Thanks Dennis, I'll get back to you some time after this weekend as I'm on call through Tuesday so I can't stray far right now.
I have two designs I'd like to compare to this mask. One of them, upon inspection, functions using a very similar mechanism to Bahtinov's design, however his will likely work on dimmer stars due to it only masking about 50% of the light.
One difference that I am speculating on is behaviour during poor seeing. I find that the design that is similar to Bahtinov's dances around a lot due to seeing, while my other design is much more stable with the trade-off being slightly less precision. I'm not sure how his design handles seeing, given I have not tried it out yet.
I'll PM you the designs and a focus movie to look at.
Suggested prototyping materials:
A4 card stock, two sheets wide with some overlap, joined using spray adhesive. You can print to the cards and then cut with a sharp knife after joining them.
3mm or 6mm Depron foam (or the card stock backed foam mentioned elsewhere in this thread) can be used in conjunction with the above to add some rigidity to the design without adding significant weight. The spray adhesive works well here. Sharp corners can be a bit harder in Depron, however if backed with the card stock, precision cuts in the Depron become less critical as the card stock will mask the light anyway.
An observation/speculation on focus being slightly out after using the mask: Collimation can dramatically impact the performance of these masks, as could slight imperfections in the curvature of your optics. Based on information in a few Astrophotography books that I have, the Bahtinov design is similar to mirror testing Hartmann masks which are designed to highlight the imperfections in curvature of lenses and mirrors. Having open regions on such a large portion of the mask could possibly have this negative side effect, as a trade-off for brightness and contrast.
Try rotating the mask 90 or 120 degrees on the OTA and see if it still shows focus as perfect. If it does, then my speculation above is likely incorrect
One difference that I am speculating on is behaviour during poor seeing. I find that the design that is similar to Bahtinov's dances around a lot due to seeing, while my other design is much more stable with the trade-off being slightly less precision. I'm not sure how his design handles seeing, given I have not tried it out yet.
An observation/speculation on focus being slightly out after using the mask: Collimation can dramatically impact the performance of these masks, as could slight imperfections in the curvature of your optics. Based on information in a few Astrophotography books that I have, the Bahtinov design is similar to mirror testing Hartmann masks which are designed to highlight the imperfections in curvature of lenses and mirrors. Having open regions on such a large portion of the mask could possibly have this negative side effect, as a trade-off for brightness and contrast.
Try rotating the mask 90 or 120 degrees on the OTA and see if it still shows focus as perfect. If it does, then my speculation above is likely incorrect
Regards,
Eric
I think you will find that the seeing conditions causing the Bahtinov mask to "dance" is more useful to capture the best average focus over a mask that is not a sensitive. My method is to focus on or near your image target. Use the same camera settings to focus as you plan to image, this will yield the best average focus in your image. In my opinion this is the best feature of the Bahtinov mask. Focusing on dim stars.
Collimation will affect the mask, just as it affects the final image. Rotating the mask 90 and 180 degrees will test collimation. However, if your mask is not centered or lines are not a mirror, then this will also magnify the error.
The best method to test various designs is to use a deep sky target image and measure the results with something like CCD inspector. Look for the best FWHM value. After all, this is intended to be used for imaging.
Lots of people have claimed the device works just because they see diffraction spikes that move with the focus knob. Just my 2 cents worth.
>snip
An observation/speculation on focus being slightly out after using the mask: Collimation can dramatically impact the performance of these masks, as could slight imperfections in the curvature of your optics. Based on information in a few Astrophotography books that I have, the Bahtinov design is similar to mirror testing Hartmann masks which are designed to highlight the imperfections in curvature of lenses and mirrors. Having open regions on such a large portion of the mask could possibly have this negative side effect, as a trade-off for brightness and contrast.
Try rotating the mask 90 or 120 degrees on the OTA and see if it still shows focus as perfect. If it does, then my speculation above is likely incorrect
Quote:
Originally Posted by jerry3672
>snip
However, if your mask is not centered or lines are not a mirror, then this will also magnify the error.
Hi Guys,
I think that I have bottomed out the reason for the slight focus variation in my original A4 transparency cut-out, Bahtinov style mask.
The mask wasn’t glued around the entire circumference as the narrow dimension of the A4 sheet of transparency did not quite stretch all the way to the edges and the unsupported edges introduced some flexing into the mask.
I inspected the templates I drew and discovered that the “snap to” function had aligned the bottom and top edges of the bars at 20° and not their centre lines, so as Jerry comments, the two sides were not a mirror reflection.
The focusing error did not appear to be noticeable by eyeball inspection. It only became apparent when I fitted the SBIG ST7 and used the CCDSoft focusing routine which I includes a FWHM and maximum value function.
I have since made a new mask from A4 Transparency that fixes the above issues and I am just waiting on an opportunity to test it out.
I have found that with poor seeing (I had some 4-5/10) the Bahtinov mask appears to still perform very effectively, even with exposures of around 2 secs to average out the seeing.
Had an afternoon of heavy rain today so I dabbled about in the shed and produced a prototype mask from aluminium and fishing line. It's 250mm square inside the outer flange of the aluminium so it will slide snugly over the C8 objective with the dew heater in place.
The frame is made from 20x20X1 Al angle. The two cross pieces in the middle are from 12x3 Al flat. Filaments are 12mm apart. The angle differs from a true Bahtinov Mask a little bit - I made it 1:3 to aid marking out.
I ballsed up the threading of the mask though. The filaments should come in the front of the outer frame and the back of the cross bars... but I'm not about to re-string it yet, I'll try it out first.
That looks like an interesting design; I would be very keen to see the results. Good luck with the weather and subsequent testing.
Cheers
Dennis
It'll be an interesting experiment, for me at least. Maybe I'm over simplifying things a bit, but I'm interested to see if it will do the same thing. If it works, it should maximise the amount of light entering the scope so focus should be possible on a wider range of stars or less change to camera settings... maybe...
Jerry's comments about symmetry could be a cause for concern though. As you can see from the photos the angled filaments are not quite a mirror image (bit of a fitting error there).
that looks like it would work.. I'm interested to hear your results on that..
How difficult is it to make clean cuts in foam core board? I think that would be the cheapest way to test many of the different designs, then once you've settled on the one that works best for your setup, take that foam core mask to the local lazer cutters and get one cut out of acrylic or delrin..
In terms of constructing prototypes, I’ve found the most flexible and efficient technique so far (for me) is to draw up the templates in a drawing application, such as Corel Draw and then either:
Print off the design on an A4 transparency and then cut out the gaps with a sharp Stanley knife and steel ruler, or
Print off a mask outline on plain, white A4 paper and use strips of tape to create the mask, aligning the strips of tape to the printed out template.
I have used the magnetic tape out of an audio cassette and some 6mm tape from my wife’s sewing kit. This way, you only measure once at the drawing stage and don’t have to mark out the work.
Have been very interested in this thread - thanks Dennis et al.
I finally managed to make one my self and will test it tonight on the 5" to see how it performs. Should be very interesting.
Have been very interested in this thread - thanks Dennis et al.
I finally managed to make one my self and will test it tonight on the 5" to see how it performs. Should be very interesting.
Hi Allan
Some of the nice features about the Bahtinov Mask are that you can quickly establish which side of focus you are on and then converge to focus quite rapidly.
For my SBIG ST7, I usually tweak the final focus using the CCDSoft focusing routine. For the DMK and Canon 40D, I just use the Bahtinov Mask as I don’t use software assisted focusing with these cameras. Remote Live View on the 40D is a great asset!
i've found focusing the qhy8 through maxim or nebulosity ok, but i did feel at times that i was a little uncertain and went back and forth a bit to find good focus.. I'm working on a mask for the c11 and the refractors at the moment.
I'd love to see someone make a template of some sort for a variety of scopes of varying apertures etc.
eg: ED80, 6", 8", 9.25", 10", 11", 12" etc
Hi Mike
Some bright guy on CN has written an on-line Bahtinov Mask Generator. Basically, you type in certain values for your ‘scope and then the application generates a file, with an extension of .svg (Scalable Vector Graphic) which most drawing applications can open.
However, I have tried this free on-line generator and the .svg file it produced gives me an IO error in my drawing application, Corel Draw.
Whether I have over protective Anti-Virus, Anti-Phishing and Firewall software and settings on my PC, I’m not sure. However, if you want to type in your parameters, generate the .svg file and e-mail me a copy, I’ll try to open in and if that succeeds (unlikely!) I can convert it into a PSD, TIF, BMP, etc.
Here is an image of Nunki using the Bahtinov Mask on the Mewlon 180 with the DBK21AF04.AS colour CCD. I had a TeleVue x3 Barlow plugged in. The attached file shows the separate R, G and B Channels as well as the original RGB image.
It looks like the mask is acting as a diffraction grating that splits the white light into its Blue, Green and Red components, presumably generating mono chromatic diffraction rings located at the points indicated by the respective coloured arcs.
Remember, whilst this particular mask was designed with the formula of Focal Length (=2160mm)/180, resulting in a Bar Width=6mm and Bar Gap=6mm, the image was captured at x3 the focal length, 6480mm.
Hi
I tried the link and it works well. Make sure you dont use decimal places in your specs
frank
Quote:
Originally Posted by Dennis
Hello,
Here is an image of Nunki using the Bahtinov Mask on the Mewlon 180 with the DBK21AF04.AS colour CCD. I had a TeleVue x3 Barlow plugged in. The attached file shows the separate R, G and B Channels as well as the original RGB image.
It looks like the mask is acting as a diffraction grating that splits the white light into its Blue, Green and Red components, presumably generating mono chromatic diffraction rings located at the points indicated by the respective coloured arcs.
Remember, whilst this particular mask was designed with the formula of Focal Length (=2160mm)/180, resulting in a Bar Width=6mm and Bar Gap=6mm, the image was captured at x3 the focal length, 6480mm.
Well, I just tried this out visually on my Megrez 90 against my other masks, and have to say that it does definitely work as advertised. I was able to tell with precision where the critical focus zone was within at least 1/8 of a turn on my 10:1 Crayford focuser, even with my non-perfect eyes. It was so much better that, when the wind blew my old mask under the lawn mower, I didn't bother going after it
I do have one unresolved curiosity. I notice that a lot of people are using a circular mask to define the outside edge of their diffraction slits. The ends of these slits are not squared off, and I'm curious how much of an impact this will have on the diffraction pattern. Attached is the shape of the mask that I made.
Thanks to Dennis's motivation and Houghy(David's) post of the mask for the 100ED which I re-scaled using the photocopier at work, then pasted the mask onto slightly thicker card, painstakingly with ruler and scaple cut out the bits. Now to make a frame for regidity
Call me tight but $82 for one is a bit steep and here is the finished product hopefully it works.
I copied this jpeg and then in Irfanview rescaled it for printing out the appropriate size onto A4 paper. It gave me masks that worked perfectly on my ED80 and 127mm scope without any alterations except for the size.
What a really pleasant surprise and focus is absolute. I was never sure before where exact focus lay but this nails it perfectly. Just so easy to use.
Again thanks Denis and Mr Bahtinov.
Allan
>snip
I do have one unresolved curiosity. I notice that a lot of people are using a circular mask to define the outside edge of their diffraction slits. The ends of these slits are not squared off, and I'm curious how much of an impact this will have on the diffraction pattern. Attached is the shape of the mask that I made.
Any comments?
Regards,
Eric
Hi Eric
The inside edge of the outer circular rim of my Bahtinov Mask is of a larger diameter than my mirror, so my mirror never “sees” this edge, although it does "see" it's own edge, i.e. the edge of the mirror.
The inside edge of the outer circular rim of my Bahtinov Mask is of a larger diameter than my mirror, so my mirror never “sees” this edge, although it does "see" it's own edge, i.e. the edge of the mirror.
Cheers
Dennis
Thanks Dennis.
On further thought, the diffraction pattern is going to be spread along the slits width wise, not length wise, so the ends probably don't come into play anyway.
Ah, my wife is done with the camera, time to go steal it back!
. The filaments should come in the front of the outer frame and the back of the cross bars... but I'm not about to re-string it yet, I'll try it out first.
or less change to camera settings... maybe... 
