Image Acquisition

Good image acquisition is the secret to great astro images. It's as simple as that - if you don't have good data to start with, no amount of processing will help. What do I mean by good data? It's pretty simple:

  • Good Guiding and Polar Alignment
  • Optimum Mount Performance
  • Accurate Focus - minimum FWHM
  • High Signal to Noise Ratio (SNR) data
  • Effective object framing

Simple, yes, but getting there is another story. Read on for the details of my still-evolving process.

Guiding

I use PHD Guiding from Craig Stark (free from Stark Labs, donation highly recommended) for guiding. It's simple, Mac and PC, and just works. My main camera is a QSI583wsg, so I have the luxury of using off-axis guiding. The 583wsg has a guide star pick-off prism in front of the filter wheel, so there's never a problem funding a good guide star. The best PHD settings are highly dependent on your specific setup - focal length, mount, camera, etc, but I think one of the most important settings is the exposure duration. Too short, and you will be chasing the seeing - I use a minimum of 2 seconds, and sometimes longer.  On the Tak, I'm using an old Meade DSI Pro for a guide camera, and it seems to work just fine. In the past, I guided my Meade LX200 GPS with a piggy-back mounted guide scope, and was always fighting flexure. If you are serious about guiding and tight stars, use an off-axis guider.

I recalibrate every time I start up, or after a meridian flip, and generally get a guiding RMS of about 0.10 pixels or better. At the DSI image scale, that's about 0.35 arc seconds RMS. Just as a point of reference, I checked back in some notes from a year ago, and at that time I was getting a guide RMS of around 0.2 pixels, or 0.7 arc seconds. Persistent and continuous tweaking really pays off, and I think I can continue to improve my guiding.

With the Planewave CDK 12.5 and the Lodestar guide camera, guiding is a bit more of a challenge, given the focal length of 2541mm. By tuning the PHD parameters, I've gotten down to an RMS of about .35 pixels on the Lodestar, which equals approximately 0.25 arc seconds. That's well below my seeing, and if PHD allowed it, I would probably bin the Lodestar 2x2.

Another important PHD factor is getting enough steps during calibration - I aim for at least 20 steps. I always turn the Graph on (under Tools) and watch the RA and Dec lines carefully during imaging, to make sure that nothing is going off track. Note that the RMS reported on the graph is for RA only - the Dec line can be all over the place without affecting the RMS number. I don't believe that there's any way to see the Dec RMS.

At last year's OMSI Astrophotography Conference in Portland, OR, Tom Carrico (noted for his high quality data) had some additional great guiding tips:

  • Start with a good polar alignment, less than 1 arc minute
  • Try for 1 minute unguided exposures with little or no drift
  • Aim for less than 1 Dec correction per minute
  • Keep the minimum move small, less than 0.1 pixel or 0.01 seconds of time
  • Keep the maximum move small, less than 0.2 seconds of time, 3 arc seconds
  • Recalibrate after every move in Declination

 Finally, if you have great mount performance, great polar alignment, etc, etc, do you even need to guide? Some people claim that they are able to take 30 minute unguided images with great results - I'm not one of those. Of course, it depends on the mount and the telescope and imaging scale, but even then there are effects of differential refraction as the mount tracks the objects, plus other random effects. Unless you are really good, and maybe using a sky modelling program like TPoint, I say guide.

For Polar Alignment I use Pempro, from Ray Gralak. Great program, easy to use, and fast to check or adjust Polar Alignment. Also, it comes with the AP900 mount. My mount and telescope arrangement has a bit of non-orthogonality, which means that slews on different sides of the meridian don't always exactly center the target on the camera chip. Remember, Polar Alignment aligns the mount, not the telescope! Rather than mechanically adjust out this non-orthogonality, I'm waiting for Astro-Phyics to release APCC. This new software package will function like TPoint, and allow the user to create a sky model that takes into account small remaining polar alignment and mount alignment errors.

Optimum Mount Performance

Obtaining optimum mount performance is pretty simple - spend a lot of money and buy a good mount! Seriously, if you have an AP or Paramount or similar, you've solved most of your problems right away. At that point, it's a matter of maintaining reasonable balance, checking for minor backlash adjustments every year or so, and keeping the mount properly polar aligned. My periodic error on the AP900 was very low, but I leave PE correction turned on in the controller. As discussed earlier in the guiding section, the fewer guide corrections the better, so removing small Periodic Errors helps. I've had my mount for over 2 years now, so plan to remeasure PE next summer, using Pempro, as it can change slightly as the gear train wears and adjusts.

High SNR

Coming soon

Object Framing

Coming soon