Dave Wallace's Adventures in Astronomy

02/07/2004

One problem with the Meade Lunar / Planetary Imager camera is that it makes focusing the telescope difficult. The LPI is sending about three frames a second in "LIVE" mode and the number of pixels in each frame isn't really enough to judge the focus easily. It would be much easier to focus the telescope while using an eyepiece and then substitute the LPI. But for that to work, the LPI and the eyepiece have to focus at the same setting (be "parafocal").

The last time I experimented with the LPI, I found that I couldn't make all of my eyepieces parafocal to the LPI camera. The 26mm eyepiece had to be extended so far that there wasn't enough barrel left over to engage the set-screw of the moon filter or Barlow. I was contemplating buying another eyepiece specifically for use with the LPI or having a longer barrel made for the 26 mm eyepiece. (The barrel of the 26 mm eyepiece was about 1/4" too short. While I could buy an extension tube for the eyepiece from Scopetronix, the extension tube is 1" long, which seems excessive.) I was also wondering if using the LPI on a visual back at the rear cell port might solve the problem, but I don't have a visual back to experiment with.

Then I had the bright idea to try making the camera parafocal to an eyepiece in the Barlow. When the 15 mm eyepiece is in the 2X Barlow, the combination has just about the same field of view as the LPI, which would help with "framing" the photography. And I guessed that inserting the Barlow into the optical path would make it possible to get the eyepieces parafocal because I knew that using the Barlow required me to re-focus by about the same amount as was necessary to get the LPI to focus. (I just hoped that the focus correction was in the same direction!)

Anyway, my guess turned out to be correct. In fact, I wound up using the parafocalizing ring that Meade supplies with the LPI -- but I had to attach it to the camera instead of to an eyepiece. After a few minutes worth of experimentation, I found that the LPI with the ring tight against the camera body was parafocal to the Barlow plus the 15 mm eyepiece if to the Barlow I added two of the O-rings I use to make the eyepieces parafocal. (These are BUNA O-rings 1-3/16" in diameter and 1/16" thick. I bought a bag of 100 of these from McMaster-Carr last year for under $10.)

So now the eyepieces form a mutually-parafocal set, thanks to O-rings on the 26 and 40 mm eyepieces and when the eyepieces are inserted into the Barlow, any combination of Barlow plus eyepiece is also parafocal to the LPI. The 26 mm eyepiece now fits all the accessory equipment again and getting the LPI to focus is now a snap.