A focus mask primer

[rcheshire]

I found this while doing a bit of research into aperture masks. It's a nice primer about focus masks and a worthwhile read, particularly if thinking of a Bhatinov mask.

http://brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/forum/Fraunhofer-primer.pdf

[multiweb]

Thanks for the links. Very interesting read. :thumbsup:

[RickS]

:thumbsup::thumbsup: thanks, Rowland.

I might have to try some 3D printing... A brighter equivalent of the Bahtinov mask would be useful.

[RyanJones]

Very interesting read Rowland. Thanks for posting.

Makes me wonder why the traditional Bhatinov mask design has been considered the go-to as opposed to a Y mask. The only reason I can come up with is that the multiple lines are intentionally used to dim the defraction lines so that only the brightest object comes through rather than a multitude of lines from surrounding dimmer stars ?

Certainly could be helpful in situations where it would be handy to focus close to the object but there are no bright stars close.

:thumbsup:

[raymo]

The brightness is not usually of much concern, as one can usually find a bright enough star to use. I think a more likely reason is that a Y configuration mask
would be much more flimsy, having only three asymmetrically arranged
points where the Y is attached to the outer frame; this would especially apply to larger apertures.
raymo

[RyanJones]

I did think of this but then I also thought that the slits don't need to reach the absolute edge or centre just as the exitsing bhatinov pattern doesn't. The Y wouldn't be literal where all three slits would conjoin . Given there is more material left on the mask, my thoughts are that unless is was made out of a significantly heavy but thin material, the mask would maintain greater structural integrity with this pattern. This is when my therory moved to the fact that brighter defractions would introduce multiple defractions from other dimmer surrounding stars, hindering the focusing task.

[rcheshire]

Thanks Ryan and Raymo. I must confess to not thinking the concept over too much, as I found the 'Y' effective with lenses and refractors in the shorter focal lengths. Easily cut from a sheet of black card or 80gsm at a pinch or laser cut from plastic.

[RickS]

Bahtinov masks don't work very well at short focal lengths so it would be very handy if Y masks are better for that...

[rcheshire]

Horses for courses, it would seem.

[RyanJones]

There's no doubt that different people will have different tastes and different needs Rowland but what I love about your post is that it includes explanation as to why and how things work. Good on you for posting what you did and by all means keep it coming :)

[RickS]

I have a work colleague generously 3D printing a Y mask for me and I have a Bahtinov mask of similar size so I will do some comparisons...

[RyanJones]

I'll be interested to hear your results :thumbsup:

[rcheshire]

I find under sampling a limiting factor. Despite the inherent difficulties of good focus with dslrs, the image is invariably 'soft' or gives an appearance of out of focus. A consideration when making comparisons.

[RickS]

Here's a test Y mask printed by my colleague. He's a member here but I won't out him unless he wants to be mentioned... and possibly plagued by further requests :lol:

[deadsimple]

All good Rick. The key dimensions of the drawing are attached.

Angle of the Y split from centre: determined from zooming in on the aperture mask in the PDF until it fit my whole monitor and calculated the rise/run, then arctan'd it, so 15.6 deg. Maybe I should have read up on the bahtinov first and stuck to 20 degrees, and assumed that the image in the PDF may have been squished.

Width of the obstruction: a value between FL/200 to FL/150 (though that should technically equal the bar+gap, not the bar? there's no equal gap here, so ....?) that fell onto a multiple of 0.4mm (the X/Y resolution the printer works well at), so 1.6mm. Maybe this should be halved.

Ok now that I think of it, a 20 deg angle (40 deg between bars) and 0.8mm obstruction might be the way to go if it doesn't work.

Well at least a photo of the first printed version of the mask wasn't posted here :)

[RyanJones]

And this is where my reading of the link missed something. I was under the impression that the Y was the cut out section. Great looking piece there ash. Well done.

[ChrisV]

+1 Ryan. I thought the Y was the cut out. Might warm up my printer and try it

[xelasnave]

You can make a disk of app size and put in two holes say the size of a ten cent piece...opposite each other on the same diagonal and say a distance apart similar to the radius...when outta focus you see two disks thru the ep or viewer and when focused you see only one disk.
You can use two triangles such that when in focus you see a star.
You can make these easily.
I am going to make one even though I have the Bat mask.
Alex

[RickS]

Here are a couple of test comparisons. The first is 30 seconds luminance through the Borg 55FL (200mm focal length) and the second is 60 seconds of Ha. The star selected is not especially bright. The three images in both cases are 1. Bahtinov mask, 2. Y mask (thin strips of material), 3. Negative Y mask (thin strips of no material.)

In the case of the lum the scope is fairly close to good focus. For the Ha the focuser was left in the same position but is not well focused with the Ha filter.

Cheers,
Rick.

[multiweb]

Much easier to read than the standard bahtinov, less interference in the middle. So what does the final mask look like? Got a pic? Is that the red one? I'm thinking there is a use for this as well for a larger aperture at longish FL and only medium to low brightness stars in the fov. Saves a trip back and forth to a bright star nearby off field. :question:

[RickS]

Here's a couple of pics of the negative Y mask, Marc. Ash (deadsimple) made me a new one with some small tweaks but it has been too cloudy the last couple of nights to try it.

[multiweb]

oh wow, yeah that won't work, it doesn't let enough light in for me.

[RyanJones]

I like the negative "y" with its clearer core. I wonder what difference it would make if the slots were opened up say double that thickness. As Raymo mentioned earlier, on a larger scale, having the cut outs meet centrally would make it a little unstable. Maybe a common point in the centre is the way to go so the cutouts stop just short of centre ?

[deadsimple]

I've been thinking about that. I wonder if it'll remove some of the the core glow, so that the intersection of the lines is easier to distinguish?

The next version in the pipeline ... (see attached).

[RyanJones]

Keep refining the idea Ash and you'll have at least one customer :thumbsup:

I don't have the equipment you have to create the ideas but I'm loving the bouncing around of ideas :)

[RickS]

Here's a shootout between a Bahtinov mask and two different versions that Ash 3D printed. 2 second subs with a 3nm Ha filter, Borg 55FL f/3.6 scope and ASI1600mm Pro camera.

[raymo]

I have never seen the horizontal lines displayed by the Bahtinov with any
of the three Bahtinovs I have owned. Maybe it is to do with the scope being very fast[f/3.6], or maybe the Ha filter. I'm speculating that a positive Y
would be better; I imagine that you'd have to use a pretty bright star with the negative one.
raymo

[RickS]

Hi raymo,

We tried a positive Y earlier and I didn't like it much. Too much energy in the central blob and not much in the lines.

It doesn't matter too much what sort of mask you use. The 55FL needs a bright star and/or a long sub to show much of a pattern with the Ha filter.

Cheers,
Rick.