Monday, May 05, 2014

ED Doublet Versus FPL-53 Triplet

I just received my (used, $350) Stellarvue SV80ED doublet refractor.  I'd sold off a bunch of off-axis guider stuff and my ST80 to buy it.  My main goal was to have a decent guide scope, but also a portable imaging telescope because the Astro-Tech AT90EDT is too heavy and bulky for air trips.  The hope is that the SV80ED plus the Vixen Polaris that I am refurbishing would make a decent portable astrophotography rig.

I had thought of buying a Takahashi FS-60C to serve as a posh guide scope and portable imaging rig, but the FS-60C is quite expensive and the accessories and adapters to actually make it usable add even more to the cost.  So I went for the used SV80ED even though I knew it would not have perfect color correction.

So how bad, exactly, is an f/7 ED (probably FPL-51) doublet?  to answer this question, I piggybacked the SV80ED on my AT90EDT, and transferred my DSLR (a Canon EOS 40D) from one tube to the other.  In both cases I used the Altair Astro Lightwave 0.6X reducer/flattener.

First off: Sirius.  Admittedly a tough test for an ED doublet.  SV80ED on the left, AT90EDT on the right. 10-second exposure.

Second: M44, the Beehive Cluster.  This image is from the SV80ED:

and this image from the AT90EDT. Also a 10-second exposure. Because the AT90EDT is only f/6.7 the focal length of the two scopes are very similar (540mm and 600mm).

I was unable to see any violet fringing on the Moon at prime focus on the SV80ED, however. It was very instructive to do fine-focusing with the DSLR Live View at maximum magnification.  On the FPL-53 triplet the star remained pure white with beautiful diffraction rings inside or outside focus; while with the ED doublet I immediately saw color aberrations once out of focus.

So the quick answer: even a non-critical imager will quickly find fault in the SV80ED, namely the blue halo around bright stars.  For visual, or for deep space objects, I expect the ED doublet would perform perfectly adequately.

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