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The Joy of Unguided Imaging
Richard S. Wright Jr.'s Blog

When I first started imaging with a Paramount ME, I had no way to guide on my Mac and so I prided myself at going unguided. As time passed, I started writing more and more camera plug-ins, and I needed to guide to test things out, and honestly, I'm not sure I didn't get a little lazy. The siren song of guiding is so appealing. It's like training wheels on a bicycle... most children are very nervous about giving them up, but eventually they realize how much they are holding them back and after some time, they never look back. Ah... but we want to be "safe" don't we... no, let it go!

I'm starting to feel the same way again about guiding.

I have two Paramounts at my dark sky site, and a MYT that follows me around and is often in my backyard. My latest testing system is exceptional. My own RH-200 paired with a loaner FLI Microline 16200. I recently put an Atlas on my RH-200, and focus with @Focus2 is perfect and repeatable to a micron or two. The image at right shows the setup, and I'll point out that the light shield is actually past the front element. The entire imaging train from front lens element to chip is actually less than what you see as the apparent length of the telescope itself. With such a rigid system, you'd expect unparalleled ProTrack performance (also note the rings... those are an essential upgrade as well). You'd be right.

In fact, this just may be the perfect unguided setup and I hesitate to post this as the results are almost unbelievable. ProTrack's largest adjustments are to account for polar misalignment, and you really don't need a great number of calibration samples to compute polar misalignment. The reason you need "hundreds" of TPoint samples is for longer focal length systems with significant flexures that need to be modeled. So, with a rigid system in place just how few points can I get away with? 100? 50? Actually, I've gotten decent 5 minute unguided results with only 20 samples, but 30 samples seem to be the sweet spot for 5 to 10 minute unguided with this setup. 30 samples. Let that sink in for a second...

A few provisos to consider:

  1. System is exceptionally short and rigid.
  2. Pixel scale is near 2 arcseconds.
  3. PEC is 0.9 arcseconds.
  4. Focal length is 600mm, f/3.
  5. There is ZERO cable drag.
  6. This is not an SCT... no mirrors are moving around.
  7. This is not on grass or soft earth.
  8. TPoint samples must not be clustered, rather distributed all over the sky.
  9. Target was high in the sky where refraction is almost negligible.
  10. Of course it's on a Paramount ;-) 

Check out a 600 second (10 minute) sub without and with ProTrack enabled taken back to back shown at right. Clearly, ProTrack is doing some work here. Oh, by the way, my polar alignment was off 11.6 arcminutes in altitude, and 3 arcminutes in azimuth. I left it this way.

A fast system with no guiding is a ton of fun to use. At f/3 I'm well above the read noise in just a couple of minutes exposure time, and even the narrow band is usable at only 5 minutes. Using the unguided dither script I've posted before (now included in TheSkyX's /scripts folder in the daily builds), I can setup a dithered run without guiding. No guide star to lose when clouds come through, just toss the bad subs. Also, an automatic meridian flip occurs because the dither script is actually performing a small slew between exposures. I also don't need Closed Loop Slew or any of that nonsense... TPoint at this focal length is going to put me right where I need to be, and the flipped image is a sort of "free dither" operation in and of itself when I go to align and stack the images (other than for framing purposes, I personally just don't see much need for rotators).

Guiding is such a pain the behind... ;-)

Other mitigating factors to consider if you "try this at home"...

  1. Use filter focuser offsets so you can change filters without refocusing.
  2. My Veloce changes focus very little with temperature changes, and in Florida I actually don't have steep temperature changes most nights. Not refocusing is pretty nice... you can also use TheSkyX's built in temperature compensation for focusing.
  3. I've gotten similar results with some refractors, but you have to have a REALLY good focuser that does not slip or bend. Most refractors aren't as fast as f/3 either, so you're likely to need at least 5 minutes baseline, but at f/7(ish), that should be well above the read noise for most good cameras.

Remember, your sub exposures only need to be long enough to get all your interesting signal above the read noise of the camera. Take a bunch of them and stack 'em. Almost nobody really "needs" 20 minute exposures... other than to show off. If you're doing narrowband at longer focal lengths and slow focal ratios, you're going to have to have a better model, or guide.

I'm also doing some experiments with a HyperStar system. Two minutes is a wealth of data at f/2.2, and I'm finding the polar alignment can be quite a bit off, and the number of TPoint samples quite low at these focal lengths and optical speeds. More to come on that front later...

Here's the final result by the way...  

I can be happy this this!


Posted 08-22-2016 10:17 AM by Richard Wright
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