Details for a lord mask

[Kevdan]

Hi all,

being as im bored and got a stiff neck from looking at the clouds, Ive decided to make a lord focusing mask to tryout compared with the bahtinov mask I made for my 10" newt.

But simple question is what thickness should the "arms" be on the mask?

Kev.

[JamesF]

I have some details on this somewhere. I'll hunt them out. There is however a school of thought that "it doesn't actually matter"

James

[JamesF]

Right, there is a range of acceptable values for the arm thickness. For a "first order" mask, the lower limit is given by the focal length divided by 400, and the upper limit by the focal length divided by 300 (preserving the units in each case). So, a 1200mm focal length scope would work out with an arm thickness of between 3mm and 4mm. If that works out too small, you can use the "third order" version, calculated by multiplying the focal length by 3, giving 9mm and 12mm as the limits for your 1200mm focal length scope.

James

[Kevdan]

Thanls James,

3mm would be a bit thin and very breakable with card as the medium. so 10mm arms it is or in old money 3/8"

Kev.

[glowjet]

Some while ago I wrote an article on the Bhatinov Mask for inclusion in our Astro Clubs quarterly magazine. You could do no better than read Chris Lord`s PDF Primer on Fraunhofer Diffraction, which covers the development of the masks http://www.google.co...DMX04STu4dSjdXQ. if that does not find it, it is easy to find in the PDF.

Basically as James has said it revolves round the focal length of the scope, Diffraction which is related to a bar and space having the same width, was used by Pavel Bhatinov to create this formulae : A space and width is regarded as one unit, using the numbers 150 or 200, this would be divided into the focal length. the resulting figure would then again be divided by 2 to give the width of the space and bar.this was known as a first order mask. If the resulting bar and space width became to narrow for practical purposes, then the results could be mulitplyd by three to give what Parvel called his third order mask. The rest is History, Chris Lord proved that a simple Y mask was just as efective and provided at least a one f stop brighter image. The fact was the commercail ball was rolling and most were now quite happy to spend money on a manufactured unit, when in reallity three bits of black plastic strip would have done the job, even 1/8" fence wire has been used successfully. there is much written about this and a trip to the USA Cloudy Nights forum will give you, I think on the last count, some 26 pages of threads on this subject, as to how it all came about in the States. Have fun

John.

[JamesF]

It may well have been one of your postings that originally brought the Y mask to my attention, John

James

[Kevdan]

Thanks john,

The link does work, Ive had a quick run through it an dit does explain in detail how it works.

will have a play later.

Kev.

[glowjet]

Kev, if you do experiment it would be nice if you could let us know the outcome. Just remember the rules, that a strip placed anywhere across the aperture of the scope will produce a diffraction pattern on the central optical axis and be at right angles to your obstruction, quite amazing is it not. Chris Lord explains all in his paper, try placing strips across the aperture and see what effect you can achieve to aid in focussing, essentially a web cam or camera with live view will help with the results, have fun:)

John.

[davew]

There is however a school of thought that "it doesn't actually matter"

James

I go to that school but it is under Special Measures.

It must be cloudy. How complicated does a Y mask need to be when made out of paper or card ?

Have I compared an any old order Y mask with a Bhatinov mask ? Yes.

Was there any change in the focus point ? No.

Did it seem to matter how thick the bars were ? No.

Did the same mask work regardless of focal length ? Yes. ( This was a test on one refractor and a few camera lenses )

The most important aspect I found was to not let the Y mask get wet because it will sag and then fall apart. The same applies on larger scopes where the mask will sag or fall.

Here is a photo taken through a 460mm refractor using a mask produced with a pair of scissors and keeping the bars at something like 3 to 4 mm. Would you be able to use it ?

I've just re read this post and it could sound aggressive. It's not meant to be !

[glowjet]

That is quite informative Dave, and your image only goes to prove a point, I am told one of the best materials is 1mm tin plate, sprayed matt black

John.

[RogerTheDodger]

Nice post Dave, answered all the questions I was about to type

[Kevdan]

Hi,

Ok Armed with new enthusiasm and a few ideas, I made a Lord mask from some plate and here it i snext to a home made cardboard bahtinov mask.

Now all I need is a star to test them both out, Has anyone seen any stars lately ?

Kev.

[davew]

As someone with a penchant for ice cream the water proof version was made out of an ice cream tub lid. More accurately cut but not sprayed black. As well as being a tight wad, I'm lazy.

Looking at that mask Kev, make sure the split arms line up with the centre line of the top arm. The trouble with soft masks on large apertures is that they sag.

I think the bottom line is, if you have it then use it. If you can make it for free then make it. Try it and see. There are plenty of people around who swear by a proper laser cut mask,

Dave.

[Kevdan]

I think the bottom line is, if you have it then use it. If you can make it for free then make it. Try it and see. There are plenty of people around who swear by a proper laser cut mask,

Couldnt agree more, I won't know wether it works until the stars re appear sometime in 2018 (well thats what it seams like) I know the laser cut ones look professional and nice £23 for my scope but if it works I've now got £23 towards a new eyepiece or filter or something I havent thought of yet but want.

Kev

[glowjet]

As someone with a penchant for ice cream the water proof version was made out of an ice cream tub lid. More accurately cut but not sprayed black. As well as being a tight wad, I'm lazy.

Looking at that mask Kev, make sure the split arms line up with the centre line of the top arm. The trouble with soft masks on large apertures is that they sag.

I think the bottom line is, if you have it then use it. If you can make it for free then make it. Try it and see. There are plenty of people around who swear by a proper laser cut mask,

Dave.

Just to put you all a little more in the picture, Chris Lord actually raised this issue on the Cloudy Nights web site, now there are some really experienced Astronomers over the pond, as there are in this country, with a lot of very expensive kit, some of the more experienced guys on the forum ran a Fourier Transformation program on Chris Lords Y mask in comparison with a Bhatinov mask and there was almost no difference with the results, the only noticeable one being the Y producing a brighter image by about an f stop. and very slight reduction in the brightness of the arms due to the reduction of the multiple 1st 2nd 3rd order images and so on, produced by the many bars of the Bhatinov, The general consensus was there was very little in it, to the end that some now use both types, the Y is very good for producing small masks for your camera. Chris Lord did make the comment that if you want to brighten the arm images, then just make them slightly wider, but by doing so will produce a shorter image. If you want to read the whole history, then read the 26 pages of threads on the American forum. Lets face it I bought a nice Bhatinov mask before I delved into this subject more thoroughly and I am fond of DIY, I wish now, in hindsight, I had not wasted my money

John.

[Kevdan]

I bought a nice Bhatinov mask before I delved into this subject more thoroughly and I am fond of DIY, I wish now, in hindsight, I had not wasted my money

If you've used it and were happy with it then it's not really a waste of money.

Kev.

[glowjet]

If you've used it and were happy with it then it's not really a waste of money.

Kev.

It is when you find out you get the same results from a Y mask, and from a DIY point of view I know of the two types which is easier to make. I will bet if the manufacturers were aware of this from the very onset, I wonder if they would have been any cheaper. Dave`s post with his resulting image has covered the comparisons very well

John.