the Bahtinov mask; is it alignment sensitive? In other words, must it be accurately centered to work well? The reason I ask is centering a mask is easy unless you're using a dew shield on a Cat, then it can get tricky.
This looks like a neat design and I'm looking forward to trying it.
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-John
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Homebuilt scopes from 4.25-16.5"
Meade LXD75-N6/SN6/SC8, DSX-90, ETX-60BB, ETX-125PE, DS-2130
Orion StarBlast, BinoViewers, Coronado PST
Rebel XT/XTi, DSI Pro (I, II, & III), DSI, LPI, Electronic Eyepiece, Phillips SPC900NC
Tasco 60mm Refractors
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suburbanskies
sage
Reged: 12/18/04
Posts: 244
Loc: New Jersey, USA
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Yes, recognize the inventor and pay a royalty. It is best for you, despite what is "allowable" within the rules.
Mark
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John Wunderlin
Vendor - Spike-a Focusing Mask
Reged: 10/01/04
Posts: 655
Loc: Mineral Point, Wi
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Quote:
One question about the Bahtinov mask; is it alignment sensitive?
It does not appear to be. I tested moving the mask's center point around and it did not appear to affect the image significantly. Of course, rotating the mask will rotate the pattern. If it is too far to one side or the other, it may affect the brightness of the affected diffraction spikes, but the center location appears to not change.
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John Wunderlin
Vendor - Spike-a Bahtinov Focusing Mask
Empire Ranch Observatory
Iowa County Astronomers
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Nocturnal
Pooh-Bah
Reged: 09/14/05
Posts: 1004
Loc: CT, USA
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This thing is absolutely AMAZING! I have an F/5.95 110mm refractor with a 0.8x FR. I guestimated the spacing of the mask to be 10mm or 5mm wide slits. Quickly drew one up in the drawing program mentioned above, printed and cut out the slits. I glued the paper mask on a piece of foam board with a hole that turned out to be bigger than the dew shield so the resulting pattern doesn't center. Bam! focus patter is exactly as 'advertised'. Instead of spending 20 minutes with Neb2's HFR focus with numbers jumping all over the place I was focused in about 2 minutes.
My advise: don't worry about the details. Make one from cheap material and try it out.
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Best,
Sander
C11-HyperStar on Atlas EQ-Q driven by EQMOD
William Optics M110 With FR-III/TRF-2008
DS2090 guide scope
QHY-8, DSI-Pro and DSI cameras
watec 802h video camera with KIWI OSD
Astro stuff: http://www.tungstentech.com
My Astro Photos
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Tomal
member
Reged: 03/24/08
Posts: 95
Loc: Norway
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Quote:
My advise: don't worry about the details. Make one from cheap material and try it out.
I second that!
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Celestron C8 203mm f/10
Meade 5000 series APO 80mm f/6
Sky-Watcher Newtonian 203mm f/5.9
Sky-Watcher Maksutov-Cassegrain 80mm f/12,5
Sky-Watcher Finder/Guide scope 50mm f/4
CG-5 GT
Canon 40D (Unmod)
SSAG
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snowdragonusa
Pooh-Bah
Reged: 09/04/07
Posts: 1125
Loc: Denver, CO
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Wow! What a sensation this little guy is! I wasn't sure what the inventor's last name was so I just stuck with Pavel. However, Bahtinov Mask really is appropriate.
So... The Amazing Bahtinov Mask it is! 
Anyway, I just sent out an email to the folk who wanted a scalable drawing I had down on my first design. I modified the drawing to work a little better for a refractor since those scopes do not have the center obstruction to worry about.
I did make a bit of a blunder though. I had asked folks to send me their aperture sizes instead of the full scope opening. oops 
In any event I hope you all find even better and more inventive ways to make, cut, paste, tape, glue (oh, maybe not glue), sew, tack, or otherwise adhere your new Bahtinov Masks to your scope.
A HUGE thanks again to Dennis for first introducing us all to this incredible new focusing aid. And of course, we need to heartily thank the inventor of this awesome design and for his generosity at giving this knowledge to us open source!
Thank you Mr. Bahtinov!!!
Here is the refractor modification.
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Adam
12" LX200R, AT80LE
Lunt 60mm Ha & CaK solar scopes
DMK41.AS, DSI Pro, DSI-C, LPI
Denver Astronomical Society
Brighton Astronomical Group
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Galaxyhunter
Pooh-Bah
Reged: 01/02/06
Posts: 1227
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O.K. Just so I understand the formula correctly, FL*3 / 175(mean)= the total width. Then the width is divided into a slot & a bar? My scopes FL is 2024. So that would be 2024*3=6072 /175=34.6mm. The slot would be 17mm & the bar would be 17mm, correct?
Does the first 20° slot intersect the circle center, cause from what I can tell, it looks like the two angeled slots intersect short of center, Or is this not critical?
Does the number of slots matter? 
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Carl
My lousy skies at Hawkeye Observatory
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Nocturnal
Pooh-Bah
Reged: 09/14/05
Posts: 1004
Loc: CT, USA
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Did you put the center strap off-center on purpose? I put mine in the middle. I suspect it doesn't matter but I'm curious if you had a reason.
Here are some pictures of my prototype and focus images:
http://gallery.tungstentech.com/main.php?g2_itemId=816
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Best,
Sander
C11-HyperStar on Atlas EQ-Q driven by EQMOD
William Optics M110 With FR-III/TRF-2008
DS2090 guide scope
QHY-8, DSI-Pro and DSI cameras
watec 802h video camera with KIWI OSD
Astro stuff: http://www.tungstentech.com
My Astro Photos
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snowdragonusa
Pooh-Bah
Reged: 09/04/07
Posts: 1125
Loc: Denver, CO
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Quote:
O.K. Just so I understand the formula correctly, FL*3 / 175(mean)= the total width. Then the width is divided into a slot & a bar? My scopes FL is 2024. So that would be 2024*3=6072 /175=34.6mm. The slot would be 17mm & the bar would be 17mm, correct?
Does the first 20° slot intersect the circle center, cause from what I can tell, it looks like the two angeled slots intersect short of center, Or is this not critical?
Does the number of slots matter?
Nope - leave the 3 out unless you are getting very small numbers (i.e. small focal lengths).
Yours would be 2024/150-200 = 10 to 14 mm. Now take that number and divide by 2 so you would have an alternating 5mm cutout and a 5mm strip etc.
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Adam
12" LX200R, AT80LE
Lunt 60mm Ha & CaK solar scopes
DMK41.AS, DSI Pro, DSI-C, LPI
Denver Astronomical Society
Brighton Astronomical Group
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snowdragonusa
Pooh-Bah
Reged: 09/04/07
Posts: 1125
Loc: Denver, CO
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Quote:
Did you put the center strap off-center on purpose? I put mine in the middle. I suspect it doesn't matter but I'm curious if you had a reason.
Naw, not really. I just quickly reworked my existing drawing to lessen the overall light blocking. The off center bar does not affect anything. It just makes a center brace for the overall structure and 'covers' where all the cutouts would otherwise meet.
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Adam
12" LX200R, AT80LE
Lunt 60mm Ha & CaK solar scopes
DMK41.AS, DSI Pro, DSI-C, LPI
Denver Astronomical Society
Brighton Astronomical Group
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Nocturnal
Pooh-Bah
Reged: 09/14/05
Posts: 1004
Loc: CT, USA
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Quote:
Naw, not really. I just quickly reworked my existing drawing to lessen the overall light blocking. The off center bar does not affect anything. It just makes a center brace for the overall structure and 'covers' where all the cutouts would otherwise meet.
Cool, thanks. I noticed when I refocused just now (DeepSkyStacker Live was showing a slight increase in FWHM) that there is a bit more focus travel from just outside to just right than I want. I'll have to look at optimizing this a bit more to see if I can make the pattern 'sharper'.
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Best,
Sander
C11-HyperStar on Atlas EQ-Q driven by EQMOD
William Optics M110 With FR-III/TRF-2008
DS2090 guide scope
QHY-8, DSI-Pro and DSI cameras
watec 802h video camera with KIWI OSD
Astro stuff: http://www.tungstentech.com
My Astro Photos
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snowdragonusa
Pooh-Bah
Reged: 09/04/07
Posts: 1125
Loc: Denver, CO
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Quote:
I'll have to look at optimizing this a bit more to see if I can make the pattern 'sharper'.
Yup, I am guessing that is where the focal length formual may come into play.
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Adam
12" LX200R, AT80LE
Lunt 60mm Ha & CaK solar scopes
DMK41.AS, DSI Pro, DSI-C, LPI
Denver Astronomical Society
Brighton Astronomical Group
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Gus_Smedstad
Pooh-Bah
Reged: 07/02/06
Posts: 1108
Loc: Boston 42° 16' N 71° 08' W
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Quote:
I glued the paper mask on a piece of foam board
Good idea. I was thinking about my struggles with my cardboard Hartman mask, and when you mentioned this I remembered I had a big piece of leftover foam board from a different project. I'll probably still have trouble cutting it out, but not like dealing with corrugation.
- Gus
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Gus Smedstad image gallery
Orion Atlas 8 EQ-G
Celestron Onyx 80EDF (guiding / widefields).
Orion Starshoot Autoguider
QHY8
Philips SPC900NC
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Nocturnal
Pooh-Bah
Reged: 09/14/05
Posts: 1004
Loc: CT, USA
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Quote:
Good idea. I was thinking about my struggles with my cardboard Hartman mask, and when you mentioned this I remembered I had a big piece of leftover foam board from a different project. I'll probably still have trouble cutting it out, but not like dealing with corrugation.
- Gus
Cutting a circle in foam board is tricky, especially when you're impatient to try things out Mine was 5mm thick so it worked OK with a thin and narrow X-acto blade.
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Best,
Sander
C11-HyperStar on Atlas EQ-Q driven by EQMOD
William Optics M110 With FR-III/TRF-2008
DS2090 guide scope
QHY-8, DSI-Pro and DSI cameras
watec 802h video camera with KIWI OSD
Astro stuff: http://www.tungstentech.com
My Astro Photos
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Gus_Smedstad
Pooh-Bah
Reged: 07/02/06
Posts: 1108
Loc: Boston 42° 16' N 71° 08' W
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I'm thinking I'm going to make a square mask, with tabs for the edges of the OTA. I don't want it to ride on the mirror / spider vanes anyway, so if it's on the outside, why does it have to be circular?
- Gus
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Gus Smedstad image gallery
Orion Atlas 8 EQ-G
Celestron Onyx 80EDF (guiding / widefields).
Orion Starshoot Autoguider
QHY8
Philips SPC900NC
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Nocturnal
Pooh-Bah
Reged: 09/14/05
Posts: 1004
Loc: CT, USA
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Quote:
I'm thinking I'm going to make a square mask, with tabs for the edges of the OTA. I don't want it to ride on the mirror / spider vanes anyway, so if it's on the outside, why does it have to be circular?
- Gus
It doesn't need to be round at all I just wanted it to fit the dewshield on my refractor. Post a pic of what you came up with!
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Best,
Sander
C11-HyperStar on Atlas EQ-Q driven by EQMOD
William Optics M110 With FR-III/TRF-2008
DS2090 guide scope
QHY-8, DSI-Pro and DSI cameras
watec 802h video camera with KIWI OSD
Astro stuff: http://www.tungstentech.com
My Astro Photos
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snowdragonusa
Pooh-Bah
Reged: 09/04/07
Posts: 1125
Loc: Denver, CO
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Quote:
I'm thinking I'm going to make a square mask, with tabs for the edges of the OTA. I don't want it to ride on the mirror / spider vanes anyway, so if it's on the outside, why does it have to be circular?
- Gus
Really the only reason would be to conserve on material if you are working with more expensive stuff. Also to optimize the amount of cutting you are doing.
But if you have extra material and time why not square, or triangular, or perhaps oblong! 
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Adam
12" LX200R, AT80LE
Lunt 60mm Ha & CaK solar scopes
DMK41.AS, DSI Pro, DSI-C, LPI
Denver Astronomical Society
Brighton Astronomical Group
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Gus_Smedstad
Pooh-Bah
Reged: 07/02/06
Posts: 1108
Loc: Boston 42° 16' N 71° 08' W
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I imagine that any material we'd make a mask from would be rectangular initially. So if you're making it a circle, the savings of material would only give you neigh-useless corner pieces. And I'm thinking square because I'm clumsy and I imagine the cutting will be easier.
I'd think the main reason to go with a circle is that it's much, much more aesthetic.
- Gus
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Gus Smedstad image gallery
Orion Atlas 8 EQ-G
Celestron Onyx 80EDF (guiding / widefields).
Orion Starshoot Autoguider
QHY8
Philips SPC900NC
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cvedeler
Carpal Tunnel
Reged: 12/20/05
Posts: 2124
Loc: Scottsdale, AZ
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Mine ain't pretty, but it sure is good.
I made mine from a plastic For Sale sign. I grabbed the image Adam was kind enough to post here, scaled it to fit the outside of my dew shield, printed it, taped it on the back of the plastic sign and with the help of a straight edge and a razor blade cut out all the parts.
I went with the thin plastic sign as I knew all those cuts would be a chore and I wanted it to cut easy. After it was all cut up it was much more flimsy than I had anticipated. That was fixed by using Popsicle sticks and a glue gun. As a testimony to the wide tolerances of the design, one of the popsicle sticks blocks most of one of the openings. I had some old plastic window blind parts that act as angle bars I glue gunned on with some black felt that hold the mask to the dew shield. It works great as long as the scope is pointed up. Luckily for my design, that is where most of the stars are.
I didn't worry about the size or spacing. I just scaled Adams image to fit my scope. The details of this design aren't that important I think as mine worked flawlessly as advertised. $2.50 for a For Sale sign and 30 minutes of my time and, vola a highly precision focusing instrument.
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Chris Vedeler
Astro-Physics 160EDF
Astro-Physics 900GTO
Q453HR / QHY8 CCD camera
Canon 450XSi
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www.aznightsky.com
Scottsdale, AZ
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