Bahtinov mask?

[TheThing]

Does anyone know if Bahtinov masks are specific to certain type of scope or a specific focal length? I.e. if one is suggested as being for an 8" SCT can it also be used with an 8" Newt?

Thanks,

[lukebl]

I have one which was designed for my 200mm Newt, and I use it on my 250.

It's too small for the larger scope, so it's now sellotaped to a large disc of cardboard, but it still works fine! So presumably it's not too critical.

[Beyond_Vision]

They are tailored for different focal lengths etc. There are calculators for the layout of the mask available online. If you search the forum there will be links.

Regards

Kevin

[Bizibilder]

As far as I'm aware they are focal length specific - If you use the mask generator:

astrojargon - Bahtinov Focusing Mask Generator: Version 0.4

You will get a different mask for a Newt or SCT of the same "diameter" but different focal length.

[TheThing]

Darn, not the answer I was hoping for!

Thanks for the replies though gents.

[davew]

When Bahtinov masks are described as being focal length specific, what exactly is meant ? Does it mean that the spikes aren't clear or perhaps the focus point moves ?

I have a slight understanding of energy spikes and really can't see what effect a mask with bars a few thousanths of an inch different would make.

Take a " Y " mask. No slots and yet it works just the same. Pieces of string across the front of the scope. Works.

I've included a photo of a Y mask I've made. It works on an 80mm f5 scope. A 78mm f8 scope and various camera lenses from 50mm to 200mm. Not a slot in sight.

Or am I missing some thing ?

Dave.

[davew]

See if it loads this time

[Demonperformer]

Another vote for the Y-mask ... I resisted the idea like anything when first introduced to it, but now use nothing else ... so simple to make/use

[Pete Presland]

nooooooooooooooooooooooooo i spent over 6 hours filing all the slots out on my home made mask please dont tell me that Y shaped thingy works

[ollypenrice]

Focus is SO important in imaging. I'm a little doubtful (perhaps wrongly) that anything woud bealt a good Bahtinov other than painstaking work with FWHM... I use tailor made ones from Starsharp.

Olly

[davew]

You could well be right.

Would the quality of the mask play any part ? Most of the masks I've seen are not razor thin. They are made from a fairly thick plate. If this works well then all the quality refractor builders are wasting their time using sharp edge rings down the tube.

The Y mask ( Which uses the same angles as Bahtinov ) can be very easily made from fairly thin plastic that doesn't deform. Well it doesn't seem to in my case because it's small.

No one yet has been able to explain to me why energy spikes from a Y mask don't work. They, like so many others won't take the risk because the received wisdom ( Could read correct wisdom ) is that you must make each mask to suit.

Also, do people use some thing like " Al's focus aid ?" The center spike possition is often a guess.

Maybe one for Durham University.

Dave.

PS. I re read my first post and it could sound confrontational. It's certainly not meant to be.

[ollypenrice]

Not at all confrontational, fear not! Interesting point about the thickness of the mask; this had struck me as well but I get an excellent spike pattern from my thick masks. One thing I have yet to try is compare the result from the mask with that from FWHM. I'm always too hungry to started to bother with a comparison!

Olly

[davew]

Comparison tests would be a nightmare. You'd lose a whole night's imaging I'm sure. First wait for a night of good seeing. Next use the correct Bahtinov mask, then FWHM, then a properly made Y mask, repeat using a focal reducer. All on the best equipement using the best motor focuser and digital readout. Not for me.

Also take into account your SCT owners. Minimum 3 masks. One for native and one each if you have two FRs. Then you find that due to the camera spacing what you thought was f6.3 turns out to be f5.5. Do you really want a fourth mask ? All starts to sound far fetched.

But then, don't let me put anyone off buying four masks for one scope.

There will be cons to a Y mask. If you have a large scope the thing may sag. Also the Bahtinov is more sturdy to store. Frizbee by day, focus aid by night.

Thanks for looking and listening.

Dave.

[Rob_Jn]

nooooooooooooooooooooooooo i spent over 6 hours filing all the slots out on my home made mask please dont tell me that Y shaped thingy works

I'm in the same position, spent 2h yesterday cutting out a 16 gauge piece of aluminium for my 14"!!

I wonder are the diffraction spikes as strong on the Y shape as a Bhatinov mask? The multiple diffraction spikes may reinforce and make them more visible? How strong are they on the Y shape?

Rob

[Demonperformer]

I use mainly the canon, so I do an initial focus using live view (enlarged) and then add the mask and take a 20s exposure and have no problems identifying when the spike is central.

Of course, never having used a 'proper' BM, I have nothing with which to compare in terms of ease/accuracy on my setup .... .

And if you were to compare my images with Olly's ....

[adamsp123]

I have made several masks for various size scopes, non of which have been absolutely spot on for focal length, I cut them with fairly wide slots out of card, and they all work fine!

I think there is too much made about the focal length/ratio slot width etc. you need a few slots done in that Y shape pattern and you will get good diffraction patterns especially on bright stars.