Hello,
With the seeing so poor last night, 4-5/10 at best, I decided to just have a play around with my home made mask and whacked a x4 PowerMate into the Mewlon 180 F12 to give me an efl of a whopping 8640mm at F48.
Using the SBIG ST7 CCD camera I picked up a mag 5.16 star and was still able to focus using the mask, although I had to push the exposure out to 2 seconds. What I did notice, is that the focusing assist function in CCDSoft (the ST7 camera control software) gave me a slightly different “best focus” indication compared to the mask.
So far, I can only think of 2 reasons for this:
- My mask is made from a cut out overhead transparency and is therefore floppy so all the diffraction elements do not lie in a single plane.
- The weight of the ST7 CCD camera and CFW8 filter wheel dangling on the end of the 2” x4 PowerMate was causing the motorised focuser (Moonlite) to flex slightly so the plane of the CCD chip was slightly angled.
The effect is more noticeable in the right image of this pair.
Incidentally, I was still able to use the CCDSoft focusing assist function with the mask fitted, to find the best point of focus due to the sensitivity of the ST7 even at an fl of 8640mm at F48. Also, having the mask fitted made the overall focusing routine so much more efficient in terms of converging quickly to the optimum focus point.
So, even despite some quite poor seeing conditions, the mask was able to perform very effectively with 2 sec exposures at F48 – an impressive result for a piece of cut-out overhead transparency!
Cheers
Dennis