Perfect focus each time using a Bahtinov Mask

[NightCal]

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?

[Dennis]

Hi Michael

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.

Cheers

Dennis

[Michael Clark]

Hi All,

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:2thumbs:.

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:)

[Omaroo]

Great stuff Michael :) :thumbsup: 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.

[Michael Clark]

Thanks for the comments Chris,

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.

Cheers
Michael

[Andrew C]

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?

[jerry3672]

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.

[Dennis]

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.

Cheers

Dennis

LINKS:

CN post with program link (http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/2961332/page/0/view/collapsed/sb/5/o/all/fpart/1)
Original post by amateur showing his application. (http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/2950416/page/1/view/collapsed/sb/5/o/all/fpart/1)
3 string approach by amateur (http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/2926119/page/0/view/collapsed/sb/5/o/all/fpart/1)

[bojan]

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...:thumbsup:

[allan gould]

There is also a 3 holed hartman mask variant with grid lines in the holes to give a bahtinov mask.

[Dennis]

The Mewlon has a 3 vane spider which effectively acts in a similar manner to the Bahtinov Mask when the OTA is pointed at bright stars.

The image generated by the 3 vane spider is not as clearly defined as the image generated by the Bahtinov Mask, especially on fainter stars, so focus is easier to establish and far more certain due to the multiple diffracting elements of the Bahtinov Mask.

For laymen such as myself, who wouldn’t know a Fast Fourier Transform if it hit them on the head, regardless of whether it was in the time or frequency domain, I find the diagrams and explanations both on IIS and CN very educational and fascinating to read about.:thumbsup:

Cheers

Dennis

[Dennis]

I was browsing the Bintel website (https://www.bintelshop.com.au/Catalog.aspx?category=imaging%20and %20accessories&page=3)and noticed that they now offer the Bahtinov Mask in a range of sizes from 14” down to 80mm, with some ‘scope specific sizes such as the C9.25.

Prices range from $25 – $59 and they look like the aluminium product from the Kendrick range, who also have a flexible plastic film range (http://www.kendrickastro.com/astro/kwikfocus.html)due out in Feb 2009.

Cheers

Dennis

[iceman]

I bought one from Bintel at SPSP - it's the flexible plastic one made my Kendrick.

I got one for my ED80 for $25. I haven't used it yet, but it looks good!

[Dennis]

Thanks Mike – I was interested in the 80mm thin, flexible plastic film model for my camera bag for the 400mm F5.6 but I only saw the alu models on the Bintel website.

Cheers

Dennis

[Dennis]

The developer of the “Bahtinov Grabber (http://www.njnoordhoek.com/?p=330)” application (Niels Noordhoek) has now added an auto focus routine to his (free) software!

The Bahtinov Grabber application overlays a 3 line grid on top of the close-to-focus diffraction spikes and produces a directional indicator (+ or -) and a numerical read out of focus accuracy to assist with finding optimum focus. Link (http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3142242/page/0/view/collapsed/sb/5/o/all/fpart/1)to post on CN.

Cheers

Dennis

[peter_4059]

Hi Dennis,

Were you able to download the focus control version - the link seems to be to the older version.

Peter

[Dennis]

Hi Peter

I had planned to build an artificial star over the next couple of days and use it in the back garden with one of my OTA’s, so I can play with the auto focus application. So, although I have downloaded the application from the link, I haven’t unzipped/installed it yet.

I just unzipped the exe as I was writing this reply, and it shows a date of 23/05/2009 – is that what you get?

Cheers

Dennis

[peter_4059]

I get 22/5. When you run the app do you see the focus control buttons as per his screen shot?

[Dennis]

Hi Peter

No – I don’t see the buttons, but I was wondering if they only activate when it recognizes an ASCOM compatible focuser is in the loop?

Cheers

Dennis

[peter_4059]

Hi Dennis,

I tried it tonight with the FCUSB ascom driver installed and the FCUSB connected but no sign of the focus control buttons?

Let me know if you manage to get it working.

Peter

[Dennis]

Hi Peter

I just saw a post on the Stark Labs Yahoo Groups that indicates the auto focus version does not yet seem to be available for download.

Oh well, there goes my artificial star project for this weekend – I’ll have to build something else instead!

Cheers

Dennis

[peter_4059]

Thanks Dennis,

I noticed the same thing here

http://www.njnoordhoek.com/?p=330

[Dennis]

The chap from The Netherlands whose nom de plume on CN is “amateur” has posted a new link to the Bahtinov Grabber software and I think it is the auto focus version?

Website (http://www.njnoordhoek.com/?p=330)
CN Post (http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3189307/page/0/view/collapsed/sb/5/o/all/fpart/1)

Cheers

Dennis

[peter_4059]

have you tried it Dennis?

[Dennis]

Not yet Peter – I’m currently investigating sources of flexure on my side by side set and killing them off one by one – nasty vermin they are! So, the few clear nights we’ve had have been given over to that project.

At the moment, manually focusing with the Bahtinov Mask gives me great results, but I will try the SW somewhere down the line. I’m thinking of the Canon 40D in Remote Live View Mode as a starting point.

Cheers

Dennis

[RB]

Thanks once again for the heads-up Dennis.

:thumbsup:

[stephenb]

I bought the Kendrick 'flexible plastic ones" from BinTel for $29 for the 8". So far I have used it only for visual and a bit of experimental imaging, but I am amazed that it worked so well! I wonder why I even bothered to attempt to make my own for this price. Thanks Mr. Bahtoniv, I do hope you are getting some financial recognition from this?

[Dennis]

Here is a 5 second exposure of Antares at ISO800 with the Canon 40D at the prime focus of the Mewlon 180 F12 (2160mm) with my Bahtinov Mask fitted. The image is an 800x600 crop from the centre of the frame. Seeing was estimated to be 5-6/10 for the session.

Cheers

Dennis

[ozstockman]

Guys,

I bought a laser cutting/engraving machine for my business and can cut any vector or even raster graphic from acrylic or mdf panels. This includes these Bahtinov Masks. It will be very cheap if it's just cut from 3mm MDF in a round shape and you are ready to paint a mask yourself and find a solution for mounting a mask onto your scope. The maximum diameter I can cut is 11"(working area of my laser is 24x12") and let say for easy pricing it will be $1 for each inch of your scope aperture + real postage cost. Usually it's from 55c to $1.10.

I can also cut it from foam panels but have no idea how much they are.

[stephenb]

Can you cut foam, for example, round holes in a foam block for an eyepiece box?

[ozstockman]

Stephen, I have just sent you a PM.

[ozstockman]

Sorry, I have to correct myself about pricing for masks. It takes a bit longer than I have expected to cut such a complex design where cuts are very close to each other. Plus there is an extra time to give it a good sanding for stunning look.
However it is still super low price of $1.5 per each inch of your scope aperture. So for example this mask cut from 3mm MDF for Vixen VC200L is just $12 + $1.10 for shipping.
I have figured out that I cannot ship it for 55c as mask can be bent or broken if it's sent without any protection. I have to put it on another sheet of 3mm MDF and bubble wrap it which makes the whole package more 5mm(55c postage cost) thick but it's still under 20mm which is just $1.10.

So if anyone interested please PM me. I have one for Vixen VC200L(full focal length) ready that fits perfectly into the OTA without even worrying about any mounting solution. I am gonna test it today if weather is ok

[ozstockman]

I have just posted an ad here (http://www.iceinspace.com.au/forum/forumdisplay.php?f=30) with prices for both mdf and acrylic masks and pictures.

[h0ughy]

this was the first use of my cusom made mask that i drew up and designed from the formula. Acrux was the target. Taken at Qld astrofest 2009. it was a joy to focus:thumbsup:

[Dennis]

Hi Dave

Very nice! From the image scale, this looks like it was taken through your main ‘scope, not a smaller ‘scope such as an ED80? Was taken with the modified 350D?

Since Pavel “invented” these masks, I actually enjoy the whole focusing process, something I dreaded pre-Bahtinov!:)

Cheers

Dennis

[h0ughy]

yes Dennis, it was through the 127ED with the cooled 350D.

it is just fantastic to click off a 6 second shot, preview it and see the instand result, then slightly adjust the focus and do it again. I never would have been able to do this normally:D

[sheeny]

What's a bloke to do on a rainy weekend but to tinker in the shed?:shrug:

If you like to build your own stuff or can't afford to buy masks for all your scopes and lenses or you aren't happy with the amount of light your existing Bahtinov mask lets through (or maybe you're just looking for an excuse to spent some quiet time in the shed;)) then maybe a Bahtinov Racquet is the go for you.

I used a jig saw to cut the frames from some scrap 3mm MDF, but there's a couple of tricks to making these.

Firstly I drew them full scale in CAD, including the lines for the strings and the centrelines and PCD for the holes. I printed the patterns full size, and used "Clag" school/paper glue (from the newsagents) to stick them to the MDF.

Then use the jigsaw to cut out the frames.

I drew the holes in the pattern at 3mm diameter, but only used a 2mm drill. This meant I had a circle left drawn around the hole after drilling so I could check the holes was accurately in the right place. To help achieve this I used a centre punch and pushed a "centre pop" where each hole was to go. If the centre pop wasn't quite right, it's easy to fix by just just working the centre punch in the direction the hole has to move until the pop mark is central to the hole circle on the pattern:).

Once it is cut out and drilled, just peel the pattern off. The wax in the MDF doesn't allow the clag glue to permanently stick to the board:thumbsup:. I'm not sure if it will work the same with other materials like Ply, or plastic, but it's worth a try.

Sand it up and scrape the holes a little with a reamer of some sort (I used an old hacksaw blade I ground to a wicked little concave point:)) and spray with clear lacquer if you want. In my usual impatience I didn't let my lacquered frames dry long enough:rolleyes:... they were dry to touch but after the handling required to string the things, they were tacky and covered in finger prints (they are definitely mine!:lol:).

The ED80 racquet I cut and glued a piece of thicker MDF to the frame to support it on the scope, but for the C11 and my camera lenses racquets I used some recycled shelf supports:thumbsup:.

I'm yet to string the camera lens racquet, that'll probably be today's job.:)

The centre bar to support the strings I made 8mm wide in all cases, though I could've possibly gone as narrow as 6 for the camera lens racquet:shrug:.

I used the last of my supply of fishing line when I built the racquet for my C8 ages ago, so I had to buy more line. I bought the lightest stuff I could find, but with my eyes that's not necessarily the best thing to do:lol:. I don't think the weight of line matters much to how it works, but thicker line would've been easier for me to work with.:P

Oh yeah, the fishermen among you may have better ideas, but I tied my strings off with surgeon's knots and applied a dab of "Multigrip" general purpose glue to the knots just to be sure...

Al.

[mill]

That looks very good and will let a lot more light thru than the original bhatinov mask :thumbsup:

[sheeny]

That was the big motivation I had to try the racquet idea in the first place;). More light through means it can work on dimmer stars. After using one for a while I find I don't need to adjust camera settings to view the diffraction spikes, so checking focus is a snap:thumbsup:. Throw the mask on, check focus, adjust if necessary and then take the shot without adjusting settings.

Al.

[Dennis]

I just love these variations on the design – top stuff Al! Be sure to take some photos using the mask and post the results so we can compare the traditional thick vane shots with the new, beaut, “Al-racket-mask” shots!

Cheers

Dennis

[sheeny]

Yep... when the clouds clear...:rolleyes:

Al.

[sheeny]

For anyone who can use them, here are PDF copies of the patterns I used for the racquets:).

The C11 racquet is in 2 parts. I suggest you print both and join them or glue them together before gluing to the board. I tried to squeeze it out of 1 A3 sheet and couldn't quite do it. I dropped the extreme strings, but still had some issues with lack of paper on the edges.

Al.

[allan gould]

Really helpfull article Al. I also was wondering how to get the Bahtinov mask to work with dimmer stars and it looks as if you have solved the problem.
Excellent solution and the PDFs will be most helpfull
Allan

[middy]

Wow :eyepop: Wow :eyepop: Wow :eyepop:... let me say it one more time
... WOW :eyepop:

I finally got around to making one of these Bahtinov masks and I have to say it is the best invention since the wheel!! It took me all of two seconds to reach perfect focus, and this was with a clunky rack and pinion focuser which shifts the image around the field of view while racking in and out. The spikes stand out like the proverbial dog's privates. Two hours of my time making the mask well spent. Thank you Mr. Bahtinov :thumbsup:

[Ron Garrett]

Guys, I know nothing about laser cutting but I wonder if the cut can be angled very slightly to produce a cut with a 'knife edge'. Needs two cuts along each line. For a 12" f/10 you only need .5 degree tilt. You can then use any thickness and get cleaner, knife-edge diffraction. (??)

Ron

[h0ughy]

dragging this thread out again - how do you make a bhatnov mask for a C11 hyperstar scope?

[bojan]

With a big hole in the centre ;)

[Dennis]

I’d make it with a strategically placed slot/cut-out, so you can ease the cables through the slot when fitting the mask to the OTA, with a large enough central hole cut out, as mentioned by Bojan.

Cheers

Dennis

[kinetic]

You are a bloody legend Al,:thumbsup:
I vote you most worthy IIS member of 2010 for the 2nd year in a row.
Making one of these today for the ED80.

Steve

[multiweb]

Just use a normal bathinov mask and fit it against the camera. You'll need a long enough dew shield to do this.

[Adrian5AW]

Hey Al, how did the lens mask work out for you? I am trying to use your template with a Canon 30-55 zoom lens, and not getting much effect. I am trying different distances from the lens front. Thanks,
Adrian

[Makronti]

Works well! First you have to find a bright star to create rays around it.What is the material you use?