OTA and Guide Scope

My original OTA was a beast. The 10-inch Schmidt-Newtonian with a thick glass corrector plate and mounting rings came in at 32lbs. This was allegedly one-pound shy of the maximum weight for the LXD75 mount. To hear the motors strain, even when well balanced, said it was about 10lbs over the limit. I would imagine that it was a decent scope, optically speaking, but I didn’t really know any better. I do know that the 1.25-inch rack and pinion style focuser left a lot to be desired.

When I decided to get back into this hobby, I was sure I wanted a Schmidt Cassegrain OTA. They were all the rage when I attempted this before. The planetary views and deep-deep sky views were amazing, and they still are. However, I think at that time, the lack of large imaging sensors in an obtainable price range didn’t allow us to see the amazing wide-field shots that fast refractors and reflectors now provide when using a big imaging chip. Back then it was about looking deeper at little things, rather than looking wider at bigger things. Now both are accessible, and I prefer the beautiful wide views. It doesn’t hurt that the bigger, wider things can be more forgiving as far as equipment precision goes.

Luckily, I read up (a lot) on what had changed and gotten better in the last twelve years. The advancement of DSLRs in the use of astrophotography had opened a lot of doors for wide field imaging. I knew that an Apochromatic Refractor would offer the best images with the least fuss, but they were also immediately out due to money constraints. You can’t touch a good true APO for under $1000 and I was looking to max out at about $400.

When I moved onto the Newtonian options, I thought I had made my final decision very quickly. GSO has a line of imaging specific F/4 Newtonians in six, eight and ten-inch varieties that seemed perfect for what I wanted. They have a nice large secondary mirror and the added feature of a two-speed focuser. The six-inch version even has an extended tube length that serves as a dew shield. However, it’s very difficult to use these visually. I had seen some attempts with the use of extensions that put the eyepiece about two feet away from the focuser. On the other end in the six-inch variety is the F/6 scope. While easy to use visually, the field starts to narrow with the longer focal length and using it at prime focus would require a different focuser, not to mention increased exposure time. The F/6 OTA also becomes longer (35-inches) and harder to lug around as the focal length goes up. The F/4 astrograph was very compact and easy to transport at 22 inches long and only 10lbs. GSO six-inch F/5 Newtonian did just about everything right in-between. You can image with it, and use it visually with only one small extension. It’s a little longer than the F/4, at 27-inches, and a little heavier at 12lbs, but still very manageable. For me, it was a happy medium. It was also $100 less than the F/4 Astrograph. It’s hard to believe that you can obtain some decent quality optics for barely much more than a Toys-R-Us child’s telescope, but you can.

Two things of note with the GSO scopes:

  1. I don’t know if this is typical, but it seemed that several of the F/5 scopes had different sized secondary mirrors despite being from the same manufacturer. You want a somewhat larger mirror for imaging. Don’t be afraid to ask the retailer for the information if it isn’t on their site or on the specs. Mine is 55mm, but I’ve also seen 50mm and 52mm. I am looking to maybe increase mine to the 62.5 mirror.
  2. You will be told that you need a two-speed focuser to image. I don’t find this to be the case. I find the GSO single-speed focuser to be very smooth and have no problem reaching perfect focus rather quickly with the use of a Bahtinov mask. You will need a focus lock!

Guide Scope

This is a short section, but it shouldn’t be ignored. For my guide scope, I had an opportunity to save $60 or so by modifying an existing Meade 8×50 finder. I cut down the focal length by about 30mm and used a Dremel tool, modifiying a 2-inch to 1.25-inch stepdown plumbing fitting ($0.80). It’s not pretty, but it is effective.

If buying new, try to stick with a 50mm objective lens to give you a decent field of view and decent sized stars. Orion offers options with and without a helical focuser, and there are also ebay specials for about half the price. You can still achieve focus on the model without the focuser by manually sliding and locking the camera in place, it’s just a bit harder to get it perfect.

Two notes on guide scopes:

  1. Make sure you get the correct bracket and mount to fit your OTA base mounting bracket, or make sure the base mounting bracket is included so you can change it. Guide scope mounts are definitely not created equal.
  2. Align your guide scope with the field of view of your main OTA. Some people adjust it to obtain a better guide star. I prefer to keep it dead centered to provide the best tracking/guiding unless it’s absolutely necessary to move it a degree or two to find a star.