How to use a Bahtinov mask

[starman1969]

Whe I got my first telescope, about a year ago, it came with quite a few extras including one of these masks. Although I've managed to see quite a lot & image stuff through it, I've never actually used the Bahtinov.

This is probably because I don't have a clue how to use it. I did try a couple of weeks ago, but it didn't seem to do anything. Do I have to use it with a computer screen or something?

From what I have seen, the mask seems to be a very useful tool for focusing.

[Rustmonkey]

You don't need a PC or camera to use it....that's only if your imaging. You can use it for visual too. Point your scope at the brightest star you can see in your sky and line it up in the centre of the Eyepiece. Focus it as best as you can then pop the mask over the front of the scope. You will see that the star has 3 straight line diffraction spikes running through it. The centre line may not appear completely centred. If not, then refocus slowly and you will see that it starts to move. Get it so that the spikes are perfectly centred nd bingo....perfect focus. Remove tha mask and you are away to go stargazing.

[Russe]

If you are using a comp, then you might think about downloading bahtinov grabber - a program that helps you achieve better focus yet.

[Russe]

Oh and: DONT FORGET TO REMOVE THE MASK...

Obvious for visual, but if you slew over to a dark object you won't notice it anymore...

[starman1969]

OK thanks. I can't see any way it can fit to the front of the scope. There are some plastic screws that came in a bag. Do I just put these through the holes on the mask, then hang on the front of the scope?

[Cornelius Varley]

The mask just hangs off the front of the tube using the plastic screws.

Peter

[ollypenrice]

One small refinement; your bright star should be as close as possible in altitude (height above the horizon) to your target. The atmosphere does affect focus, though the effect is minor in visual use.

Olly

[Russe]

Yes, the screws confused me as well. Hang the mask on the front of your scope, with screws lying on top of your scope. No hard connection.

[starman1969]

Ok. Thanks for the replies. I must start using it next time i get the scope out.

[memoryman]

Does it make a difference if you are using a dew shield? Do you hang it on the dew shield instead?

[Russe]

It's not really designed to go on top of a dewshield.

[IanL]

It's not really designed to go on top of a dewshield.

So long as the dew shield is solid enough to take the weight it makes no difference. If you think about it logically, you can use a mask with a Newtonian where it is the whole length of the tube away from the primary mirror, or you can use one with a refractor where it is a few mm in front of the objective and it will work effectively in either case. All that matters is that there is an appropriately shaped obstruction close enough to the scope to create the required diffraction pattern. For a typical amateur scope, it makes no difference if that is right in front of the scope's primary/objective or a fair distance in front, so long as it is thick enough to produce a visible pattern and covers the whole aperture.

I made my own Y-mask (the simplest version of a Bhatinov; easier to make DIY and better for imaging since it obstructs far less light) and sit that on the dew shield of my ED80. Combined with APT (which has Bhatinov Grabber built in as a tool) I can get perfect focus for imaging.

[Steve Ward]

As Ian said .... makes no odds with it on front of the dewshield , the shield on my Mak180Pro is as long as the tube itself to eliminate neighbours lights as well as dew and focuses precisely with my "StarSharp" mask ....

[JamesF]

The other reason for remembering to remove the mask of course is that otherwise when you slew to your target it will probably fall off. On grass you might get away with that. On concrete you'll probably end up with a broken mask.

James

[glowjet]

The mask is nothing more than a device to cause an obstruction to a wave front of light so as to create a diffraction pattern on the optical axis of the scope, so long as this is placed across the aperture of the scope, or the aperture extended a short way by a dew shield, should not make any real difference