Had to post in here... I used my mask on the M102 tonight... first time I've used it on the refractor... Its just.... Bliss.... normally I'd spend at least 25 minutes getting it just right, and even then I'd be questioning it from time to time... Not tonight...
I never thought I'd say it... focusing is one of lifes simple pleasures !
Question - is anyone making these at all now? Silly trying to get one from someone before Lostock - so I guess I'll attempt to borrow one up from some poor sap there (I'm coming for you h0ughy!!) and see how they work.
Anyways, just checking to see that someone either is or isn't producing any more....
you are a legend, you are super sleuth of the year for finding this one
David,
Did you say you have some of these mask made if you did how do i go in getting made .I have a Meade lxd75 200mm sn can you help or point me in the right direction
David,
Did you say you have some of these mask made if you did how do i go in getting made .I have a Meade lxd75 200mm sn can you help or point me in the right direction
Ian C
Ian read the entire thread and look at the attachments - if you cant figure it out by then then I will draw something up for you - but you then have to get it made (see previous posts)
OK - a bit behind the times, but I received mine yesterday arvo in the mail. Trying it briefly last night on a couple of bright stars I suddenly see everyones' point - that's the quickest perfect focus I've ever managed. Three seconds and it was all done. Thank you Pavel. Brilliantly-simple ideas work the best.
Yep – here’s a single raw frame from the Canon 40D showing the Bahtinov Mask hard at work, in full, glorious technicolour! The example is a full sized crop from the centre of the frame, no processing performed.
This idea of using different holes to use one mask for multiple small refractor scopes is quiet a neat idea. So you can use the same mask from 80mm to 110mm http://www.focus-mask.com/MORE_INFO.html
Mine only works for me on bright stars. It's very hard to see the bands on anything but the brightest of stars. However, I only use it with live view on the 40D at the rear end of VC200L.
This idea of using different holes to use one mask for multiple small refractor scopes is quiet a neat idea. So you can use the same mask from 80mm to 110mm http://www.focus-mask.com/MORE_INFO.html
Hi Fahim
I saw this website on CN in the DSLR Group, where it generated a bit of comment as the site originally didn’t appear to acknowledge Pavel Bahtinov as the originator of the mask. I think the site was then soon updated to mention his name.
Mine only works for me on bright stars. It's very hard to see the bands on anything but the brightest of stars. However, I only use it with live view on the 40D at the rear end of VC200L.
Rather than just looking on live view, try taking a 3 sec exposure (obviously with tracking on) I found the same thing with my CCD, doing the short 1sec preview exposure on dimmer stars it was near impossible to make out the diffraction spikes, however with a 3sec exposure, then inspecting the image at full resolution, it is rather easy to see even on dimmer (mag 4~5) stars..
Over the last two days I've read various thread on numerous forums about Bahtinov masks with great interest. These discussions have raised a couple of questions in my mind which I don't recall have been fully explored in these threads.
1) My 8"LX200 has a very long dew sheild. Does it make any significant difference how far in front of the scope mirror/lens the mask is placed.
2) I have an Orion 80ED and an 8" LX200. The LX200 can have anything from a f3.3 focal reducer to a 4x barlow shoved on the end meaning the focal length can vary from 660cm to 8000cm. The formulae for designing these masks includes the focal length of the system, yet the one of the commercial retailers for these masks (http://www.spike-a.com/) just sells them by OTA diameter. So is it really necessary for me to have 5 different masks for my LX200 (f3.3 focal reducer, f6.5 focal reducer, normal, 2x barlow and 4 x barlow) or is a single mask adequate?
3) In relation to question 2 ,are the sensitive of masks to different focal lengths more, less or insensitive to whether the mask is a first order or a third order mask?
4) What are the advantages of a 1st order mask over a 3rd order mask?
I have used a single, scope specific Bahtinov Mask on my ‘scopes using Reducer/Correctors, at their native focal length and using x2.5 and x4 PowerMates and have had no problems with focusing. The ‘scopes are a Celestron C9.25 at F6.3, F10 and up to F40 and my Tak Mewlon 180 at F9.6, F12 and F48.
My understanding of diffraction is somewhat piecemeal, so I welcome any corrections if the following is incorrect:
At focus, the edges of the bars in the Bahtinov Mask produce a diffraction pattern (like a diffraction grating) and as there are a many edges, the pattern is very strong and noticeable compared to that of an unobstructed circular aperture, such as a refractor.
In an unobstructed telescope (e.g. refractor) most of the light from a point source, such as a star, forms a bright central spot, called the Airy disc. From memory, some 80% plus of the light is concentrated in this bright central spot. Surrounding this bright central spot, you may see some fainter concentric rings. With my Vixen 4” refractor and Tak Mewlon 180, it is very easy to see the 1st and sometimes the 2nd in-focus rings on nights of steady seeing. Note that I’m referring to the in-focus Airy rings, and not the de-focused intra- and extra focus rings we see when close to focus.
The central bright spot is normally called the Airy disc. The subsequent concentric diffraction rings drop off in intensity quite rapidly and I understand that the first ring is referred to as the “1st order” and the 2nd ring in referred to as the “2nd order” diffraction ring.
If the above is correct, and the use of FL/N=S and FLx3/N=S formula caters for the 1st and 3rd order diffraction rings, then I have seen both effects in my test images and focus is still precisely achieved by making the central “moving” bar bisect the shallow “X” stationary bar.
The Pavel Bahtinov mask thread has been very interesting to me, even though I am using a Newtonian I thought it may come in handy as my eyesight ain't the best these days and any help is worthwhile.
So I figured I'd make one myself but not in the usual print out and cut out manner.
Here's what I did.(please refer attached photos)
1. Down to Bunnings, purchased some 9mm and 3mm mdf.
2. Marked out required dimensions on the 9mm (for me to suit a 12" dob)
3. Used a router to cut out the basic round mask shape including a shoulder or lip on outer circumference so mask fits nicely into OTA without dropping in.
4. Used a jigsaw to cut out the circle centre segments.
5. Used a drop saw to cut many 9mm slats for applying to mask frame (a cicular saw could be also used).
6. Applied 20 degree slats with hot melt glue (nice and quick). Used every other slat as a spacer for equal spacings.
7. Applied the perpendicular to centre slats.
8. Trimmed slats overhang by turning mask over and running router around it again.
9. Quick sand and screwed to an old broomstick hammered into the ground then painted with White Knight black chalk board paint.
10. Attached to scope. Worked first up, amazing! perfect.
Total cost around $10. $10 for the mdf (enough for two masks for a 12" so plenty for smaller scopes), $16 for chalkboard paint but only used one quarter of a can for this mask.
(above costs of course assume you have the tools)
Total time, around 3 hours
Thankyou Pavel, no more guesswork, YIPEE
Last edited by Michael Clark; 01-12-2008 at 05:10 PM.
Reason: wanted to indicate time taken
Great stuff Michael Looks great, and will last forever.
I used mine in anger last night - and I have to say that it was a very difficult thing to do. A 102mm refractor coupled with a digital SLR doesn't let a whole lot of light through to the viewfinder. It was quite difficult to actually see any diffraction spikes whatsoever. Everything was quite small and very dim. Very hard to see.
It was fine with an eyepiece - worked a treat - but with a camera it was a different matter. Maybe live focus will be the only way because I can enlarge the image. I'll go with Alex's reccommendation to do 3 sec exposures. At least then the spikes will give me definitive focus information rather than "less fuzzy" or "more fuzzy" that you get with just a naked star and no mask.
I have not tried mine yet with the camera, no doubt it will be different to looking through the eyepiece.
I haven't had any experience with an SLR, just video, but I would imagine that to get a decent image to check focus you would need to do a few processed exposures, but I think it's worth it, "less fuzzy/more fuzzy" will become a thing of the past.
I haven't looked throught the entire thread, but has anyone commented on the possibility that once focussed the central spikes (i.e. the pair that move back and forth according to the focus) appear to move back and forth from the middle at an almost measurable slow rate and amplitude according to the seeing? Seems to offer the prospect of a way of comparing seeing on different occasions?
Let me see if I can answer a few of the questions:
Nightcal-
1-You can place the mask over the dew shield. It will work just fine.
2-It is not critical to make a mask that is optimized for focal length. The calculation for the cutout is to minimize the distance to the first order spectrum. In reality most use the third order anyway.
3- Great question! According to Pavel Bahtinov The first order is the most accurate to focus by because it is closer to the center. However, the third order works just as good if you over expose the image like you would do using live view anyway. I have an image that shows the first and third order on what you would see using live view.
4- The shift in the first order and third order move equally with the focus. So no real advantage on either.
Andrew-
Yes the spikes do move as the focal point moves, therefore as seeing changes the spikes move. This is an excellent point and a method to judge seeing.
A CN Forum member (amateur) has written an application for measuring the focus accuracy of your Bahtinov Mask! There is nothing quite like introducing an elegant element of computer complexity into such a mechanically simple mask!
It looks like the (free) application overlays the 3 close-to-focus diffraction bars over a live image and then provides a numeric readout (pixels) of how close to focus you are.
I have downloaded the application and will try to find an old focusing AVI to test it.
On the third link is a variant of Bahtinov mask with threads..
I was always wandering if I can build something similar to tennis racket (but with thread pattern that will produce diffraction spikes useable for focusing.
This guy was a bit faster than me...
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