Re: [APML] focus

[Aplanatic@aol.com]

Scott,

>I am looking for comments about the focus of this image. This is my
>first attempt with the KEF technique. I think it still needs some work.
>www.creatorsview.com/Pages/testimage.html
>This is an unprocessed image.
>
>Scott Hammonds

It doesn't look well focussed to me.  This could be due to poor focusing before the start of the exposure, or it could be due to a change in temperature of the OTA during the exposure, or both.

The Meade 10" f/10 has, I believe, a secondary magnification of 5X.  As the aluminum tube changes length with temperature, the secondary moves with respect to the primary.  This motion is amplified by the square of the secondary magnification, about 25 times in your case.  This causes about a 12 mil (300 micron) change in focal position for a 1 degree C change in temperature, and even this small change (1.8 F) will defocus the image significantly.  All commercial SCTs, with their large secondary magnification, suffer from this effect.  The 9.25" Celestron is a bit better in this regard, having a secondary magnification of 3.3.

This problem can be mitigated by allowing the OTA to cool for at least an hour or two before exposing.  Also, I've found that the air temperature is most stable after approximately 1 am on an average night/morning.

Do you remember if the air temperature was dropping during the exposure?  When using a commercial SCT for prime focus photography it's a good idea to record the air temperature at the start and end of the exposure, or better yet, record the tube temperature directly by using a readily available indoor/outdoor thermometer having an outdoor probe.  Attach the probe to the tube with some tape and then tape some foam over the probe.  Making this measurement can be a great way to eliminate some of the uncertainty when diagnosing problems with fuzzy images.  Even though I use a CSCT camera with a carbon-fiber tube and low secondary magnification, I make this measurement myself, and have found it to be useful.

As Matt has stated, make sure that your KE is parfocal with the film emulsion, and practice with the KEF.  Move the focal plane inside of focus and outside of focus by 0.5 mm and notice how the aperture darkens from the left or right.  This will give you a good sense of when you are very close to perfect focus.

Make measurements, experiment and have fun!  A decent SCT can take very nice, even spectacular images, but they're a bit trickier to use than a refractor.

Dave Rowe




--  APML Archives at <http://astro.umsystem.edu/apml/>  ---
             Unsubscribe at <majordomo@seds.org>