Fun with coma correctors…

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.

Astrophotography Equipment

telescope-setup

Mounts

I started getting into astrophotography using a few different mounts. I tried using a really cheap EQ1 style mount that had a motor drive, but despite my best efforts to align it with the Southern Celestial Pole, I could never get it to work quite right. A lot of my frustration with this mount was due to the drive motor, it had a variable speed that you could change using a potentiometer and really no way to tell if you were close to the right speed.

Next, I got a hold of a Meade DS-2000 mount. This basic Alt-Az mount is fine for pointing a telescope at objects and looking through an eyepiece, but it’s useless for much else. There are instructions on modifying the firmware for the hand controller which will allow you to switch the mount from Alt-Az mode to EQ mode. If you put the mount on a wedge, you can get some level of accurate tracking as long as you don’t put much weight on the mount. A DSLR with a small-ish lens is probably as much load as you’d want to put on it before the clutches start to slip. It won’t be accurate enough for exposures of more than a minute anyway. You might get away with 250-300mm focal lengths with fairly short exposure times – enough to get an OK shot of the Orion Nebula.

Barn door mounts are quite a bit of fun. The basic ones are not too hard to make and you can get some decent results, but again don’t expect to take any really long exposures. You’re still limited to wider fields, or shorter exposures at longer focal lengths. I’ve had a few goes at making these with various levels of success. They’re a good project, and if you motorise them with say, a stepper motor controlled by an arduino, you can learn some programming and electronics along the way.

Ultimately all of this effort (and money) led me to buying a decent mount. The mount is key to getting good pictures. There are many, many mounts available but ultimately you will want an equatorial mount if your goal is astrophotography. I found that the HEQ5PRO was the best value for money at the time, and it has served me very well.

Guiding

If you’re starting off, you can probably skip an autoguider. Don’t get me wrong, they’re fantastic things and have helped me go from 20-50% keepers to 90-100% (weather permitting), but they are an added complexity. I currently use the fantastic and cheap Orion Starshoot autoguider package. You just fit it to the finder scope mount on your OTA, align it with the OTA, plug it into a computer and  the mount, and you’re good to go. It’s such a small package that it won’t add any more weight and it works well enough as a replacement for the finder scope.

OTA

AKA Optical Tube Assembly. This is where you’re really spoilt for choice. There are thousands of options to choose from, and researching your options will really pay off here. I went with a package deal for my first setup, HEQ5PRO mount with a Guan Sheng Optical f/5 8″ Newtonian telescope. It’s a decent package, I’ve taken some good shots with it, but I think if I was going to buy a new OTA I would spend the money on a good quality Ritchey–Chrétien telescope.

Imaging

I use a Canon 450d with the IR filter removed. It’s great for just taking shots without having to worry about filters. It has its limits – after all it’s still an entry level DSLR, but for now it’ll do.

Accessories

  • A small powered USB hub for connecting the autoguider and camera to.
  • A good electronic spirit level, for levelling the tripod and helping set the mount to the correct latitude.
  • Velcro cable ties are handy for keeping the mess of cables at bay.
  • It’s worth getting a Bahtinov Mask too – but I’ve found you need a fairly bright star to make use of it.
  • A laptop is essential for controlling everything too, especially if you’re like me and not permanently set up in a dome or roll-off-roof shed.
  • Get a laser collimator – this makes collimating the telescope quick and easy.