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Home / Imaging with the Astro-Tech 66mm Refractor
by George Kolb 10/21/09 | Email Author

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Background

I consider myself a weekend amateur astronomer of 30 years with a main interest in imaging under the skies in Arizona. My first imaging experiences were with the Olympus OM1 film camera, followed by use of the Starlight Xpress MX5C and MX7C one shot color cameras. Feeling tied to using a laptop in the field for imaging; I divorced myself from the entanglements of wires and connectors a few years ago and settled on DSLR imaging with my TEC 140, C9.25 SCT and piggyback using my Losmandy G11 mount. There are so many imagers out there today and I know I will never compete with them, especially with the high level of hardware available. But, I still enjoy composing and capturing images the way I want and attempting to climb the image processing learning curve. Wide field imaging has always intrigued me but the cost of high-end equipment has been prohibitive. The alternative was using camera lenses on CCD cameras but focusing was always an issue. For the record, I have no affiliation with any telescope or astronomy accessory manufacturer.

The Move Toward a Small Refractor

With the introduction of relatively low cost, near APO, small objective diameter refractors in the last 3 years, I found myself attracted to purchasing one for wide field imaging. Knowing that the William Optics and Astronomy Technologies product lines were at the forefront in providing short focal length, fast refractors, I began my research in picking my next telescope. I immediately realized how quickly these scopes were introduced and then discontinued, presumably as part of a natural evolution in perfecting the design driven by competition. Special Dispersion (SD) glass, Extra Low Dispersion (ED) glass, doublets, triplets and Petzval designs were all available to entice the consumer to purchase one of these little gems. Aside from the different objective lens design, all were solidly built and finished, most with two-speed Crayford type focusers with SCT threads terminating the focusing tube.

So, I narrowed my search and zeroed in on a nearly new, AT66ED offered for a great price on Astromart. The AT66 is a 66mm diameter cemented doublet, 400mm focal length, f6 instrument. There are many reviews available that detail the visual capabilities of the AT66 but I wanted to concentrate here on describing the imaging attributes. It is well known that field curvature is an issue when using larger chips with refractors, so I planned on purchasing a field flattener/focal reducer as part of the imaging train. Several options are available, but good reviews of the Williams Optics FF/FR 0.8X Version II convinced me to purchase one. The FF/FR can be used either by inserting the 2 inch barrel into a telescope or by utilizing the native built-in SCT threads to fasten the FF/FR directly to the AT66.

Although supplied with a SCT to 1.25 inch barrel adapter, you would need to purchase a SCT type 2 inch diagonal if you wanted to utilize any 2 inch eyepieces - not something you would want to do if you already owned a refractor type 2 inch diagonal. Fortunately, Williams Optics sells a female SCT threaded diagonal adapter that simply screws into your refractor diagonal in place of the 2 inch barrel tube enabling it to be used on these small scopes. The William Optics adapter not only fits their own diagonals, but they advertise it will also fit WO clones such as the Astro-Tech diagonal, which I own – and it fit perfectly. William Optics also offers a SCT to 2 inch barrel adapter, which is very difficult to find. One would think you could use a refractor type diagonal with this adapter, however, in checking with most telescope accessory suppliers, they claimed you would not be able to reach focus due to insufficient in-focus travel. I took their word for it.

This combination would provide a flat field on my QHY8 APS sized chip and converted the AT66 into a 320mm focal length scope at f4.8. A quick call to a leading telescope accessory dealer provided me with the proper combination of spacers to achieve the prescribed spacing of 56mm from FF/FR to CCD chip. The combination of the FF/FR and spacers guaranteed a rigid set up with no flexure.

Imaging with the AT66

Mounting the AT66 piggyback on another scope can be challenging, as there doesn’t seem to be any exact fit rings available. There are guide scope rings for sale with the Delran tips that would work but that would add substantial cost to the set-up. I chose to utilize the mounting L-foot installed on the scope, fastening it to a Losmandy camera adapter via the ¼-20 thread. After tightening the screws sufficiently, a quick test of trying to twist the scope on the adapter proved to me that no movement was evident and that the mounting would not slip during imaging. The Losmandy camera adapted was then coupled to the dovetail plate on top of my C9.25 allowing me to position the AT66 to minimize weight of the camera wires pulling on the camera. This added to the stability of the set up considerably.



The first light that passed through the AT66 onto the QHY8 One Shot Color CCD camera was M8 and M20, beautifully captured in a very wide field. I used Nebulosity 2.0 for image acquisition, flats and pre-processing and PSCS2 and Noel Carboni’s Astronomy Actions for post processing. I was concerned that with the very small objective size of 66mm, it would necessitate me to acquire images with lengthy sub-exposures but quickly realized when composing the image and focusing the camera that the AT66 was providing sufficient light. I made the normal exposures of 10 x 10 minutes each.

Becoming excited about potentially capturing some very nice images with this little scope, I quickly moved onto the Antares region before it became too low in the sky.

Finally, the next night I captured the Great Andromeda Galaxy when it was near the Zenith.

All in all, the AT66ED in combination with the WO FF/FR provides a great wide field imaging platform with very good color correction and nearly no blue haloes around bright stars. In my opinion, this set up proves to be much more capable than a fixed focal length or zoom camera lens attached to the CCD camera due to the ease of focusing with the two-speed focuser on the AT66. If you have ever tried to focus a camera lens on a CCD camera you know what I mean. At an incremental cost of $375 (used AT66 and WO FF/FR), I added wide field imaging capability to my existing platform. Add another $40 for the SCT diagonal adapter and I now have a grab & go scope for wide field visual use under Arizona skies. And with the summer Milky Way on showcase, the views are spectacular.

Pros

Lightweight

Low cost / high quality ratio

Very smooth two speed focuser with lock (no creep with CCD camera)

Good color correction

Cons

Focuser needs to be extended nearly to end of range to achieve focus

SCT threads at terminiation of focusing tube vs. 2” opening for accessories

Summary

While the Astro Tech 66 is no competition for the hight priced / higher quality true APO’s, you will get very pleasing images both visually and with CCD / DSLR cameras. The field is very large and with a focal reducer such as the WO 0.8X, focal length drops to 320mm @ f4.8. Stars are generally small with an even focused field across the QHY8 6.1 MP chip. I was thoroughly amazed at how much light the little scope could capture and how good the finished images came out. For the price, I can’t see how you can beat it.


Clear Skies

George Kolb

Phoenix, Arizona

www.pbase.com/geokolb



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