Long FL focusing and Y mask.

[ollypenrice]

With our refractors I've been happy to use FWHM for focusing, though I do an initial focus with a Bahtinov and sometimes tweak it afterwards.

However, focusing Yves' 14 inch using FWHM has been a nightmare because at the long FL the readings leap about insanely. Focus has often been disappointing.

So I made a Y mask using three flat strips of aluminium with the Y angle copied from a Bahtinov. It works. I went to a bright star close in altitude to the target and centred the middle cat's whisker. I then did a longer exposure and found no splitting of the diffraction spikes produced by the scope itself. Cross checking with the FHM showed a very low value, better than I usually get. Excellent.

What I need to do is make the arms of the Y a little thicker so that it will be easier to see when the central whisker is central, but for a minimal outlay and effort I think we've cracked the nut.

Olly

[andrew s]

Hi Olly - Strange you found the long focal length difficult to focus. I would have expected it to be simpler as the slower the telescope the bigger the "focus zone". What was the FL or F involved?

Regards Andrew

[Willarnold]

I'm going to start imaging with my SCT at f 7.7 using the Starizona focal reducer/corrector, so that's really interesting.

How do you know how wide the Y mask should be?

[ollypenrice]

The focal ratio of the ODK is 6.8 and the focal length just under 2.4 metres. The depth of field (so degree of focal tolerance)is only related to F ratio so F6.8 is easy from that point of view. However, the problem with the long focal length is simply that it is far more affected by the seeing than a short FL. Perhaps the 14 inch wide incident beam is also far more easily disturbed than a narrower one.) This causes the FWHM to vary wildly from sub to sub, even when using 4 second subs to let the seeing avarage out a bit. Bahtinovs and Y masks can beat the seeing by sampling the sky along the length of the diff spikes rather just around the star itself.

With a Y mask the importance of the angle is, I guess, that it determines the shape of the X in which you then centre the moving central 'cat's whisker.' In order to judge when you are central this angle needs to be fairly shallow but I don't think it has any other significance. I copied mine from a Bahtinov but since I can adjust it (the Y being made of three alloy strips bolted together) I can suck it and see.

Cheers,

Olly

[andrew s]

Thanks Olly - I know what you mean I had significant difficulty at 2m Fl when trying to use my 300mm Newtonian with an F10 spectrograph. Getting the photons through the slit was a real challenge. Similarly with my OMC200 at F20 giving 4m FL makes focusing accurately difficult in the face of turbulence.

To a degree if the turbulence is that bad focusing accurately is not that meaningful for long exposures as the turbulence shifts the effective focal point all the time as well as blurring the image. Regards Andrew

[ollypenrice]

Yes, like politics, imaging is the art of the possible!

Olly

[Jessun]

Les Granges is turning into DIY heaven, Olly! Good to hear it works a treat!

[JamesF]

I believe Chris Lord suggests different widths for the strips depending on whether you want to use the first order or third order diffraction pattern. Did you worry about that at all Olly, or did you just knock it together and it worked?

James

[ollypenrice]

I believe Chris Lord suggests different widths for the strips depending on whether you want to use the first order or third order diffraction pattern. Did you worry about that at all Olly, or did you just knock it together and it worked?

James

I'm just experimenting with widths at the moment and not really finding anything very coherent! To be continued...

Olly