Staring at the Sun, 17

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Hand-me-down parts.

8/18/2010: Or, "I got your recycling right here." Rick Auerbach upgraded to a 4-inch FeatherTouch-equipped focuser for his A-P 155 and he put his 2.7-inch FeatherTouch-equipped focuser on AstroMart at a good price (though more than I wanted to spend). David Lim bought Rick's old focuser and put his relatively modern but non-FeatherTouch focuser on the same astronomical board at a steal of a price. I jumped at David's offer and used his focuser to replace the ancient and slightly sloppy cast aluminum focuser that was shipped with my 5-inch F6 A-P in 1987 (which I bought used from Larry Price 17 years ago). I used a (new) D&G Optical backplate to mate the focuser to the tube. As expected, the new focuser is such an improvement and works so smoothly that I think I can forego the 0.8-inch FeatherTouch add-on drawtube that I've been using routinely for photography with the A-P since 2003. That matters, and makes this chain of aluminum begats suitable for the "Staring at the Sun" pages, because it means I can move the FeatherTouch focuser to the solar telescope as its primary focuser.

I removed the 2-inch insert adapter from the add-on FeatherTouch, chucked the adapter in the lathe, shortened its snout by a harmless 1/4 inch to clear the internals in the Lunt, and turned a very shallow shoulder on the adapter's outer diameter. I only needed to shave a few thousandths to slip fit it into the tailpiece of the Lunt LS60Ha-T. When I had it whittled to size, I tightened the set screws that held the original focuser and now hold this one. And now I have... clouds.

Later that same day: yes, the telescope reaches focus visually (although it's near maximum extension). The camera focuses well inside that point, so I'm sure this will work fine for photography. I adjusted the foam in the Lunt's case so the FeatherTouch can be stored as part of the "official" configuration. Now we need a clear day to see about getting some really focussed solar images. In the event I want to remove the FeatherTouch for other duties, I need only loosen 3 set screws, put the original focuser back, and retighten the screws. Hard to beat (except maybe with thumb screws which are available from McMaster-Carr, if I can guess the threads).

I need to work out another way to find the Sun in the Lunt because my previous technique depended on the shadow of one of the lock nuts on its original focuser. The Sun-finder I made way back in the beginning works well enough but could work better.


8/21/2010: Because there's no free lunch, the jury is still out on where and how various accessories and cameras will focus behind the A-P with its new-to-me focuser and backplate. This configuration is both an inch longer and an inch shorter than the previous one. What? In some configurations, I always used the Feathertouch, and that provided a couple of inches of extension. With and without the FT, the new configuration is both longer and shorter than the one before. Don't forget that extensions are easy to come by and there is perfectly good one on the 6-inch achromat to use for trials. Here are the results of brief glances at the Sun on mostly cloudy days:


F6 w/Diagonal
35mm Pan
Focuses near outer limit w/o extension, 2-inch extension also works.
16mm Nag
works well w/2-inch extension.
12mm Nag
works well w/2-inch extension.
+ Barlow
Should work fine, not tried (clouds!)
F4.5 w/Diagonal
35mm Pan
focuses comfortably.
16mm Nag
focuses comfortably.
12mm Nag
focuses comfortably.
F6 w/Binoviewer
& diagonal
  Use Barlow behind the diagonal, then focuses nicely. Needs about another 3/4 - 1 inch of in-focus to work without the barlow. Not worth more tube shortening, but consider some shortened diagonal options?
F4.5 w/Binoviewer & diagonal
  no chance
Binoviewer straight-thru
  Not tried at any FL.
A-P DSLR:   (All tests using wide mouth adapter and T-ring; the mini-extension ring machined for previous focuser arrangement is removed.)
F6 w/Diagonal
  Focuses OK (1430 pixels = 1/2 deg) (F6)
F6 straight thru
  With 2-inch extension, focuses near outer limit
+ Barlow (@ F6)
  Barlow works behind the diagonal at F6 (~2300mm FL, 3.0x, F18)
F4.5 w/Diagonal

Insufficient in-travel (not particularly close, an inch?) The light path through the diagonal constitutes an extension which is too long now.

Think about a shortened diagonal (see binoviewing note above).

F4.5 straight thru

Focuses near outer limit. 1240 pxiels = 1/2° (0.87x) (F5/2).

w/2-inch ext & wide, short PF adapter: 1130 px = 1/2° (0.79x) (F4.7)

w/same components, ext'd by partial insertion: 1096 px = 1/2° (0.76x)
focuses near infocus limit. (F4.6)

Note that A-P PF adapter will not insert in Blue Fireball extension, something's off by a few thousandths. Will likely over-extend the camera and make sufficient in-travel impossible.

A-P CCD:    
F6 straight thru
  Not yet tried
F4.5 straight thru
  Not yet tried (presumably, the custom extension will be too long, so be prepared to shorten it or use Nikon extension tubes)
F6 + barlow
  Not yet tried

Happy 90th birthday, Ray Bradbury. The Lunt with the 0.8-inch FeatherTouch add-on used as its primary focuser requires a short extension to reach focus comfortably -- it just focuses with the B600 tube extended so far that it seems insecure. I'll routinely use it visually with an extension in the eyepiece holder instead (this extension is the shell of the shorty barlow whose lens I stole to screw into the prime focus adapter). All my favorite solar eyepieces (20mm and 10.5mm Plossls, 7mm Nagler) focus nicely there and are almost parfocal. The 50D still will not reach focus without a barlow. But with single and double barlows, it focuses very solidly. I could shorten the B600 tube a little and the camera would surely focus without a barlow now, but why would I want it to? I have a full-disk image with the single barlow. I could probably use the T-threads on the eyepiece side of the B600 and also reach unbarlowed focus.

I'm on the verge of making a big change to the way I acquire H-a images through this telescope, so I'll refrain from lots of detailed case studies (like those for the A-P above). More when I know more.

In the meantime, here are two images of a very quiet sun. Focus was painless with the Feathertouch -- no more rubber bands for assisted weight-lifting, so fumbling, no fuss. You just blow up a detail in LiveView and turn the FeathetTouch's fine-focus knob back and forth until you're satisifed. That's all.


whole disk

Single barlow, 3 x 1.6s, ISO 400. THe usual Bayer plane
extraction, manually aligned in Photoshop.


single frame

1 frame, carefully focused. 0.4s ISO 400.


Those are not bad images from what was supposed to be mostly a gear-testing session. I wanted baseline images for the 50D and shot a range of exposures from 0.02s to 1.6s, three each, at two different EFL's. All were ISO 400. And I wanted to see if the FeatherTouch made a significant difference in sharpness. Maybe. It was certainly much easier to achieve sharpness similar to what I have previously enjoyed. The difference owing to seeing was much starker than usual among the close-up frames, so either the bar has been raised or the air is very unsteady today. Either way, the ability to select the best frames from a long series becomes apparent.


8/23/2010: And happy 89th birthday to my dad. I did a tour of the Sun with the solar outfit on the G11 set up in the front lawn in Johnson City. Preston Wolfe, Eric Job, Bill Hyers and, later, Van McClellan, all got an eyeful of filaments and prominences. In the process, I discovered that the G11 has no decent balance point for the Lunt: one counterweight is too much, none is too little. The brass extension on the counterweight bar is not enough by itself, either. If putting the solar outfit on the G11 gets to be a habit, you'll need either more weight on the telescope side or less on the counterweight side. Don't react to this with a design and fabrication fit or a used gear search until and unless it becomes obvious that Sun will be a regular target for G11-mounted glass. The Giro might be better on the road and the Mach 1 gets the nod for use at home.

Oh, and another thing: store the stepper motor cables with the electronics and the 7AH traveling battery. Why would I keep them anywhere else? Maybe I had a reason but I can't imagine what it was. The 6-inch achromat should balance on the G11 easily (given that it is similar in weight and size to the 5-inch A-P), but in light of the experience with the Lunt, make sure before investing travel time and funds in it. Sheesh.

Over lunch the systems group kicked around the free C++ compilers available from Microsoft and the practicality of using an SDK supplied by Point Grey Research to support their high-resolution monochrome cameras. One reason the cameras are so affordable is the rudimentary software with which they ship. If the basic capture utility is sufficient, great; if the SDK can be used to make it sufficient, better. I get to learn C++ and do high rez imaging as my lab exercise. Via email, Al supplied his favorite references for C++, and then had Barnes & Noble send the top of the heap to me (thanks!). I added a more basic text by the same writer (Bjarne Stroudstrup, who should know all there is to know about the subject). So all that remains is the hardware, and I'll get that on the way soon.


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                   © 2010, David Cortner