Wednesday, June 11, 2014

Astro-Tech 8" Imaging Newtonian, Part VI

The never-ending saga of the Astro-Tech imaging newtonian continues.  After my last update, I

  • purchased a Feathertouch focuser and motor, and made an Arduino focuser controller
  • used a much larger, 2mm thick aluminum sheet to reinforce the tube, and epoxied the aluminum sheet to the inside of the AT8IN steel tube
  • purchased an AstroSystems 2" laser collimator, because the $30 Seben was hopelessly miscollimated
  • purchased a CatsEye XLKP autocollimator and TeleCat cheshire, and learned how to critically collimate using the autocollimator, because the AstroSystems laser wasn't accurate enough

In spite of these efforts, and in spite of collimating with the OTA in the final position on the mount, I could still not get round stars across the CCD. So after eight months of fighting the ASA Keller reducer, I simply gave up.  I could not control the flex in the tube, and the thought of spending another 500 Euro to purchase a carbon fiber tube - with no guarantee that things would work - simply did not appeal.  So I sold the Keller reducer and played with the bare newtonian for a while.

Thanks to my well-honed collimation skills, the newtonian produced perfect, round, small stars - in the center of the CCD sensor.  Just 10% off-axis, massive amounts of coma reared its ugly head.  Now I know what coma looks like! and it's not pretty.

I found a Type 1 photographic Paracorr for a ridiculously low price on astromart, so I purchased it. Unfortunately.. it required a tremendous amount of extension to reach focus. That's a 2" extension tube there.  The Keller did not require much extension (i did have to add a 35mm focuser base extension to the Feathertouch, which is visible in this photo below).

I had known that the AT8IN had this issue; in fact when using it visually, the extension tube must also be in place and the focuser racked out quite a bit.  But that long imaging train bothered me, and it can't help the stability of collimation (incidentally this ridiculous setup produced nice round stars almost to the far corners of the ST8300M).



Based on some suggestions from the Cloudy Nights forum, I did some additional experiments (not at infinity, but pretty close - two kilometers or so away).  With no Paracorr in place, the DSLR does reach focus
(below) with the focuser racked out about 70% or so.  The 35mm focuser base extension is visible here as well.



The DSLR also reaches focus without an extension tube (unlike the CCD) but the focuser is racked out almost completely, and I had to raise the Paracorr as well (it isn't sitting flush on the shoulder of the focuser).  This obviously was a non-ideal situation and resulted in the extra-long imaging train in my first photo.



I figured that the focuser base extension was 35mm, and the extension tube 50mm.  So if I lengthened the tube by 85mm, I could get rid of both the base extension and extension tube, and significantly shorten the imaging train.  However I decided to be a bit more ambitious, and lengthened the tube by 100mm.

To do this, I hand-rolled some 2mm aluminum sheet into an extension.  Since the aluminum I had on hand wasn't long enough to form a full circle, I had to do some ugly joining to form the complete 360 degrees.  I also epoxied the aluminum at strategic points, and riveted everything to the existing tube (the aluminum extension is actually two coaxial cylinders).  I also learned how to use a pop rivet tool.



The end result is that eyepieces now can reach focus nicely without an extension tube; the CatsEye XLKP autocollimator is much easier to use because when it is resting on the shoulder of the focuser, it is nearly at the focal plane; and, the DSLR reaches focus with very minimal focuser out-travel with the Paracorr in place (below).  I expect the ST8300M will require more out-travel but well within the range of the focuser.  I also removed the 35mm focuser base extension, which makes the setup very low-slung indeed.


The only downside is, the DSLR won't reach focus anymore without the Paracorr in place.  But I question who would use a DSLR for imaging on a newtonian without a coma corrector anyway.

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