My Issues with Long Focal Length Imaging

[BlueAstra]

Interesting learning experience over the weekend. I have been taking narrowband images using my Meade 5000 APO operating at 384mm efl, F/4.8 (with a x0.8FR), typically taking 8min exposures. This gives me workable results, considering it’s a mobile setup, and is a reasonable compromise between setup time, imaging time, sub loss due to planes, etc, having to get up for work and the weather limiting nights out. I’ve recently significantly improved my polar alignment setup with the use of Alignmaster software so I thought I try a challenge with my C8 SCT and narrowband imaging. I’d also recently read that the x0.8FR gave better results than the x0.63FR normally used, so I thought I would include that.

The setup was C8 SCT+0.8FR+(SXOAG+SXLodestar)+SXFW+SXH16. This was now operating at 1625mm efl, F/8. The good news was that the polar alignment software and MaximDL guiding was spot on, and it seemed I could guide for however long I wanted. The bad news was F/8, implying x4 exposure times to get equivalent results so my 8m exposures become 32m ! In practice this was too long for me, and shorter exposures were very grainy. So perhaps a compromise was to go back to the 0.63FR, giving 1280mm efl, F/6.4. This would only be about x2 exposure time so perhaps not so bad. However, found that the FR+OAG+H16 arrangement only really suited the 0.8FR. If I screwed on the 0.63FR, stars were badly distorted and hard to see in the OAG, possibly due to the prism location in relation to the beam cones. I could re-arrange the imaging train to put the OAG closer to the FR and refocus it, but I didn’t want to mess with a good setup, so I think I’ll have to stick with the 0.8FR.

I persevered with the 1625mm, F/8 SCT system, but in the end decided that it was possibly OK for RGB, but not for narrowband. I’ve enclosed an example of a 10x10m Ha image from this system, and you can see its not long enough. Guiding did not seem an issue with the x0.8FR+OAG, and I could have gone much longer. However, the 384mm, F/4.8 refractor obviously suits NB much better. So in the end I think I’ve re-invented the wheel and discovered the short focal length,F/Number systems are much more suited to astrophotography than long focal lengths,F/Numbers, especially if you have limited time to image. I may be looking for a slightly longer focal length refractor to frame some other targets better, but watch that F/Number!