Bahtinov masks for telephoto lenses

[Stefan Buda]

Good quality 3D printed Bahtinov masks for telephoto lens. Slits are 2mm wide, so they are suitable for focal lengths of over 200mm. The smaller one fits 67mm thread size and the larger one is for 72mm thread. They are slight push fit, no need to mess with screwing them in.
The red rim is for red torch visibility.
If there is demand I can make these for all sizes up to about 140mm clear aperture.
You can have these for $30 or I can separate them for $20 each. If you are not from Melbourne and can't pick up then a $5 postage charge aplies too.

Update: The size 72 is sold, the 67 is available for $20.

[LewisM]

Stefan, if I send you my mask diagram as an .svg (I generated it using the Baht generator online), can you custom make one? It is for a 78mm f/6.4 scope.

[kkara4]

^^This, specifically the .svg part?

[Stefan Buda]

Hi Lewis and Krishan,

DXF file would be better but I should be able to convert the svg to a format that my CAD understands.
Whichever the format, I still have to produce a 3D model for printing.
At the moment I'm working on a model to fit 77mm diameter filter threads and on the weekend I'll be working on one to fit an 80mm refractor.

Cheers,

Stefan

[raymo]

Is it not possible to make them with narrower slits for the myriad of smaller lenses out there like my 18-200mm which would need one of 62mm diam? I'd buy one immediately.
raymo

[Stefan Buda]

Hi raymo,

I will try and let you know if it is feasible.

cheers,

Stefan

[leon]

Hi Guys,

Maybe I am missing something, however when I used to use my Canon 500mm lens for any astro work, I would auto focus with the center focusing point only on the camera, then turn the lens to manual and image away.

During the night if the temp fell dramatically I would do an other auto focus and go again,

So why the need for a Bahtinov Mask on long lenses. :shrug:

Leon :thumbsup:

[Stefan Buda]

Hi Leon,

For top results people attach DSLR lenses to CCD cameras and in that case, at least with the current generation of CCD cameras, the auto focus feature is lost. Also there are old telephoto lenses that don't have auto focus.

cheers,

Stefan

[leon]

Thank you for that Stefan, I should have realised, :rolleyes: I did only used Canon 5D.

Leon :thumbsup:

[Stefan Buda]

I can now answer raymo's question about finer slits: Yesterday I made a 55mm one with 0.9mm slits for my 135, f/2.8 lens and a 58mm one requested by a well known Victorian astro imager, also with 0.9 slits. Tomorrow night I hope to test mine on the sky.

cheers,
Stefan

[raymo]

Looks promising Stefan. Having not looked into the technicalities of
Bahtinov masks, I don't know if one would work with a zoom lens, or
if they are limited to prime lenses.
raymo

[Stefan Buda]

This post should really be in the ATM section but the thread I started there, to share my experiments, got promptly locked - probably because I mentioned potential commercial activity.
Anyway, last night I did some testing using various combinations of lenses and masks, with interesting and somewhat surprising results.
Lenses used were my trusty old Pentax 50 f/1.7 and a Mamiya 135 f2.8 attached to a QHY8 camera.
Bahtinov masks with 0.9mm and 2mm slits were tested with both lenses.
Main things learned:
2mm slits work well for 135mm focal length which came as a bit of a surprise.
0.9mm slits produce too much diffraction with the 135mm lens and the patterns are hard to interpret, also a surprize.
2mm slits are not quite good enough down at 50mm, as expected.
0.9mm slits work very well with the 50mm lens.
One needs to choose bright stars for the diffraction pattern to overcome light polluted sky background.

It will take me a while but eventually when I've done all the cutting, pasting and labeling required, I will post the images from last night.

Cheers,
Stefan

[Stefan Buda]

The 72 is pending.

[Stefan Buda]

The size 72 is sold.

[Joshua Bunn]

Hi Stefan,

Would the 67mm one work for a 70-300mm lens?

thanks
Josh

[Stefan Buda]

Hi Josh,

It will certainly work for the 100 to 300 range but having not tried it down as far as 70, I cannot be sure.
Most likely that if you have a bright enough star in the field and the sky is not too bright you should be able to see a good diffraction pattern.
I will soon follow up with the test pictures I promised a few days ago.

Cheers,
Stefan

[Stefan Buda]

Finally I have processed some of the test images. What a pain! I had to debayer them one at a time in DSS then develop and crop them in Star Tools, and then assemble them in Paint.

The fourth panel is with the mask removed after focusing was achieved.
That panel shows quite well that the lens was designed for film, as it produces red rings around reddish stars.

Tomorrow I'll try and present the images captured with the 50mm lens through a mask with 0.9mm slits.

[Joshua Bunn]

Thanks for the visual aid Stefan!

[Stefan Buda]

Here are the test images with the 50mm lens and a mask with 0.9mm slits. It is obvious from both sets of images that my sky is very bright and these masks should work even better under a dark sky.

cheers,
Stefan

[christian.ckr]

I very recently bought a 72mm bahtinov mask from Stefan, I am yet to test it out with Melbourne's inconsistent weather but the quality is fantastic

[Stefan Buda]

Thanks Christian,

I always try to do my best.

Clear skies!

Stefan

[Joshua Bunn]

Hi Stefan,

Can I place an order for a mask for a 70-300mm lens f/4 and a 35mm f/1.4?

thanks
Josh

[Stefan Buda]

Hi Josh,

The 67 is available, in fact I printed 4 specimens, but I would need to know the filter thread size of your 35mm lens to be able to print one for that lens. I've never heard of anyone using a mask for such short focal length but I'm sure that is because no one has been making them with fine enough slits.
Based on my tests I believe that a mask with 0.9mm slits would work on a bright star with that lens. If that fails I can go to extremes an print one with 0.5 slits - the limit of my printer.

Regards,
Stefan

[Joshua Bunn]

Of course, sorry for not including that. They are both 67mm threads.

Josh

[Octane]

Stefan,

Any chance of making one for a Takahashi FS-60CB?

Cheers.

H

[Stefan Buda]

I guess a clear aperture of 50mm should be sufficient for the 35mm lens.
Is that ok with you? I don't want to compromise the mechanical integrity by making the slits longer than necessary.

cheers,
Stefan

[Stefan Buda]

Hi Humayun,

No problem if you are prepared to wait in the queue a few days as I already promised someone a mask for a 5" refractor as well as Joshua's 67 with narrow slits.
I will need the size of the inside diameter of the dew shield measured quite accurately for a good fit if you decide to go ahead with it.

Cheers,
Stefan

[Octane]

I'll get back to you tomorrow. :)

H

[Stefan Buda]

Well, I added another size to my model portfolio: 67 with 0.9mm slits.

[Joshua Bunn]

Hi Stefan, my apologies for not getting back to you, I'm subscribed to this message, but didn't get an email.

That looks great! So 1 67mm @ 0.9 slits and 1 67 @ 2mm slits.

Josh

[Stefan Buda]

Hi Josh,

I just sent you a PM. If you get back with a delivery address within 20 minutes the I can still catch the post office open.

cheers,
Stefan

[Joshua Bunn]

PM Replied :)

[Stefan Buda]

Hi Humayun,

I should be able to tackle your mask, over the weekend, if you give me the size of the inside diameter of your scope's dew shield.

Cheers,
Stefan

[Jen]

Hi Stefan :) I was wondering if you could make one to fit my ED/80 please :)

[Octane]

Stefan,

I haven't had a chance to measure. I'll private message you as soon as I do the measuring.

Cheers!

H

[Stefan Buda]

Hi Jen,

No problem, just give me an accurate (to better than a quarter of a mm) measurement of the inside diameter of the dew cap and a few days to get it done.

Regards,
Stefan

[pmrid]

Hi Stefan,
I hope you won't mind my adding a positive feedback here. The mask you made for my ED127 arrived today and I'm more than pleased with it.

The things I find particularly pleasing include (in no particular order):
1. The frame and lip around the mask itself - this ensures the mask sits straight and is well seated in the dewshield;
2. Size - having specified 147.25mm as the ID of the dewshield, this mask fits perfectly - a nice positive fit without being tight or heaven forbid, too,loose;
3. Materials - I've had masks made in Oz that have been very brittle - fracturing or even shattering if dropped. This looks to be made of materials that will tolerate a drop or two;
4. Overall appearance - let's face it, the red lip is just plain sexy. What can I say??

Peter

[Jen]

What material is it made of acrylic :question:

[Stefan Buda]

Thanks Peter, great feedback!

Jen,
No it's not acrylic, it is ABS the same plastic that Lego blocks and many injection moulded toys are made of. It does not break if you bend it and it can take quite a bit of rough treatment - even though it may feel a bit flimsy, it retains its shape quite well.

[elken2004]

Here is a quick test of the .9mm mask

as you can see its through a kitchen window
canon EOS 60d 18-135mm lens

this image was at 135mm
it works at 18mm not as good but still highly usable

My view the .9mm pitch is perfect as it lets more light through for dimmer objects. the other mask worked but hard to see light
oh PS this was a street light about 1.5km's away

[Stefan Buda]

Thanks Clive, I appreciate the feedback.

[DavidTrap]

Hi Stefan,

What is the largest diameter you can cut? I'm interested in one for a 130mm refractor, the dew shield is 180mm in diameter.

I'm also in the market for masks for an 85mm camera lens and an FS-60 (like H). I've had other masks cut for these optics, but the slits are far too wide to make effective spikes. Your fine slits look like they'll do a much better job.

Ta
DT

[Joshua Bunn]

I tested the masks last night that Stefan made for me, on a 35mm f1.4 and a 70-300mm f4-5.6 lens, the 0.9mm slits worked better than the 2.0mm slits, on both lenses.

I'm very happy with their build quality.

[Stefan Buda]

Hi David,

See the attached image for the smallest and largest I'm able to print.
I can go smaller but not bigger, as 150 is the width of my build table.
The large one on the picture is for a 127mm refractor and fits the inside diameter of the dew shield - 147.25mm.
For your 85mm lens I would need the filter thread size and for the FS-60, the inside diameter of the front of the dew shield.

cheers,
Stefan

[Stefan Buda]

Thanks Josh for the feedback!

When I decided to start experimenting with these masks I had only prime lenses in mind as zoom ones are not supposed to be very good for AP, but looks like there is demand from zoom lens owners. I should develop a set with 1.2mm slits specially for variable focal length lenses, as that size would appear to be a good compromise.

Cheers,
Stefan

[Jen]

Hi stefan :hi: I measured the inside of the dew Shield on the ED/80 (tricky task haha) :) I got 351mm does that sound right :)

[Joshua Bunn]

Hi Jen,
That does not sound right, It might be in the order of 110mm, but definitely not 351mm. Are you sure you did the diameter and not the perimeter?

Josh

[Jen]

:eyepop: oops :scared3:
Ok how does 113mm sound :P
Thanks Josh :)

[Joshua Bunn]

Sounds about right, 113mm * Pi = 355mm. :)

[Stefan Buda]

Hi Jen,

That sounds close but I'm not sure if it is close enough. We both would be unhappy if I made you one that was a sloppy fit or too tight.
I found the outside diameter specs for the Orion and the Vixen versions of the ED80 and they are both 116.12 including the paint. I guess yours would be the same. Assuming 1.5mm wall thickness for the dew shield, the inside diameter should be a fraction under 113, allowing for the paint thickness too. But if the wall thickness is 1/16" (1.59mm) then we are looking at something like 112.75mm.
Perhaps we should put out an appeal to find an ED80 owner who also owns a vernier caliper, presuming that you used a ruler or a measuring tape, and plead for an accurate measurement:lol:.

Cheers,
Stefan

[Maurice]

Hi Stefan

Just to make things more confusing........
I have the Saxon branded variant of the ED80 & the ID of the dew shield varies from 113.1mm to 112.6mm on mine.
It has what appears to be a velour-type tape on the other end that presses onto the lens cell.

regards
Maurice

[Jen]

Arrrhhh ok I am known to rounding things off quite a bit ;)
I will measure it up again tomorrow and take a pic of it lol lol

[Stefan Buda]

Hi Maurice,

Thanks for the measurements! I think it is safe for me to aim for 112.8 and the out of roundness can easily be taken up by the flex of the locating legs.

Regards,
Stefan

[Jen]

:D yep I just dialled a friend who said he got 113 also but it might be just a little tight :)

[Stefan Buda]

Hi Jen,

I've done your mask so you can PM me with the pic up or delivery details.

cheers,
Stefan

[Jen]

Woohoo thank you :D

[elken2004]

Here are two tests at 135mm on canon eos60d
Second pic is the .9mm, it is very clear and distinct, they work down to about 70mm and at 18mm you just see the mask

[Stefan Buda]

Great feedback again, Clive, I really appreciate it!
I think its time for a bit of theory now.
For one to understand what's going on here, one needs to understand the concept of entrance pupil, as it applies to camera lenses.
In the case of a normal refractor the diameter of the entrance pupil is the same as the clear diameter of the front lens.
With camera lenses the entrance pupil diameter is the focal length divided by the f-ratio. So at 70mm and f/2.8 it is 25mm and at 18mm and f/2.8 it is only 6.4mm So, from all the light that gets accepted by the large front element only a bundle of 25mm or 6.4mm respectively, ends up forming the image of any particular star and diffraction pattern. Now that in itself should not be a problem, however only the bundle that corresponds to the optical axis goes through the centre of the first lens (and the centre of the mask). That is a problem that becomes more severe as the focal length gets shorter.
At 18mm the useful 6.4mm beam of light will go through only one set of slits rather then all three if the star is off centre.
Because for zoom lenses the front element is further from the image plane than in the case of equivalent prime lenses this entrance pupil offset is larger.

In conclusion, it is very important to pick a bright star that is smack in the middle of the field when using a Bahtinov mask with a wide angle lens.
Also 18mm is probably going a bit too far, but I'm going to try and design a mask that can do it.

[DavidTrap]

Hi Stefan,

Here is an image of the 85mm lens that I would like a mask cut for. There's not much of a ridge to clip a mask over, or do you build the lugs to grip the inside of the filter thread?

This filter thread on this lens is 62mm. The focal length is 85mm and I stop it down to f4. Clear aperture is 48mm. I've got another mask that was cut with wider slits and doesn't really give a useful diffraction spike.

The outside diameter of the FS-60 is 80mm. Focal length is 372mm at f6.2.

Can you send me a PM with a quote please.

ta
DT

[elken2004]

I got a test 67mm mask at .5mm to try out below is the link to ATM thread

http://www.iceinspace.com.au/forum/showthread.php?
p=1182574#post1182574

where are the clear skies?

[Stefan Buda]

Clive,

The experimental mask I sent you (the top one in your post) has 0.5mm slits, rather than 0.2mm.

[Jen]

:thumbsup::thumbsup: thank you Stefan I received mine safe n sound :D it looks very sexy in the red hehe ;)
My scope is all aligned tarped up waiting for the weekend so I will check out how snug it fits then :D

:thanx:

[elken2004]

A perfect fit on 80mm

Now for a star test tonight clouds permitting

[Phil Hart]

I can confirm that these little Bahtinov masks are damn handy.

I've tried them with 24mm and 80mm lenses and they do help get sharp focus quick. At 80mm I can focus in liveview on a good star but at 24mm it's better to take very short exposures and review - but the mask shows really quickly when you're at the sweet spot and if not which direction to go. With lens aberrations, when trying to focus very fast lenses seeing the correct focus point just with Live View without a mask is not as easy or as robust as you might think.

With longer lenses it's a snap to get sharp focus real quick. They also make life much easier if you're trying to use lenses on a camera system that doesn't have Live View.

Thanks Stefan :thumbsup:

Phil

[elken2004]

I absolutely concur with that, after my tests last night with my canon 60d and 18 -135mm lens, even at 18mm little harder to see it, live view at 10x gives you a little show. but using the three part objects, once they line up, focus is achieved, and with a pic taken you can see the pattern.
and that was using the experimental .5mm mask.
:)

[Stefan Buda]

Thanks Phil and Clive,

I think we broke some new ground with these tests, as I never heard of anyone using focusing masks for wide angle lenses. In fact when I set out to make these things, I only had telephoto lenses and small refractors in mind.

[Jen]

:thanx::thanx:
Mine fits beautifully woohooo :)

[DavidTrap]

Collected mine today from my office - look fantastic. Will report back when I've had a chance to try them out.

Thanks,

DT

[Regulus]

Thanks for that Stefan, it was useful info I have never had before.

Cheers - Trev

[Stefan Buda]

Thanks Jen, David and Trevor for the feedback!

I now have 3D models for the following filter size/slit size:

55/0.9 55/2 58/0.9 62/1 67/0.9 67.0.5 67/09 67/2 72/2 77/2 82/1.2

Also models for the following refractors:

FS60, Zenithstar 70 ED, Megrez II 80 and 5" refractor with 147.25 ID dew shield.

I would also like to develop models for 4" refractors, so please don't hold back if you require one.

[Stefan Buda]

Another size added to my list: 77mm filter size with 0.9mm slits.

Suitable for lenses down to 50mm focal length.

[gavcol]

G'day Stefan

I am interested in getting a couple of masks.

Are you still making them?

Looking for one for an SW ed80 Pro, SW 150/6" Achro and a couple of camera lenses.

Can you give me an idea on cost and what further information you might need for lenses etc?

Thanks

Gavin

[Stefan Buda]

Hi Gavin,

Yes I still make them and I have most sizes for camera lenses in stock for $20 a piece or less for multiple orders.
If you tell me the focal lengths of your lenses and the filter thread sizes, then I can choose the right masks for you.

The 80mm refractor size is $25 and I need a good measurement of the inside diameter of the dew shield.

Sorry but I can't help with the 6" refractor size as the largest diameter I can print in one piece is 150mm.

Cheers,
Stefan

[gavcol]

Hi Stefan

Thanks for the reply.

Will send you an email in the coming week, once I figure out exactly what I need.

Thanks

Gavin