Tuesday, May 13, 2014

Dual-Axis Custom Tracking Rates for Long-Duration Unguided Exposures

I have refined my plate-solving and path-modeling software for my Astro-Physics Mach1.  The principle remains the same: the mount and SBIG camera are under the control of Linux software that I wrote (in Perl).

I documented a previous effort here. However, that version used a polynomial fit (that proved inaccurate) and actively sent guiding commands. This version simply uses the variable-rate tracking feature of later AP GTO controllers to cancel the computed RA and DEC drift.

Performing long-duration unguided exposures requires the following:
  • getModelData2.pl which captures ten (10) images each of 30 seconds, one after the other, at the RA/DEC of the target object.  No slewing is done in RA, hence the successive images show the drift in RA and DEC due to polar misalignment.  The assumption is that periodic error is minimal (which it is for my mount). This script only works for SBIG cameras and Astro-Physics GTO mounts, and relies on the Astrometry.net blind solver (local installation) for the plate-solves.
  • calcCustomRates2.pl takes the solved RA/DEC from the previous script as dependent variables, with the HA (hour angle) as the independent variable. It then does linear fitting of RA versus HA, and DEC versus HA, yielding the slope in RA and DEC which are then converted into custom RA and DEC tracking rates for the AP mount.  The script then programs the custom rates into the mount similar to Ray Gralak's PulseGuide.
  • the curve-fitting algorithm only applies the custom rate if the R-squared (correlation) parameter in the line-fit is sufficiently high.
Here is a local model captured of M13 with the telescope on the west side of the pier. The first seven columns are all captured by getModelData2.pl. The RA and DEC rates at the top-right are calculated by calcCustomRates2.pl (the R-squared value is also displayed).  I also calculated the RA and DEC slopes with Excel (in the box).  The DEC rate calculated by the script is of opposite sign to that of Excel because the DEC rate needs to be inverted when the telescope is on the west side of the pier, in the Northern hemisphere. Note that the confidence level for RA is not that high.

This is a ten-minute (600-second) unguided exposure of M13 at approximately 400mm focal length. The trailing in both RA and DEC are apparent.

The same object M13, but with DEC custom rate enabled (calculated from the model data), which should eliminate declination drift:

And with both RA and DEC custom tracking rates enabled (calculated from the same model). RA correction is still non-ideal; probably the calculated RA rate is not correct due to the 0.75 confidence level in the fit.

A meridian flip was then performed, and another 10-minute exposure done, without changing the tracking rates.  There is some trailing present.

Another set of model data was captured with the telescope on the east side of the pier, data is below. Note the better confidence level in the RA data.

Another 10-minute exposure, with RA and DEC custom rates enabled and with the new model on the east side of the pier. Better results than using the west-side model and not recalibrating.

And for validation purposes, a 10-minute unguided shot with the telescope on the east side of the pier, and custom rates disabled. Trailing is very obvious compared to the above exposure.

Source code is available here.  You will need the latest Linux developer kit from SBIG for the "testapp" program that captures images from the SBIG camera.

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.