Vernier calipers for focus setting?

[DavidH]

Hi All,

Has anyone here tried using a set of vernier calipers to set their focus for imaging?

May not be able to set focus precisely, but should be able to get a pretty good starting point if you have a measurement between two reference points from a previous imaging session. I guess that SCT's will have no real reference surfaces so it would not work with them.

Any input appreciated.

Regards,
David.

[gbeal]

I think this is how Uncle Al does it with some of his scopes.

[Dennis]

Hi David

I use an eyepiece with a "Parfocal Ring" fitted and clamped in place. Here is how it is done:

• Insert e.g. ToUcam ccd camera fully into the focuser and as a one off operation, obtain a good focus (by trial and error).
• Remove the ToUcam from the focuser without adjusting the focuser.
• Insert a 20mm Plossl (with a loose Parfocal Ring fitted) and without moving the focuser, slide the barrel of the eyepiece into the eyepiece holder until the image is in focus.
• At this point, tighten the Parfocal Ring on the eyepiece body so it acts like a stop preventing you from sliding the eyepiece in any further.

At this position, your eyepiece is said to be Parfocal with the ccd chip. This gets me to a good focus, but not a precise focus. I then use e.g. CCDSoft with the ST7E ccd camera and the motorized JMI focuser to fine tune the focus.

Unfortunately, for each different ccd camera (e.g. ST7E, DMK, Pentax DSLR) you will need a separate Parfocal eyepiece and I find with my DMK that I have to use an extension tube on the eyepiece to fit the Parfocal Ring onto. Note that this works on my refractor and SCT, so it is independent of ‘scope.

Cheers

Dennis

[DavidH]

Thanks for the input, guys. I'll test it out under the stars, and may be try a parfocal ring Dennis.

Regards,
David.