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.
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.
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
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.
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.
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.
Hi stefan I measured the inside of the dew Shield on the ED/80 (tricky task haha) I got 351mm does that sound right
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?
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.
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.
Cheers,
Stefan
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.
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
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.
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
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
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.
Last edited by Stefan Buda; 12-06-2015 at 11:18 AM.
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
Last edited by DavidTrap; 13-06-2015 at 04:20 PM.
Reason: added clear aperture for 85mm lens
I measured the inside of the dew Shield on the ED/80 (tricky task haha) 
I got 351mm does that sound right 
oops 
.
yep I just dialled a friend who said he got 113 also but it might be just a little tight 
