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.
Bintel Coma Corrector
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.