Bahtinov mask and barlow lens

[Keith1]

Hello,

After quite some time trying various ways to focus my telescope with a DSLR and / or webcam, I finially took the plunge and built a Bahtinov mask.

Being the sceptic that I am, last night I tried it for the first time, and was totally surprised how easy it was to focus using this simply device. It also worked visually as well as through the netbook with CCD. I would recommend this to anyone struggling to focus on CCD's.

I have a Skywatcher ED80 telescope and I cut out a mask to match this telescope. However, one question that I have is as follows...

The Bahtinov mask is cut to match the apperture and focal length of a specific telescope. My ED80 is 600mm focal length and 80mm apperture.

If I add a 2x barlow lens into the optical chain, then am I correct thinking that I would need another mask for 80mm apperture and 1200mm focal length?

many thanks in advance for your assistance with this question.

Keith

[themos]

No, you won't NEED another one. Th eexisting one may be sub-optimal in terms of the clarity of the diffraction pattern but it will work.

Almost Anything will work as long as the lines are clean and parallel.

Now, just wait until someone says I am wrong.

[adamsp123]

No, you won't NEED another one. Th eexisting one may be sub-optimal in terms of the clarity of the diffraction pattern but it will work.

Almost Anything will work as long as the lines are clean and parallel.

Now, just wait until someone says I am wrong.

Totally agree with your statement, I have made 4 now and sometimes the template gives lots of little thin cut outs, well blow that I haven't the patience so I go for thicker and less slots, they all work fine.

[Keith1]

Many thanks,

That saves me a lot of effort.

If only thar stars would come out now for a little longer than a couple of hours at a time.

Keith

[glowjet]

It is a tedious, time consuming job, making Bahtinov masks, but well worth it with the results, whats more interesting is that we apparently may have all been making the masks to complicated, Fraunhofer`s Diffraction reveals that a simple solid Y across the aperture will produce the same effect but give a brighter image, as starlight is not reduced by continuous bars and spaces across the opening. Focusing is achieved

in the same way, thicker or thinner bars still produce the same effect but give different thicknesses of diffraction spikes, what is more amazing is that it does not matter where you place the pattern in the opening, the image will still appear on the optical axis. As this is an easy thing to try out I am sure someone among the imagers will have a go at making this simple mask and report back.

John.

[glowjet]

Addendum to the above, I stated, in error, that continuous bars and spaces do not reduce incoming starlight, in effect it does, that is why the resulting image produced by the simple Y pattern is brighter. Sorry.

John.

[adamsp123]

Right! I will make a simple Y mask and see if it does the job

[blinky]

A Y mask would be interesting to see, especially if it lets more light through. When focussing with the DSLR it would enable you to focus using a dimmer star.

[glowjet]

You could experiment first with thin black plastic or card, as a base for your design you could stick pretty much with Pavels formula that is : The angle for the Y part is 20 degrees above and below horizontal or in nut shell 40°, the thickness of the Y is related to your FL, simply divide the FL of you scope by 200 or 150, these are the parameters for the space/bar width ( Fraunhofer ) the space and bar are as one unit and needs to be divided by two to give you the solid bar dimension, the two dividing factors allow for some tolerance in the bar width , for example FL 1200mm dividing by 200 and 150 will give a result of 6mm or 8mm this must now be divided by 2 to give the resulting width of bar, which will be between 3mm or 4mm, the choice is yours,

As I understand it the reason behind the tolerance is when you are faced with much smaller focal lengths, when this happens, as with fast scopes, you simply multiply the FL by three and calculate from there, this was known as a third order mask. From the information I now have it would seem you can make the mask any width, it results in thin or fat diffraction spikes, I think experimentation is the order of the day, the new mask is very simple to make and I shall try it out on my 10x50 binos to see the effect, at a rough calculation the bar width will be in the region of 2mm, that is if ever the clouds clear.

John.

[adamsp123]

Y mask done and used.

I cut 3 x 25mm slots in a Y shape for my 120ED didn't even measure the angles but roughly the standard Bahtinov angles +/- 15 degrees?? took me about 3 mins to do.

Stuck it on the end of the scope, looking at Rigel you get 3 oblong "blobs" as you focus thay merge into one quite clearly, checked it against my proper home made mask and it was spot on focus! so there you go, try it takes so little time and fuss.

[glowjet]

From what information I now have Pete, wide bars give fatter diffraction spikes, as you reduce their width the diffraction spikes become thiner and brighter, true focus is reached in just the same way though. With only three narrow bars, as against a B/mask, starlight is not so restricted, so there should be more light throughput, I am pleased it was successful, as yet I have been unable to try it due to weather and other commitments. Thanks for the feed back.

John.

[Psychobilly]

So the "y" performs just like a traditional hartman then .....

The whole point of the Bahtinov is the moving line between the fixed "X" which become centered at perfect focus...

Peter...

[adamsp123]

So the "y" performs just like a traditional hartman then .....

The whole point of the Bahtinov is the moving line between the fixed "X" which become centered at perfect focus...

Peter...

Basically yes, but I have always plotted the templates I have used with a "sensible" number and width of slots.

Although the Y worked a la Hartman OK the 3 nice diffration spikes of the B mask are very obvious when at focus, more so than the Y.

[glowjet]

So the "y" performs just like a traditional hartman then .....

The whole point of the Bahtinov is the moving line between the fixed "X" which become centered at perfect focus...

Peter...

Although unable to try it myself at the moment, focusing is achieved on the optical axis for perfect focus in the same way as the Bahtinov, the added advantage being that the resulting image is brighter as more light is allowed in, the scope opening not being blocked by a grid of bars, only the three bars of the Y.

As to the width of the bars of the Y in relation to your scope opening and FL, Pavel B`s formula is a good place to start.

John.

[glowjet]

Y mask done and used.

I cut 3 x 25mm slots in a Y shape for my 120ED didn't even measure the angles but roughly the standard Bahtinov angles +/- 15 degrees?? took me about 3 mins to do.

Stuck it on the end of the scope, looking at Rigel you get 3 oblong "blobs" as you focus thay merge into one quite clearly, checked it against my proper home made mask and it was spot on focus! so there you go, try it takes so little time and fuss.

Apologies Peter I misread your thread and it is no wonder there is some confusion relating to the Hartman Mask, you cut three holes in a Y shape which would work like a Hartman, the new mask which I am referring to uses three solid bars in the shape of a Y which should, of course, permit more starlight to enter the scope and result in a brighter image and as pointed out in my previous thread

works just the same as a Bahtinov Mask.

John.

[Psychobilly]

Thanks for clearing that up...

I find Hartman style masks pretty useless for really precise focus...

Much prefer the diffraction spikes of a Baht...

Peter...

[glowjet]

Thanks for clearing that up...

I find Hartman style masks pretty useless for really precise focus...

Much prefer the diffraction spikes of a Baht...

Peter...

Why don`t you give the Y a try Peter, it is supposed to give a brighter image than with the grid of bars of the B mask, quite simple to make I should think.

John.

[adamsp123]

Apologies Peter I misread your thread and it is no wonder there is some confusion relating to the Hartman Mask, you cut three holes in a Y shape which would work like a Hartman, the new mask which I am referring to uses three solid bars in the shape of a Y which should, of course, permit more starlight to enter the scope and result in a brighter image and as pointed out in my previous thread

works just the same as a Bahtinov Mask.

John.

A "negative" version of the type I made, I will try it

[Psychobilly]

Why don`t you give the Y a try Peter, it is supposed to give a brighter image than with the grid of bars of the B mask, quite simple to make I should think.

John.

I have access to CNC Laser cutters and used to get the masks made for people... So I can get very precise (0.01mm) masks made in various materials... I used to do the fiuner slot 1st order designs and would draw 3rd order masks for people who wanted to cut their own...

They 1st order ones give very sharp diffraction spikes...

The Y bar one sounds ideal for use with shorter focal length camera lenses though so I will knock some up and give them a try...

Peter...

[johnrt]

Could you post a pic of your Y shaped mask please?

[glowjet]

I have access to CNC Laser cutters and used to get the masks made for people... So I can get very precise (0.01mm) masks made in various materials... I used to do the fiuner slot 1st order designs and would draw 3rd order masks for people who wanted to cut their own...

They 1st order ones give very sharp diffraction spikes...

The Y bar one sounds ideal for use with shorter focal length camera lenses though so I will knock some up and give them a try...

Peter...

That sounds ideal Peter, the bar width for a camera with a 50mm aperture would be in the region of 1mm or so wide a simple Y glued across an old dust cap, the cap would need to be trimmed to give a clear opening first of course. If you want to experiment from my research one very interesting point has come to light, it apparently does not matter where you place a single bar across the aperture, the resulting diffraction pattern will always be perpendicular to the bar and always cross the central optical axis, if you think about it you realize how the B mask works, but with the many bars trying to repeat the same diffraction the resulting X with vertical will not be as bright as a single bar Y pattern, the reduction in the grid will also, as I have said previously, allow more light throughput and subsequently a brighter image.

John.

[Pete Presland]

i was going to buy one.

but having read this thread i might be able to save £20.how do i make one and what with? they look very time consuming and fragile

pete

[Uranium235]

I made mine by printing it onto 100gsm card, making the bars/slots wider to aid cutting & strength (with a scalpel). Then make a collar for it so it fits tightly over the end of your scope.... simples!

[adamsp123]

Embarrasing as the Y mask is that I made in 2.8mins you can see how crude it was, next to it is my "pukka" 120ED home made mask, my 200/300mm mask is in between the 2 ie thick paper but cut to a good Bahtinov template bit like U235s.

[Psychobilly]

Peter i think we have decided your Y mask is the wrong way round?