Astronomy Technology 8" Reflector
By Paul Temple
In December of 2010 disaster happened! My AS-GT Scope mounting a 6’ Celestron refractor with an ST-7 CCD was picked up by a rogue 125 mph (recorded on my weather station!) wind gust and thrown against the fence. Luckily the OTA and more importantly the ST-7 survived. The mount did not. The OTA and camera were a bit too heavy for the mounting anyway but it really showed it’s weakness when it hit the fence. Instead of just replacing it I did some soul searching and decided to replace the mount and OTA.
My residence is located south of Deming, NM about 5 miles, 25 miles from the Mexican border. It is fairly dark in most directions, almost 6.0 skies with just a few lights in the distance though there is a substantial light dome over Deming that reaches about 10 degrees high in the northern sky. I have been observing since the age of 5 years old, around 51 years. Over these years I have used everything from a telescope built from 2 magnifying glasses to the 24” refractor at Lowell Observatory and the 31" reflector at the Astronomical Society of Kansas City's observatory in Kansas! My all time favorite telescope to use is my 6” Celestron refractor with the Meade 90mm F/8 refractor close behind. Still my homemade 8” dob and now the AT 8” F/4 have carved a place in my heart.
The AT 8” F/4 refractor is well built, comes very well packed and is pretty much useable out of the box. It comes with a battery case for the fan on the mirror and a 35mm, 2”, extension tube. The extension tube works very well to position the ST-7E camera at the right distance for imaging. The one problem is that it only comes with one thumbscrew when there is holes for 2! This is how it is shipped from the factory! I robbed another focuser of its thumbscrew for without it the camera was a little loose. Since I use a home observatory I just use a 12 volt external transformer to power the mirror fan.
The OTA is mounted on a used Sirius mount upgraded to a tripod with 2” diameter legs. The mount is operated by a HP 3.0 mhz laptop with 1 gig of Ram. The mount is connected by an EQ-Mod connecter to a USB port on a multi port station, (I use a PCMCIA 2.0 USB adapter to speed things up!) running, The Sky pro 6, T-Point, CCDSoft for the CCD, EQASCOM to let the mount talk to the Sky and EQPAD to run a cheap 5 dollar, E-Bay PC gamepad in place of the hand controller. All of this has a pretty steep learning curve but works very well! The Sirius mount works extremely smoothly and I have been very impressed! To balance it all I have to use 33 pounds of counter weight.
Though this scope is designed for imaging you can use it for visual observations. I looked at a number of objects with a 25mm Plossl and was impressed by the large FOV and brightness of the image. Yes, there is coma but who cares when you can get a space like view here on Earth! Still this setup is obviously maximized for imaging since you have to add extender tubes to get focus.
The negatives: The focuser is a bit light. It works well but unless you lock down the focuser tube it can shift between images. This problem is not enough to warrant replacing it since I am eventually planning on using a robo-focus setup anyway so this issue will be moot. The procedure is to move the focus knob, lock it down, image, unlock the knob, re-focus, etc. Once you get focus though, it holds nicely even for a heavy camera like mine. I do have a dial gauge that I am planning on mounting on the focuser so I can really hone in on the different settings for the filters. Since I have an CFW-8 filter wheel with different brands of filters you have to refocus each time you change so robo-focus or a dial gauge really saves time.
The second issue is finding focus. Because it is a very fast focal ratio it is very sensitive to focusing. I still have not reached a critical focal point in all my efforts so far. I have a Bahtinov mask and that helps but it still seems to be slightly off and you must use the settings in CCDSoft to find the best focus. I suspect that those that use video or DSLR will find it easier. The ST-7 E is a parallel model and takes close to 10 seconds to image and then download. So you have to image, wait, adjust, image, wait, adjust and that just takes awhile. If you put it in 2x2 binning it is easy to focus but is still way out when you change to 1x1. Since I do mainly photometry critical focus is not all that important anyway!
There is also substantial coma in this design. If you want to use the full FOV you would need to have a field flattener and coma corrector. Even with my small chipped ST-7 there is a bit of coma around the edges and that effect is exacerbated by being out of focus. Since I also desire to do ultraviolet imaging I opted not to use a field flattener since it would absorb most of the UV light and I just live with it.
The other issue is a very small image scale. This is to be expected when using a fast focal ratio scope. Still, objects that had lots of detail in my F/8 refractor are seen but with not as much detail in the AT8”.
The positives: Great design and build! It is built very heavily with a good serviceable focuser and a steel tube. The baffles in front of the focuser get rid of any stray light effects.
The field of view is amazing. With an F/8 scope it was difficult to find comparison stars sometimes. This is not an issue with this scope! The FOV is around a half degree. This makes it very easy to place the object on the chip every time!
Not sure why this is but the AT8” setup maximizes detail in images even with the small image scale. With a 30 second image in infrared I captured the jet coming out of M87! Due to image scale it is not detailed but the fact that an 8” scope can do it at all is pretty amazing! I was also able to locate 2 proplyds in M42. This is also not something I have seen a lot on the hundreds of M42 images that you see everywhere.
I was very concerned about balancing issues. So far there hasn’t been any! The Sirius works well even in the awkward positions the camera assembly can end up in. It would probably benefit by putting a counter weight on the opposite side of the camera but is really a non-issue so far.
The mirror fan is quiet and non-vibrating. Just a couple of minutes of running the fan will cool down the mirror pretty fast anyway but it is nice to know it does not effect the imaging/seeing.
Conclusion: This is a great bang for the buck! It is one of the finest instruments I have used and is actually cheaper than my Celestron 6” refractor OTA. For imaging this is a great set up and I am surprised that others have not figured this out and started buying them like hotcakes! It is a good size for a Sirius/Atlas or CGEM or similar mount. The Celestron AS-GT and Meade LXD-75 would be a bit light but would probably work with this setup though it is approaching the maximum load. With the reflector, unhindered UV and spectroscopy can be accomplished. Overall I would highly recommend this scope for science or pretty picture imaging alike.
|All times are in (GMT-8:00) Pacific Standard Time Zone|