Several weeks ago, I finally set up my C-11 OTA, TCF focuser, Pyxis field rotator, AO-7 and ST-10 for imaging at f10. That’s 2880mm with only .5 arc seconds per pixel unbinned on the ST-10. Let’s just say that results were mixed. I’ll admit that seeing wasn’t great, but I don’t think I’ll try this again.

There are some good things about imaging at f10.

• The AO-7 guides very well. Round stars, no wandering, very nice even at this long focal length.
• Objects are large with good magnifications (if only they were clear — see below)
• I could use the Pyxis field rotator with my C-11

But the negatives outweighed the positives.

• Finding objects was hard. Where a sub-second focus exposre 3x binned would find an object at f6, 1 or more seconds was required for each finding and framing exposure.
• It was murder trying to get to focus. FocusMax struggled because seeing changed overwhelmed focus changes.
• It was impossible to find a guide star. With the AstroDon Clear filter, it was a struggle to find a star bright enough to guide at 1-2hz with the AO-7, minimum (in my mind) for decent results at f10.
• So that was guide stars with the clear filter. Forget color. No object I tried had a guide star in blue that was usable. (Sidebar — NGC 891 – great guide star, available only at f6.)
• No data. I felt as if I was imaging with an Ha filter. After 5 minutes, the object was hardly brighter than the backgound (100 ADU out of 3500)
• Fuzzy results. The seeing errors overwhelmed the ability to resolve. All I got were high magnification, blurry results. It reminded me of when I try to use a high magnification eyepiece or a barlow, and, while the result is more magnified, it is not a better image to see or record.
• It sure makes me wish I had a 20″ Richey Chretien. 🙂

It only took a couple of years, but now I have learned that all of the advice about long focal length imaging is correct. It is really hard and, without great equipment (no, good won’t suffice), it will not produce adequate results.

Here is the result:

So f5.95 with an C-11 is OK, but f10 is not.