A new focus mask? "The Carey Mask"

[erick (Eric)]

http://www.btinternet.com/~gfcarey/careymask.htm

Just found a Cloudy Nights thread on it:-

http://www.cloudynights.com/ubbthrea...v=#Post3007969

[Dennis]

Thanks Eric

Nice find and thanks for posting! This is an interesting variant and the author has got some nice illustrations on his website covering the diffraction patterns from spiders and the Bahtinov Mask.

Cheers

Dennis

[sheeny (Al)]

Thanks Erick!

Could be another fishing line version coming up...

Al.

[Omaroo (Chris Malikoff)]

Thanks Eric!

Clinton?

[citivolus (Ric)]

I find this one interesting, but it does leave me wondering how vulnerable it is to scintillation? The close tolerance of only 2 degrees may make it difficult to use if the seeing is not great. I guess I'll have to build one to see.

[Dennis]

Hi Ric

In terms of scintillation due to seeing, I have noticed that in poor seeing, the centre bar of the original Bahtinov Mask can appear to “wander” or “oscillate” around the judged focus point. Also, when using my Pentaz XW14, as I move my eye around the field, the centre bar moves (very slightly) with it. I suspect the same phenomena would occur with the Carey variant?

Cheers

Dennis

[GeorgeC]

I was doing a bit of Googling and found this discussion on my 'Carey Mask'.
As far as I can make out, poor viewing conditions have not been a problem. Massive scintillation will make the spikes wider, but I can see no reason for the relative positions of them to change.
To be effective the mask needs to be cut from a thin sheet. My most successful version so far is laser cut from paper card, 1.9mm thick. This is to ensure elongated spikes. 1mm or even thinner would be ideal.

If anyone is interested and has access to a laser cutter I can supply a DXF file which can be scaled to suit your 'scope.

[GeorgeC]

Originally Posted by Dennis

Hi Ric

In terms of scintillation due to seeing, I have noticed that in poor seeing, the centre bar of the original Bahtinov Mask can appear to “wander” or “oscillate” around the judged focus point. Also, when using my Pentaz XW14, as I move my eye around the field, the centre bar moves (very slightly) with it. I suspect the same phenomena would occur with the Carey variant?

Cheers

Dennis

Using the masks visually will cause some strange effects as your eye slightly changes its focus. All 3 spikes in a Bahtinov will move simultaneously, but it can look as if just the central spike is moving.
I have never tried looking through the telescope with a Carey mask in place - I will try this out when the sky is clear!

[Dennis]

Originally Posted by GeorgeC

Using the masks visually will cause some strange effects as your eye slightly changes its focus. All 3 spikes in a Bahtinov will move simultaneously, but it can look as if just the central spike is moving.
I have never tried looking through the telescope with a Carey mask in place - I will try this out when the sky is clear!

Hi George

Thanks for that and thank you for the very informative lessons on diffraction covered on your website!

Cheers

Dennis

[GeorgeC]

Here is a DXF file for a Carey Mask for a reflector http://www.btinternet.com/~gfcarey/masks/CareyMask.dxf

If you try one, please let me know what you think of it.

[GeorgeC]

Here is a DXF file for a Carey mask for a refractor (no central obstruction).
http://www.btinternet.com/~gfcarey/m...o_obstruct.dxf
Using Niels Noordhoek's simulation software it can be seen that a focus error of only 20 microns is clearly visible using a Carey mask.
The gap between the right hand spikes is larger than that at the left.
http://www.btinternet.com/~gfcarey/m...st_no_20_2.jpg

[allan gould]

Originally Posted by Dennis

Hi Ric

In terms of scintillation due to seeing, I have noticed that in poor seeing, the centre bar of the original Bahtinov Mask can appear to “wander” or “oscillate” around the judged focus point. Also, when using my Pentaz XW14, as I move my eye around the field, the centre bar moves (very slightly) with it. I suspect the same phenomena would occur with the Carey variant?

Cheers

Dennis

Dennis
I observed the same effect when the seeing was bad and I was trying to capture "Bird's stike" on Jupiter. I found that it (the central bar) did wander slightly but it was easy to judge the center. I suppose its a good way to test the seeing, but without the mask I found it very difficult to judge best focus.

[GeorgeC]

Niels Noorhoek's simulation is now very accurate. This is a Carey mask going in and out of focus:

Carey Mask Simulation