Telescope Modified: DIY Digital Setting Circle (Declination)

I like observing and capturing images of deep sky objects, but I also enjoy the hunt—just finding really dim objects has its own thrill. I don’t like “Goto” mounts that noisily slew to the perfect spot the computer tells them. I prefer the freedom to move the scope by hand, rapidly and quietly, if not terribly accurately.

But I would like a little more precision in the hunt, especially when it comes to my declination setting circle. As with other equatorial mount scopes, the fork mounted C8 has two tic-marked “setting” circles for coordinates, Right Ascension (RA) and Declination, that are similar to longitude and latitude.

RA Setting Circle

The resolution of my Right Ascension Setting Circle isn’t bad. At twelve tic marks for everyhour, it has only 288 markings, but there’s a good deal of space between the markings making it effectively twice that.

On my declination circle, there are 360 tic marks but they’re squeezed close together on a small disk. At night, with aging eyes, my accuracy in reading is at least off by a degree or two, where objects can be easily out of the field of my eyepiece or camera.

Therefore I try to avoid using the setting circles.   My usual trick with the camera attached is to focus on a brightish star on the same RA or declination as my dim object, and slew on a single axis until I’m in the right vicinity of the dim object by using the finder scope.   This doesn’t work well for some objects, and I need to use the setting circles instead.

So I spent a week thinking about options for improving my declination precision.

Option #1: Get digital setting circles and install. Pro: Not terribly expensive. Con: Not terribly cheap. That investment might be better spent toward a new mount that has a lot less error than my current clock drive.

Option #2: Get a new mount. Pros: There are some fine mounts out there in the $800 range with great features such as polar finders and low-error drives (even drives you can program the error out of). This is nice, but all the ones I could find either had Goto (noisy, hunt-free) mounts or analog setting circles. I’d like to get a nice mount at some point, but I need to really nail down my requirements and find the right one.

Option #3: Print out a really big setting circle and attach it over the existing circle. Pros: cheap and easy. Cons: ugly, in the way, and I couldn’t even find a printable circle with better than 1 degree precision. But while seriously considering this I had an idea…

Option #4: It struck me that there must be some sort of digital protractor out there. It turns out these are called “digital angle finders,” and they are relatively inexpensive.

A trip to Ace Hardware and $23 later, I had a General Digital Sliding T-Bevel with .05 degree precision. It had a power button and a zero button, and was easy to strip it down to the core. Best of all, it had a center hole in the housing that was the perfect size to tap 10-32 screw threads through—the size and pitch of the center screw that holds the declination circle to the bearing on the scope.

So here it is, my DIY Celestron C8 declination digital setting circle in action:

There’s a Moon over Grizzly Peak Tonight…

Moon over Grizzly Peak

3/17/2014 21:08 2032mm f10 1/320s  ISO 1600

The last full moon of winter.  Picture taken from Albany Hill as the moon rose over Grizzly Peak in Berkeley.  The Grizzly Peak Electronics Site, a tower used by KPFA and others, is in the foreground (a little less than four miles away).

I got both lucky and unlucky with this one.  I’ve taken full moon rising pictures from the deck off-and-on for years now, but never through the scope.  I’d always wanted one with Grizzly Peak in the foreground, but I didn’t even know the antenna tower was there!  I had no idea this moon was rising over Grizzly peak until just before it rose but I had the scope ready.  I’d focused on house lights in the hills, and meant to focus on the moon once she appeared.  After some test shots before moonrise, I settled in.

And of course, technical failure as soon as Luna showed herself.  I thought the battery had died.  I lost a bunch of seconds until I realized it wasn’t the battery, and yanked out the remote shutter release, figuring I’d sent it into program mode.  All my pics then had to be finger presses on the camera, so I jacked up the ISO for faster shutter speeds.  Never had a chance to make sure I had a crisp focus.  Good enough, though.

3/17/2014 21:08 2032mm f10 1/125s ISO 800

 

Telescope Modified: Focus Remote

Focusing has always frustrated me, especially now that I’m taking a lot of pictures.  Humans are not well designed to focus small telescopes with their fingers.  Turn the focus knob, and the image bounces around.  Wait for it to settle down, and turn it again…

So I built a motorized focuser, based largely on Brian Sumpter’s design.  It works great.  I bought the servo, switch, and rheostat, but scrounged around for the rest of the parts (iPod case, erector set pulleys, etc.).  The breakthrough for me was using a grommet (I had a box of assorted grommets from Harbor Freight) to fit in the drilled out pulley center-hole and over the .5″ focus spindle.  I was also happy to figure out a way to attach the servo mounting rail to the scope without tapping any new holes.

Supernova in M82

Some of you may have read about the (relatively) close supernova that was observed last week.  Timing was good for me, as the moon is waning and rising later, and I was considering my next target.  So without further ado, here’s the Cigar Galaxy lit up by its exploding star SN 2014J, taken last night.  ISO1600/80s/2032mm/f10

Periodic Error on my SCT drive

I took 36 five-second test exposures of a star and decided that the “glitch” mentioned in my Crab Nebula post is just normal periodic error of the drive.  That is, there’s a lot periodic error (drifting back and forth in RA), and I’m going to have to use guided tracking if I want to tighten up the images.  I would’ve thought I could get away with 30 or 60 sec exposures, but not with this mount.

Crab Nebula

Crab Nebula in Taurus 1600/1x60s/C8pf

Here’s the Crab Nebula.  Not a breathtaking pic of what’s normally a pretty dramatic supernova remnant, but it’s a single 60 second exposure.  The other nine exposures I took and hoped to combine had too much movement.  I think I have a periodic glitch in my drive of about a minute.  I have to throw out about half my 30s exposures, and almost all of my 1 minute exposures.  So I gMuess I’ll be taking my drive apart again on the next full moon, and sticking to 30s exposures in the meantime.

The good news is the star haziness was mostly gone last night, so it was the sky, not the optics, that had the problem a few nights ago.  Also, my lighted reticle eyepiece worked like a charm for drift aligning.   Finally, the altitude adjuster that I jury-rigged worked well.  I’ll post a picture of that soon.  Crab Nebula in Taurus, 60 sec, ISO 1600, 1000mm f/10, cropped.

Orion Nebula

M-42 The Orion Nebula

Last night was my first out with the reassembled scope.  I’m a little concerned with some haziness around bright stars, like I didn’t do a good job cleaning the optics, or the grease I added to the focus mechanism is off-gassing and creating a film on them.  There was similar haziness with binocs, so maybe it was just moisture in the air.

I wanted to shoot the Crab nebula, but it took me a while to find (I’d never seen it before).  Figuring it would take a while to image it, I decided on a simpler task.  The Orion Nebula is an easy target, and I didn’t take a lot of care with it, but it’s still beautiful.

Telescope Apart

Celestron C8, in pieces

I purchased some light-absorbing flocking board to line the tube with (dark tubes=better contrast).  That gave me an excuse to break the scope down, clean the optics, and do some maintenance.  I took a picture before putting it back together.

Full Moon Mod #1: Illuminated reticle

It’s that time of the month…

…when the moon is out and I do equipment modifications rather than pictures.  But here’s a picture of my latest mod, a custom cross hair reticle eyepiece.  I drilled a couple of holes in a 10mm plossl eyepiece, ran some audio optical cable on the inside, and stuck an LED on the outside.  For the reticle, I used a spider web and a drawn out filament of plastic cement.   I hung dimes from the ends of the web to keep it taut while I glued it.  Impressive stuff, spider web, but I think I like the look of the cement strand better when illuminated and magnified.

This contraption should help me drift align the scope for better tracking.  I’ll test it out in a few days once the moon starts rising later.

Saturn Nebula Observed

I did see the Saturn Nebula through the scope, but didn’t think it would make a very good picture.   Sort of a fat, slightly oblong star.