One thing I started to notice in photographs taken through my GSO 8″ f/5 newtonian was the stars towards the edge of the field were stretched out a reasonable amount. This is called comatic aberration or coma for short. I knew this would happen, but I decided I would try imaging without any corrective optics to begin with.
Eventually I decided that it was time to fix the comatic aberration in my images by using a coma corrector. I don’t like to spend money when it isn’t needed, so I figured that buying a cheap and cheerful coma corrector from Bintel would be the best idea. It says in the product description that it has a 2″ barrel, and it can be used with a DSLR. Perfect I thought! When I received the corrector, and tried to use it, I discovered things weren’t quite that easy.
Now, it’s really easy to make this mistake. If you look at the product description, it tells you that you can use a T-ring by unscrewing the 2″ barrel and screwing on a “wide” T-ring. Now here’s what tripped me up – I don’t have a “wide” T-ring. This appears to be some sort of “special” T-ring from Orion (and maybe others?) that is 48mm, versus the standard 42mm for a T-ring. I think the idea behind this size increase is full-size sensors in much more expensive DSLRs tend to experience an amount of vignetting when you use a 42mm T-ring. I’m running a 450d which is a crop sensor camera, and I don’t see any significant vignetting.
I was playing around with my current T-ring to 2″ barrel adaptor, and discovered that the barrel has a 48mm thread inside it for filters. It turns out this is the exact same thread that the coma corrector uses, and it screwed right in! I enthusiastically set up my telescope and tried to get it to focus. I failed, until I moved the combination of camera, tube and coma corrector out fairly far and racked the focus right out. Not good. I thought maybe it was because the distance between the lens and sensor was wrong (which it is), so I thought I could just get some spacers to correct this.
Back to Bintel – I found this product:
Orion T-Thread Spacer Ring Kit
This looks like it’ll do the job, but it’s for T-thread (42mm) which just isn’t going to help. The lens and sensor are already too far apart so this will just make the problem worse! I checked the Bintel site (and several other Australian telescope suppliers) but I couldn’t find a product to adapt a 42mm t-ring to the 48mm thread. I contacted Bintel about this problem, and it turns out they have a solution. They have a M48 to M42 step down ring for this exact problem. This, combined with the expensive T-thread spacers should allow the camera to mate up with the coma corrector at the right distance.
Fast forward a couple of days, the parts arrived. $50 for a few bits of metal that would hopefully make life easier. I tried several combinations of the spacers with the M42-M48 adaptor connected to the coma corrector, and I have found a combination that gives me roughly 70mm spacing. According to several sources on the Internet, this is the right amount of spacing to correct any comatic aberration in my f/5 newtonian.
As is always the case with these things, there hasn’t been a clear night since I received the spacers and adaptor so I haven’t had the chance to test this gear out. It’s looking likely that in the next couple of days there might be a few hours of clear skies during the night. I will attempt to get some before and after shots as soon as I can.