Book Mania

Thursday, February 21, 2013

How To Collimate Your Newtonian or Dobsonian

Near the close of last year's observing season, just before it got too cold for me, I had to admit something dreadful about my 6 inch Discovery telescope. I've always considered it, and the similar Celestron Omni XLT 150mm, to be nearly the perfect telescope type. Compact, relatively inexpensive, and good for a wide range of observing.

And while I love my Discovery telescope, especially after some modifications I made to it, I none-the-less had to admit that the images it presented were often sub par.

I particularly noticed this when I did side by side comparisons between it and my ETX 90 Maksutov. I used high resolution targets like Jupiter and Saturn as comparison objects. I was disappointed that the ETX, with only 3.5 inch aperture, would give as good or better images as the larger 6 inch reflector.

My rich field is an f/5, and I know of many people who successfully use similar focal ratios and get good planetary images. I was concerned about this mediocre performance. I've used Newtonian reflectors for decades, from long focus to short focus, and I was pretty sure I knew how to properly align the optics in them. So assuming I knew what I was doing, what was the problem?

Did I need different eyepieces, more expensive ones that would work better with the short focal ratio of the telescope? Did I need a new objective mirror? Did I need a new and better Barlow lens? Did I simply need to accept poor images at high power because the telescope was a rich field, and resort to only using the instrument for general star gazing? I decided to get to the bottom of the problem, hoping that some kind of affordable accessory purchase would help me improve the performance.

With some critical observing on some close double stars, I observed what appeared to be astigmatism. I did the usual tests, rotating the eyepiece to see if the astigmatic image rotated with the eyepiece -- it didn't. I used my ETX on the same objects to see if I saw the same problem, suggesting was my eyes -- it wasn't. This left me with the conclusion that my Newtonian's primary mirror may have astigmatism.

A bit of reading told me that it was possible that I still had an alignment problem, though I'd tried very hard to get the collimation right. 

I'd used a homemade Cheshire eyepiece to help me get it right. As the animation at left shows, I tried to adjust the secondary to put the dot I'd painted on the center of my primary, right on the cross hairs. Then, of course, I aligned the primary to get the reflected cross hair coincident with the centered primary dot.

Even so, I wanted to be certain about the alignment, because it had been a difficult procedure due to the short focus of the telescope. Buying a new primary mirror would have been expensive, and I didn't want to do that on incomplete evidence. So for much less than the cost of a new primary, I bought a Celestron Collimation Eyepiece to more precisely adjust my telescope's collimation.

Best  thing I've ever done. I found, using the Celestron device, that I'd been more often than not getting the secondary alignment wrong. My homemade Cheshire device was too short to reveal the error. The poor secondary alignment was causing the apparent astigmatism. Now, with the better alignment, the astigmatism was gone, and views were crisp. I checked the system critically on some close doubles, and things looked nearly textbook.

The bottom line is, if you have a Newtonian reflector -- especially a short focus one -- don't do as I did and just assume that you've got the knack for optical alignment. It's not that expensive to get a Collimation Eyepiece, or for a bit more money a laser collimator, like the Orion LaserMate Deluxe II Telescope Laser Collimator. You, like me, may be surprised that your skills aren't as good as you thought. I figure that over the years, probably 80% of the time my reflectors have been misaligned enough to cause some degradation of performance.

So don't agonize over or tolerate poor telescope performance. Get your collimation eyepiece in hand, go to the Newtonian Collimation Tutorial, review the material, and get your Newtonian performing like it should. 

Sunday, February 17, 2013

Hello Spring Moon

Lunar Stofler Region - ETX90 Photo
Well, the winter of 2013 is easing off, so I'm getting mentally prepared for another observing season. I know -- if I wasn't so frail of spirit, I'd have observed through the winter on nearly every possible occasion. But I just couldn't get worked up to brave the cold this year.

I'll start lightly, by observing one of my favorites, the moon. I love looking at the ever changing views of the craters, like those of this photo from my ETX 90 photos page. The lighting variations of the countless craters bring up fantastic details each time I look. But this year will be special, at least for me. We just lost one of my most admired space pioneers in 2012, that being Neil Armstrong, the first man to set foot on the moon. He along with Buzz Aldrin and Michael Collins were the courageous astronauts that rode Apollo 11 all the way to the moon, with Armstrong and Aldrin descending to the lunar surface.

If you're old enough, you likely remember where you were and what you were doing on that momentous occasion. I remember precisely what I was doing. I had just graduated college with my science degree, and was thinking what a lucky break it was that just as I was poised to begin my career, we were landing on the moon. I thought my science future looked exceedingly bright.  Ah, the lamenting of youth.

My parents only had the one TV, and it was black and white at that. But they knew how much the moon landing meant to me, having bought me telescopes and astronomy books since my grade school days. So they let me watch the entire landing broadcast. I was glued to the set, and took a few Polaroids of the TV screen as history unfolded.  I'll never forget it -- it was one of the biggest thrills of my life.

I don't think I'll be able to look at the moon again, now that we're moving past that exciting era, without thinking of Neil Armstrong and his crew.  Those memories will definitely heighten my excitement each time I take on the wonders of our nearest neighbor, whether it be with my modest 60mm telescopes, or my six inch reflectors.

If you haven't looked at the moon in awhile, I suggest you study up a bit on the Apollo 11 mission and the excitement it caused, then see if you aren't a bit more inspired to get out there and give your eyes the unparalleled treat of the moons mountain ranges, craters, mare, rilles, and shadows. You can start that study of Apollo 11 with the following excellent book:
Moonshot: The Flight of Apollo 11 (Richard Jackson Books (Atheneum Hardcover))