SV80S Evaluation

Background:

On Thursday, August 21 2003 I received my long awaited SV80S, serial #0003.  This is a brand new scope offered by Stellarvue in their new partnership with TMB Optical.  The SV80S has an 80 mm objective 480 mm focal length fluorite triplet designed by Thomas Back.  Details on the scope can be seen on Stellarvue's web page here.

Vic Maris of Stellarvue has been hyping this (and his other new scopes) as reflector like in color correction.  So, I decided that the first thing I would do is try and test this claim.

Let me state at this point that I consider myself a novice at CCD imaging.  I also do not know a lot about optics.  What I've presented below is based on my current understanding and may or may not be an accurate representation of what is going on.

Data:

Below are two images taken back to back with an ST-2000XM CCD camera at prime focus of the SV80S.  The Sky computes the focal length at 478.45 mm, giving an f-ratio of about f/5.98.  One image is a single 10 minute exposure through the SBIG clear filter (passing UV and IR).  The second image is a single 20 minute exposure through the SBIG green filter (488 nm - 574 nm pass band).  Both were taken at 1x1 binning with the CCD operating at -10 degrees C.  The images were dark/bias subtracted.  No flat field was done.  I imported both images into Photoshop, stretched the images  with curves and levels to achieve similar brightness, and unsharp masked the images the same.
 

Clear Filter
Clear Filter
(click image for full size)
Green Filter
Green Filter
(click image for full size)

In order to asses how well the SV80S focused the broad spectra in the clear image, I cropped 200x200 pixel regions out of each image.  I then doubled the size of each crop in Photoshop to 400x400.  Finally, I subtracted each crop of the green image from the clear image to see how bloated the stars in the clear image are.
 
Clear center
Center 200 pixels of clear filtered image (doubled in size)
Green center
Center 200 pixels of green filtered image (doubled in size)

Center difference
Green image minus clear image (doubled in size)
Minor star bloat visible on brighter stars


Clear upper left
Upper left 200 pixels of clear filtered image (doubled in size)
Green upper left
Upper left 200 pixels of green filtered image (doubled in size)

Upper left difference
Green image minus clear image
Star bloat more pronounced, but still very minor


Clear upper right
Upper right 200 pixels of clear filtered image (doubled in size)
Green upper right
Upper right 200 pixels of green filtered image (doubled in size)

Upper right difference
Green image minus clear image
Similar to upper left difference



 
 
 
Clear lower right
Lower right 200 pixels of clear filtered image (doubled in size)
Green lower right
Lower right 200 pixels of green filtered image (doubled in size)

Lower right difference
Green image minus clear image
Similar bloat still



 
 
Clear lower left
Lower left 200 pixels of clear filtered image (doubled in size)
Green lower left
Lower left 200 pixels of green filtered image (doubled in size)

Lower left difference
Green image minus clear image
Still similar bloat

Conclusions:

The SV80S is very well corrected to the edge of the ST-2000XM FOV.  There is some minor bloat visible in the difference images.  It is only slightly obvious when comparing the two cropped versions side by side.  I need to do an LRGB composite and compare it to a high resolution RGB image for further evaluation.  However, this scope performs quite well.

There is some star elongation visible in the upper and lower right corners of the image.  I do not yet know the cause of this.  Further evaluation is needed.

I have a TeleVue 0.8X reducer/flattener designed for the TV refractors that should work quite well with the SV80S.  The reducer will give a focal ratio of about f/4.8 and will possibly help the elongation visible in these images.

All in all, this looks like it will be quite a good scope.

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