Bhatinov Focus Mask

[RogerTheDodger]

You could always make a Bahtinov mask The Bahtinov Mask

[AndyUK]

Cheers Digz....

Quote:

Originally Posted by AndyUK

...and I'm sorry for being a bit obtuse...

No pun intended?

I missed that one- Well caught!

Hi Roger - Yes... you're right of course.... I've already bought two (one for MN190 and the other for the Equinox 80), but I was hoping I might be able to re-use the the 190 mask on the 180 Mak... if not, the Y mask will probably be a lot easier to make than cutting out all the slots for a B Mask... (Just being lazy!)

[Demonperformer]

In true blue peter fashion, here's one I made earlier ...

[Digz]

Excellent DP, how does it perform?

[Demonperformer]

Excellently. It produces a cross on the star and another bar that slides up and down through it as you focus. Point of focus is when that bar precisely bisects the cross. Simples.

Note I resisted the temptation to answer your question with "it preforms demonically":D

[glowjet]

Sorry for the late reply Andy but yes, the arms of the Y are set at at 20 degrees above and below the horizontal line, making an angle of 40 degrees between the arms, as in accordance with Bhatinov`s original design, I do not suppose this is rigid as any alteration, so long as you do it to both sides of the Y, will only alter the angle of the X, which the vertical line will translate each side of center as you come into focus. If you recall what Chris Lord pointed out in Franhofer`s diffraction, if you had chance to read it, any solid line of obstructing to the light path entering the aperture of the scope, no matter where it is placed, will cause a diffraction spike to lie across the central optical axis. I have read that some of our American brethren have measured the difference of being in focus and out of focus to be in thousandths of an inch of revolution of the focusing knob. Quite accurate I would say.

John.

[AndyUK]

Cheers John... And cheers DP for the picture! I was imaging getting a large piece of black plastic and attacking it with a modelling knife, but it looks as though I may have over-complicated it... In fact, at a push, it looks like something you could almost make up in the field

(And I've just looked at the weather forecast - IF it can be believed , it looks as though I have TWO sleepless nights ahead of me )

[palebluedot]

Theres a chap who has written a template generator and instructions on how to make the more complicated version.

It take the focal length, aperture and edge thickness (amongst other advanced parameters) and spits out a .svg file.

Bahtinov Mask

The template generator is here:

astrojargon - Bahtinov Focusing Mask Generator: Version 0.4

Has anyone tried it out? I was thinking of giving it a go.

[Digz]

I have used the template generator, but when saving from illustrator to open in AutoCAD the sizes were incorrect compared to what I put into the generator i.e. the mask was too big.

Plus I got confused as to what dimensions to put in, for example I know the mirror diameter is 250 but the tube diameter is larger than this - which figure do I put in? Presumably the 250mm?

I then came across this thread and saw the virtues of a Y mask so was gonna try that first

[palebluedot]

Plus I got confused as to what dimensions to put in, for example I know the mirror diameter is 250 but the tube diameter is larger than this - which figure do I put in? Presumably the 250mm?

I would take a stab and say that the aperture is the mirror diameter and the edge thickness would be the tube diameter-aperture/2

[Digz]

I would take a stab and say that the aperture is the mirror diameter and the edge thickness would be the tube diameter-aperture/2

Aye, thats exactly what I did. Im guessing there is a problem in the scaling between Illustrator and AutoCAD. Still, for the sake of 10mins in AutoCAD I can draw one from scratch

[palebluedot]

I just generated one for my 150p, opened it in chrome and printed it from there and it was only 2mm too big. I think I might get my craft knife out

[Edit] The edge thickness is the total edge thickness, i.e. both sides.

[glowjet]

No bother Andy, it just surprises me that anybody would now go to the trouble of attempting to manufacture a Bhatinov after the PDF article by Chris Lord, see website reference in Big_Bear`s thread, we know the Y mask works as DP`s splendid model confirms, if you digest the article at length it would seem that the B mask is OTT, but of course it is now firmly established and regarded as the norm, lets face it the Y pattern diffraction has long been seen in Newtonian's formed by the spider which holds the secondary, producing stars with spikes in photos, but in the wrong position for focusing, the Airy disk is nothing more than the circular diffraction of your scopes aperture edge.

What it needs is a comparison test, Bhatinov versus Y, ill health has prevented me from undertaking such a project at present, but time will tell.

John.

[AndyUK]

Cheers again John - I've also been looking at the Y mask for creating a focus check for my nifty fifty and other lenses.... Aside from the fact that a Bahtinov mask for something that small would require a microscope, I imagine the image when looking through would be quite dim(?), and DP's example looks SO easy, and scalable...!

[glowjet]

Andy there is no bother in making masks for photo and other small lenses, just remember to take the focal length of the lens and multiply by three as a third order mask, if you then divide this by the lower formula figure of 150 you may find that thin round wire will produce what you want, example a 150 fl telephoto x 3 = 450 divide by 150 = 3, halve this and you have a guide of 1.5mm, round wire sprayed black should do the trick, folks in the USA have even made B masks for their telephotos.

John.

[AndyUK]

Cheers again John... That's even easier!!!

[Demonperformer]

1. Please note that I was a (very) reluctant convert during a discussion in another thread (I think it was with Glowjet, but that may be wrong). Quite amusing really, as I originally thought the Y was the bit you had to cut out, not the bit that was left in.

2. Definitely scalable. Recently produced one to fit over a 135mm M42 lens purchased for wideangle stuff. Took roughly 15 mins (but that included getting the table out and putting it away afterwards!)

[mark7331]

Quite amusing really, as I originally thought the Y was the bit you had to cut out, not the bit that was left in.

Interesting as I've made a Y mask today made from a card disc with the Y shape cut out. The article in great bears link suggests that the Y is the bit to cut out as it starts with an open disc matching the aperture as the mask and then goes to two smaller discs and so on.

Anyway. I've just tried it and it works a treat. Really easy to do and using 2 second exposures I've managed to get focus bang on.

Because it's a solid bit of card it's sturdy too and should last a while.

[Demonperformer]

My only thought would be that your version has that a lot less unobstructed view, so that much less light is getting in, so the image that you are trying to focus will be dimmer (particularly if used visually). However, I'm a great believer in 'if it works don't fix it', so I won't try to talk you into changing.

[mark7331]

However, I'm a great believer in 'if it works don't fix it', so I won't try to talk you into changing.

Me too - but I don't mind being persuaded. I think it'd be worth reversing my cut out since your version works well DP. Plus I still have the "Y" that I cut out so I'll stick it on a surround and give it a try just to be thorough.

It's already been really useful though. I've always worried that, after focus, when I put on the focus lock it would go out slightly. Using this method I can see if it's bang on.

Mark

[G4YVM]

So, I am about to make a Y mask for my 500mm fl SW 102.

As I have understood this thread that gives me either a Y of 2mm strip suspended over the objective, OR, if I go for a third order Y, strips between 7 and 10mm wide ditto.

Have I got this right?

David

[Demonperformer]

David

I am not really up on the maths of these things, but the Y-mask I have for my 102 has strips of 6-7mm wide and they work. 2mm may work better, but I have found that making the strips too thin makes it difficult to attach them (but then, I am using black electrician tape to stick them on - there is probably a better way.)

HTH

[Horus]

it does not generate anything , I get an invalid file page ?

[glowjet]

Hi Demonperformer, yes it was me you were involved with, I seem to keep writing up on the B mask quite often from time to time, I am pleased yours was a success in the end, its so easy and simple to do, and experimentation is the order of the day, those reading this should remember that wide arms and leg of a Y will produce short but bright diffraction spikes, thin ones slightly fainter but longer and thinner diffraction spikes, if you leg of the Y is producing a nice bright image but your arms are not, then thicken the arms a bit. If at the first attempt you see three big white bloges then you have made the Y to wide, up to 150 mm aperture you might find 3mm wire could do the trick, sprayed black, one Astronomer, who is I think a member, made one out of 2mm round rubber which he just slipped over the aperture when focusing, then put the thing back in his pocket, that is the most adaptive one I have heard of yet . Embroidery frames are very good for masks, they go up in 1" increments from 4" Dia to very big, stick soft velcro on the inside, tack your mask to the front of the frame, slip over the the front of the scope and tighten the adjusting screw.

John.

[glowjet]

So, I am about to make a Y mask for my 500mm fl SW 102.

As I have understood this thread that gives me either a Y of 2mm strip suspended over the objective, OR, if I go for a third order Y, strips between 7 and 10mm wide ditto.

Have I got this right?

David

Hi David, with Bhatinov`s formula the focal length was to be divided by 150 and 200, this was to give a figure of latitude so that you could take the top or bottom figure or somewhere in between. without wishing to complicate matters further, the resulting figure chosen was for a space and bar in the original concept, so again you had to halve your chosen value. If we take your 500mm fl as a first order then by the maths you would have 1.6mm to 1.75mm bar widths or with the third order 500 x3, this would give from 3.75mm to 5mm, either could be fine but, of course the later will be easier to construct HTH

John.