Awhile back, I converted a NexStar 11 GPS OTA to forkless with the intention of using it for long-exposure deep sky astro-photos. It’s been a long road, but things are starting to work out.
First, Some History
This scope started out its life as a Christmas present in 2002. Unfortunately, a scope this size is pretty complex, especially for a first-time user, so it sat covered in a garage for 15 years. Fast-forward to 2017; the owner of the scope is now married and looking to make some room in her home for future additions to the family, and puts her husband in charge of getting rid of it.
Long-story short, I met the husband on a popular local astronomy society’s site. After meeting up with him and spending some time learning how to get the scope set up and tracking planets, he and his wife decided to give the scope to me, straight up. I couldn’t believe it.
Figure 1: My son standing under the then-new-to-me NS11, with its original fork mount still attached.
Immediately, I set out to make some minor modifications to the scope. I would take it off of its Alt/Az mount and add a dovetail bar to it to make it work on my Equatorial mount (I’ve written another blog post about the whole process; read about it here). These mods didn’t take a whole lot of time, but I found myself busy with other projects throughout the summer months. I did take the scope out for a couple of outreach events, but it sat mostly dormant over the winter, which brings us to today.
Getting Things Going Again
Last year, I did get a chance to use this scope once on a last-minute imaging session with M13, but it didn’t turn out the greatest. It’s been so long I almost forgot where I left off, so I had to relearn most things about getting it set up for imaging again. Here’s the checklist:
- Set up tripod.
- Attach mount head.
- Install bar extension and counter weights.
- Attach scope to mount head.
- Attach camera and off-axis-guider to scope.
- Balance scope in both RA/DEC axes.
- Polar align the mount and collimate the mirrors.
- Connect cameras to laptop.
- Achieve critical focus with main camera.
- Achieve critical focus with guide camera.
- Calibrate guide camera.
Having not done this in over 6 months, this took FOREVER, but I finally got things working at about 2AM. I set the cameras to shoot until sunrise, then hit the sack.
I ended up getting two hours of exposure time on M101, also known as The Pinwheel Galaxy. There were several outstanding issues I couldn’t solve (more on those later) but this is what I was able to get from the session, and I’m really happy about it!
Figure 2: The Pinwheel Galaxy (M101). Click for high-res version.
Now for the things that didn’t work out so well.
- It took FOREVER to get it polar-aligned, and the alignment was only so-so. I think I didn’t put it on firm-enough ground so it settled a little bit while I was aligning it. Lesson-learned (doh).
- The autoguider had some trouble, both finding a guide star (which is a common issue when using off-axis guiders), and then it had trouble staying locked on. I had to throw away half of my sub frames, leaving me with 34 two-minute exposures. Part of this may have to do with the settling issue above.
- Focus stayed reasonably good throughout the session, but on the whole things are a bit soft. I think this is because I’m using the cheap Celestron Reducer/Corrector; it’s just not designed for large APS-C sensors like the ones I’m using now. I think a Starizona SCT Corrector LF ought to do the trick. Bad seeing may have played a part, but we were camping in an area with no reception and I honestly didn’t think to check before things got underway.
- Not enough exposure time. I was hoping to get 4-6 hours on the target, but had to settle for just over 1 hour of decent exposure time. Part of this was the issues above and part of it was arriving late in the evening to our camping spot.
All told, I’m really happy with what I got out of this go-around. If I can fix at least one of the above issues next time I try this out, the image quality should improve by quite a bit. Stay tuned!