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Astronomy Technologies 8” F/4 Astrograph


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Astronomy Technologies 8” F/4 Astrograph

byPaul V. 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 wind 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 5.5 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 53 years.  Over these years I have used everything from a telescope built from 2 magnifying glasses to the 24” refractor at Lowell Observatory! My all time favorite telescopes to use were my 8” Meade LX200 (It is now a Robo Scope being used by an international research team) a 6” Celestron refractor or a Meade 90mm F/8 refractor. Still my homemade 8” dob and now the AT 8” F/4 have carved a place in my heart.

A few years ago I created the Temple Research Observatory (TRO) to do photometry research. It originally featured the 6” refractor and ST-7E but met with the fate mentioned above. Now it features an Astronomy Technologies 8” F/4 Newtonian reflector on a Sirius mount. It is a robotic scope and is controlled with Team Viewer 8 from my residence. The only human intervention needed is to open the dome and check initial alignment before the start of a run.

The AT 8” F/4 reflector 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 unless you change the spider and mirror placement. This was done last year (2012) to eliminate the need for the extension tube.  One problem is that it only comes with one thumbscrew when there are holes for 2!  This is how it is shipped from the factory! I robbed another focuser of its screw, for without it the camera was a little loose. Flat thumbscrews have now replaced the standard screws to increase rigidity.  Since I use a home observatory I just use a 12 volt external transformer to power the mirror fan. There are no visible vibrations on the images from 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 Dell Optiplex 260, 3.4 Ghz with 3 Gb of Ram.  The mount is connected by an EQ-Mod connecter to a USB port, The Sky pro 6, T-Point, CCDSoft for the CCD, EQASCOM to let the mount talk to the Sky and EQPAD to run an X-Box 360 controller in place of a hand controller.  All of this has a pretty steep learning curve but works very well!  The Sirius mount works smoothly  To balance it all I have to use 33 pounds of weight and hang 1lb of weight off of the bottom of the tube to counteract the weight of the camera.

This scope is designed for imaging but 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 and need a field flattener to minimize distortion both for visual and large format CCD.

The focuser is a bit light.  It works OK but even when you lock down the focuser tube it can shift between images when using a heavy camera.  Lighter equipment or visual would not create as much of a problem. This issue is not enough to warrant replacing it since I added a robo-focus setup anyway.  Before robofocus the procedure was to move the focus knob, lock it down, image, unlock the knob, re-focus, etc. Once you get focus though, it holds for a while (several hours) until you slew it to a different part of the sky or you have a drastic temperature change. For a time I added a dial gauge and that helped in quick focusing. Now with the Robofocus unit it focuses automatically.

Without auto focus it is a challenge to achieve focus. I believe the “sweet” spot for focusing is around 56 microns! A human hair is around 100 microns! This does not lend itself to “eyeball” focusing. The quickest procedure I found to get it close, is to image binned to 3x3 on a star of about 2-4 magnitude. When the spikes on the star are no longer doubled you are close. Switch to 1x1 and do the same thing until the spikes on the star are as skinny as they can be. This will get you about as close as you can get with a Bahtinov mask. To refine it further pull up several faint stars in a small box (CCDSoft) and tweak the focus until they are as small as you can get them. Magnifying the box helps with this procedure.

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.   For the small chip in the ST-7 this is not a big issue but it would be an issue for a larger chipped unit.

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 brightly, but with very little detail in the AT8”. So this is not the scope you want to get if high detail small FOV images are what you are shooting for.

Great design and build!  It is built very heavily with a serviceable focuser and a steel tube. Steel does not require as much temperature compensation in focusing as aluminum does. 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 with the ST-7.  You could achieve over a degree with a coma corrector and large chip camera. This makes it very easy to place objects on the chip every time regardless of the camera!

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.  Placing the camera as close to the axis as possible and adding weight on the bottom of the tube makes it run smoothly. Orienting the camera so it is hanging down has also improved the balancing issues.

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 accommodate the 6” version very well.  With the reflector, unhindered UV and spectroscopy can be accomplished. Wide field photometry is also enhanced. Overall I would highly recommend this scope for science or astro-photography.

Specifications for 8" f/4 Imaging Newtonian Optical Tube

 

Optical Design

Imaging Newtonian Optical Tube

 

Aperture

8"

 

Limiting Visual Magnitude

14

 

Focal Ratio

f/4

 

Focal Length

800mm

 

Optical Tube Dimensions

32" long x 9" diameter (with 9.25" diameter front and rear cells)

 

Focuser Design

2" Dual Speed Crayford

 

Weight - OTA

21.6 lbs.

 

Maximum Power

320x

 

Back Focus

34mm (1.35")

M51 20 images of 30 second length. No filters or image processing.

NGC 4258 2 images of 30 seconds each.




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