Bow tie focus mask

[rcheshire]

I've been working on a focus mask that is easy to make and easy to use with my Canon 200mm lens.

The bow tie pattern doesn't block too much light and with substantial obstructions, ~5 mm, produces good spikes. Other dimensions as required.

I followed the (Chris) Lord method of removing the grid typical of most focus masks and used the Carey mask style to produce spikes at slightly different angles. The spikes need to be separated by a degree or two - it's easier to judge the spacing.

Focus is achieved when all four pairs of spikes are spaced evenly. The bottom image shows out of focus. Turning the focus ring clockwise - in this case a very small amount - to achieve focus, as in the top image.

Cut from a flat section of clear bubble packaging, it's relatively easy to get nice sharp straight edges with a very sharp hobby knife/scalpel. Mounted in a step up ring for attachment to the lens - tape will do.

Please modify at will. I will be playing around with it until I'm perfectly happy with it's performance.

EDIT: added better images - duplicates up the page. Note the additional perpendicular spike in focus.

[ballaratdragons]

Fantastic work Rowland!

May we call it the 'Cheshire Camera Lens Focuser'?

[rcheshire]

Thanks Ken. I don't mind - I kinda liked the Bowtie Mask though:lol:

[ballaratdragons]

Your design, you can name it :thumbsup:

[rcheshire]

Ken. The "Cheshire Camera Lens Focuser" it is.

[DavidU]

Awesome ! Ok, first to ask, may I make a Cheshire mask for my 100mm refractor?

[rcheshire]

David. You may. Please let me know how it performs and any improvements you might suggest. A 1 degree splay should adequate with 100mm aperture.

EDIT: 1 degree between spikes.

[ballaratdragons]

If it works on Dave's refractor you'll have to change the name to 'Cheshire Mask' :lol: :thumbsup:

[rcheshire]

Not so much of a mouthful! I hope it works.

Now that I think of it, a Bahtinov calculator should provide the correct obstruction width for a bigger scope. The dynamics are identical for any focus mask from what I can see.

[rcheshire]

EDIT: Thought these might be better posted at the start of the thread. I'll leave them here anyway

I noticed that the gap between the spikes is filled with either red or blue. Out of focus blue (not so evident in this shot) and out of focus red is fairly obvious,

In focus it seems that the upper pair of spikes contain blue and the lower red.

I have also attached an image of the street light across at Safeway to illustrate the sharpness of the spikes, not so evident with a short lens.

EDIT: You will notice too, that the single spike perpendicular to the bow tie is only present when focused.

[ballaratdragons]

Where do I find the attachments please Rowland? I can't see them.

[rcheshire]

Sorry Ken. I hit submit - doh! Images attached to post now.

In focus blue on top red below, opposite of out of focus - made the change in the post.

[ballaratdragons]

:thumbsup:

[Amaranthus]

Sorry for digging up an old thread, but I'm looking to make a Bahtinov mask for my SCT, and if the Y-shaped "Lord Mask" works basically as well, I may as well just make one of those. In theory, it should, but has anyone got any practical experience with them?

I also wonder what the advantage of the above 'bowtie' mask would be over Lord's Y version?

[rcheshire]

This mask was designed to be easy to make and use. It generates a spike perpendicular to the splay, at focus. It works quite well with my 200mm lens. I have used it on 35 and 60mm lenses.

I'm sure a Y will be just fine. I was curious, experimenting/ playing around with Fraunhofer diffraction and came up with the bow tie shape. I've used it ever since.

[Amaranthus]

Thanks Rowland! After having a go tonight at making these with cardboard, I found the bow tie actually easier to make! Now waiting for clear skies to test...

[nebulosity.]

A great design Rowland! I will make one and see if it helps focus the 85mm.

Go Cheshire Mask! ;)

Jo

[rcheshire]

Let me know how it performed. I recall using a Bahtinov mask calculator to arrive at the width of the obstructions and since found that 4 -5mm is OK, with the mask just in front of the objective and open the lens wide to avoid IRIS spikes. No need to refocus if stopping down - with the 200mm.

[Octane]

Rowland,

Could I pay you to make me one for the EF 200mm f/2.8L II USM?

Cheers.

H

[rcheshire]

No cost to you H. I am getting a few made professionally - laser cut. Will be in touch.

[Octane]

Oh, wow.

Thank you, so much! :)

H