AS PUBLISHED IN ASTRONOMY TECHNOLOGY TODAY

by Jon Talbot

In August 2007, I was loaned a Stellarvue SV102ED by the company owner, Vic Maris, in order to evaluate the astrophotographic qualities of this doublet refractor. The version I received (the SV102ED2) had the optional 2inch Feather Touch focuser.

I had used several Stellarvue telescopes in the past, including an SV78S (Nighthawk achromat), SV 80S, and 90T (both triplet apochromats) and obtained superior results from the triplets, so I was interested in how this 4inch ED doublet would work. Being a doublet with an ED element, I was curious about how well it would correct for chromatic aberration, inherent to some degree in most doublets. I have not imaged through any ED refractors from other manufacturers so this is not a comparison as to how this scope compares to other 4inch doublet refractors on the market.

The scope arrived in pristine condition and the first thing I noticed about it was how light it was. I was expecting it to be much heavier. I mounted it on my Mountain Instruments MI250 and piggy backed my older Stellarvue SV78S on top to act as a guide scope. I also attached a RoboFocus stepper motor to the Feather Touch in order to do automatic focusing.

The cameras I used to evaluate the scope were a Starlight Xpress SXVH9, a small chip (11.2mm diagonal with 6.45micron pixels) CCD camera and the much larger chip SBIG STL11000 (43.7mm diagonal with 9 micron pixels).

Using the SXVH9

I used CCD Inspector software and the SXVH9 to measure the field curvature without focal reducer or field flattener to be 9%. Stars were only slightly elongated in the extreme corners.

The images shown on the next page are enlarged crops from the corners of images of M13 and M27, both showpiece objects in the summer sky.

It was evident that the color correction was much better than my SV78 achromat; I'd say 50% better, but not as good as the near perfect triplet lenses of the 80S and 90T. It's hard to beat a well made triplet when it comes to color correction, but you pay twice the price for this. In any case, only the brighter stars had small blue halos around them indicating the blue filtered images didn't focus as sharply as the red and green. These blue halos were again rather small so they were easy to remove for pretty picture purposes in post processing using many of the tools available in Photoshop.

Using the STL11000

The STL11000 camera offered a chance to examine the ultra widefield performance of the SV102ED. Since Stellarvue does not offer a dedicated field flattener for this scope, I used a Televue RFL4087 reducer/flattener designed for the TV 102 and other 800 to 1000mm focal length refractors. Along with the reducer/flattener I needed the TLB0375 extension to get the chip spacing correct, the STL1071 adapter and the RAD1074 2inch focuser adapter.

The RFL4087 reducer does a fairly good job at flattening the huge field offered by the 11K even though it's designed for a slightly longer focal length scope. About 50% of the field is very well corrected although some significant elongation is still evident beyond 75% of the FOV. The chip is so large that using a 2 inch focuser causes some vignetting of the edges of the field, most of which is correctable with flat field frames. The weight of the STL11K along with the extra optics of the reducer flattener assembly is significant and the weight bearing capacity of the Feather Touch was crucial with this combination.

I imaged M31 (The Andromeda Galaxy), NGC 253, (The Sculptor Galaxy) and IC2118 (The Witch's Head Nebula) with the 11K. Again small blue halos were present around medium to bright stars but easily managed in image processing software. All the images were visually pleasing once downsized to a more manageable size for web viewing.

It's evident that even with a very large chip, one can obtain respectable images using the 102ED. APS sized chips, like those available with commercial DSLR cameras, should work very well with this reducer/flattener combination. I think an optimized field flattener would go a long way in correcting the field even more when using very large CCD chips. Also, a 2.7" focuser option would also alleviate some of the vignetting issues. That said, for a telescope in this price range, very respectable images can be taken even with large format CCD cameras.

In summary, the 102ED is an excellent imaging platform for those on a budget, especially when combined with the optional Feather Touch focuser as it is cise focus very easy. The native field curvature is not a problem when using small CCD chips such as the SXVH9. When used with the Televue RFL4087 reducer/flattener much larger chips, including APS sized DSLRs and full frame CCD such as the SBIG STL11000, can be used

Reprinted with permission of Astronomy Technology Today. To subscribe to this magazine, click here.

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