Sunday, June 29, 2014

Motorizing a Vixen Polaris Mount, Part II

I finally got around to re-making the motor bracket, and made a tripod attachment to the Vixen ring mounting points (there is no saddle).  The Polaris carries the Stellarvue 80ED and DSLR quite well.  I also used an intervalometer from e-bay to allow repeated bulb exposures.  Power is (literally) provided through the USB port on the Arduino; because the stepper motor is only 4V, providing 12V through the barrel jack on the Arduino causes a lot of noise and motor heating.


I took some 10-minute long exposures with the DSLR (an exposure this long is normally not practical, as thermal noise in the DSLR becomes unmanageable).  The reason for these long exposures was to measure the periodic error.


Unfortunately, the total PE is a massive 52.5" peak-to-peak over the 10-minute worm period.  This means that the maximum error is 0.175" / second or about 0.012X the sidereal rate.  With my (old) Canon 40D and the SV80ED, the plate scale is 2.1" / pixel. Assuming star FWHM of 4" and a maximum eccentricity of 0.60, this means stars would be 4" x 7" at worst - a drift of 3".  This corresponds to a maximum exposure time of only 17 seconds, which is pretty bad.

This means that PEC is not optional, but mandatory.

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.

Tuesday, June 03, 2014

Motorizing a Vixen Polaris Mount

I purchased another Vixen Polaris mount.. for $80 (plus another $100-odd for shipping from the US). Should never have sold the first one.. (this one, with a William-Optics Zenithstar 70ED that I also sold)


The second one arrived in sorry state; rust everywhere, no counterweight (and counterweights for the threaded shaft are impossible to find); and the HA/RA setting circle was busted.  I knew all this, of course - the mount was $80! Vixen Polaris mounts in good condition usually go for $150 to $200.

I ordered a Vixen GP2 counterweight shaft (with white locking nut) which thankfully fits perfectly.  Now I can use standard Vixen (or Celestron) counterweights.


Because I intend to use this Polaris as a travel mount or at least back in the Philippines where Polaris (the star) is actually visible, I purchased a CG-4 polar scope for it.  The CG-4 polar scope fits in the bore, but does not thread in. And it isn't tight, so I had to wrap teflon tape around the polar scope tube for a snug fit.  It works now, but is obviously non-stock.


I had a bunch of Vexta PX243 gearhead steppers (expensive, high-end steppers actually) that I had lying around from my AP600 GoTo conversion (which I have since rescinded).  I hammered together a mounting plate out of 2mm thick aluminum in order to attach the motor to the Polaris.  The motor has a 1:18 gearhead and 200 ppr, giving 3600 steps per revolution of the worm, and 2.5" per step. I don't think I'll bother to half- or microstep the motor.  It is a hybrid stepper though, I hope my Arduino L298 motor shield can drive it without undue heating.


I used the existing bolt hole in the mount for attaching the aluminum bracket. I believe it is an M5 SHCS.


Unfortunately the motor interferes with the mount at low latitudes, which means that there's a large portion of the sky just to the west of the zenith, which is unavailable (kind of like Dobson's hole).  On the other side of the pier things are better, but I intend to mount a Bourns ACE 128 encoder on that side (so I have absolute indexing of the worm position) which will also limit the mount's travel.

The motor is coupled to the worm shaft with a 5mm to 6mm shaft coupler from ebay.


My eventual goal (once the basic motor drive function is working) is to implement some form of absolute PEC (using the Bourns encoder) and then further add atmospheric refraction correction.  This will eliminate the need for RA guiding altogether (and there's no DEC guiding anyway because there's no motor drive in DEC).

Hoping for first light this Saturday.