Saturday, July 16, 2016

More Stars!

While my lack of new posts may imply things are quiet here - that could not be further from the truth! It has been a zoo here, trying to get Melinda's radiation oncology treatments started... Was supposed to start Wednesday, but then she was in too much pain to continue. Thursday and Friday with a new plan in hand, the machinery was down, so reset to this coming Monday. Best laid plans...

What this post is supposed to be about is that John Davis from work, and I got out observing again last Saturday. Even though we had a 5-day-old moon, the clear sky was calling and we again went up to a site on the south side of Kitt Peak. As opposed to the weekend before when Roger and I got out and I only did wide-field stuff, this time the plan was to do more close-up work. John is a mechanical wizard, and made the mount shown here. It used to be fixed at an observatory at his in-laws place in Sierra Vista, but realized it might get more use as a trailer-mount, and this was it's first excursion!

My setup, shown at right, was the standard AP1200, shown here with the early-evening setup of the Meade 80mm F/6 refractor and a Canon 300mm F/4 telephoto. Later I installed the TEC140 for some imaging with the 1,000mm focal length that it provides...

Baboquivari w/the Pentax 165mm on 6D
Baboquivari w/Meade 80mm F/6
But in the early evening, while setting up, suitable targets are everywhere! I'm still finding out which lenses work best with the "newish" Canon 6D's large sensor. My first trial targeted the mountaintop of Baboquivari with a Pentax 165mm lens. Designed for the 6X7cm format and available for a song on Ebay, it should have no trouble covering the 35mm sensor size of the 6D. But you never know about older lenses - sometimes color correction or other aberrations show up that weren't an issue with film, but at least on this daytime shot at left it was fine. Turns out I didn't have time to try any night exposures - another time!

Same shot as above, Vignetting corrected!
At upper right is a shot of "Babo" with the 480mm focal length of the Meade 80mm F/6 with the Explore Scientific field flattener (more on that later). It also performed great and looking at the images brought back memories of hiking on the mountain decades ago, as part of the "Forbes Route" is visible in the view from 15 miles distance! Unfortunately, also ominously visible is vignetting - darkening in the corners, likely from the small aperture of the t-ring adaptor or the 2" field flattener. If anyone knows an easy way of correcting this in Photoshop on an uncharacterized optical assembly, drop me a line!

Actually, I looked it up on Youtube myself, and while the videos were less than helpful, poking around in Photoshop, I was able to figure it out myself. Compare the upper right image with the darkened corners, and this one, approximately corrected manually in Photoshop CS5...  It looks much better!

As night fell, I investigated more properties of the Meade 80mm with the 6D sensor. For several months, I've borrowed a field flattener to correct the field curvature inherent in many refractor telescope designs. For the smaller APS sensor of the XSi I've used for many years, it wasn't an issue, but for the 6D, I knew it would be. Particularly when it comes to my star images, I'm a little picky and want them to be aberration-free and in focus!

Meade 80mm, no flattener - corner clearly defocused!
Meade with Flattener - sharp to the edge!
So as soon as it got dark (though with the moon still up!), I pointed the Meade's 480mm focal length towards a suitable target, the Antares region in Scorpius. There are some globular clusters and dark and bright nebulae that are of interest and serve as a good subject. In these comparisons, I've done small section blowups and over-plotted it on the image. Shown at left without the flattener, an image where the telescope was focused at the center of the frame showed considerable defocus at the edge. 

With the inexpensive flattener ($150) at right, again focusing at the center, the stars were uniformly tiny across the frame.  Both arrangements suffer from vignetting pointed out above.

One always wonders how these small telescopes compare to actual camera lenses... Lenses like the "football telephotos" used at games are very expensive because of their autofocus engineering and large apertures. Give up the autofocus and multiple elements that would likely fix the vignetting and field curvature, the Meade might be a good alternative to a 500mm. Looking just now at prices, a 500mm F/4 "football lens" runs in excess of $8K, though a third-market maker of a 150-500 F/6.3 is less than $800, with autofocus and image stabilization (not useful for night-time use!). I spent about $600 for the Meade, and I've been happy with it - depends what sort of "bang for the buck" you would like! Spend the $8K if you can!

But a couple years back, I got a used 300mm F/4 Canon lens and this is really the first time I've gotten to use it for stars, and the 6D's larger size is a pretty severe test. Yet, 7 exposures of 2 minutes each of this same region came out great! Shown at right is orange Antares near the center. To its right is the globular cluster Messier 4, and to its upper right is the more distant NGC 6144. Most of the stars have dust clouds around them illuminated by the colors of the stars themselves, and there are multiple dark lanes in the upper part of the exposure - one of my favorite star fields in the sky! And note that while I stopped it down a stop to F/5, field illumination is very good, perhaps just a bit darker in the very corners, and the star images are tiny across the field! Looks like a winner!

After a few shots with the 300, I took these lenses off and installed the TEC140 (5.5" F/7) with a focal length very close to 1,000mm... I also attached a small autoguider and used a notebook computer to run a Lodestar camera for guiding the mount. Also for the first time, I used a field-flattener lens (another used-market purchase after getting the TEC) designed for the TEC under a dark sky. By this time (after 11pm), the moon had set and we finally had inspirational skies! So both the telescope/flattener combo was an unknown, as well as how sensitive the 6D was to the red H-alpha light that the hydrogen clouds mostly emit.

Canon 20Da and 11" Newtonian from 2010
TEC140 and new Canon 6D
I can report that the combo works beyond expectations! Even at full resolution in the corners, the images are excellent and small, the field illumination looks extremely uniform, and the 6D showed good sensitivity to the reds of Messier 8 shown here at left. This is a 7-frame stack of 15 minutes total exposure at an ISO of 3200. I could find little to complain about in these images or the resultant stack.

As for the sensitivity to red wavelengths, yes, these nebulae put out a LOT of red light, but interestingly, the eye is not very sensitive to H-alpha when dark adapted. Modified cameras that admit more red light almost go too far! Shown at right is an image of Messier 8 taken 6 years ago with my Canon 20Da and 11" Newtonian - about the same focal length, F/4 at 8 minutes with an APS sensor, thus it appears larger in the smaller area sensor... Anyway, the enhanced red-sensitivity almost makes it look "too red". I almost like the 6D version at left better - it shows the blue and "white" (combination of blue, green and red) that your eye sees better... Feel free to argue w/me in the comments!

The only weird thing I noticed in the 6D images were some sharpening artifacts, even though no sharpening was done! Shown at left here is a full-res section of a single raw frame. Nothing was done to the image other than minor levels adjustment, yet, the medium-bright stars show dark rings around them as though I ran a sharpening routine and over-did it. If any of you have 6Ds or have otherwise seen this, let me know if I am doing something wrong! Certainly when the image is downsized for web display, the artifacts disappear.

The large sensor size and correspondingly large swaths of sky encourage extending that into a mosaic. With a few other objects nearby, shown at left is a 3-frame mosaic extending north that includes M20, the Triffid Nebula, Messier 21, the open cluster above and left of the Triffid, and NGC 6544 the globular cluster below and left of the Lagoon Nebula - also visible in the lower left corner of the M8 shot above... Note this is assembled from single exposures, 3 minutes exposure each, so 9 minutes total...

I took a few more frames, but nothing as exciting as the above... John talked me into staying a little later than I was wanted, but we eventually finished up and hit the road about 1:15, my getting home about 80 minutes later - tired but excited by another great night's observing. John is already thinking to getting out again, but with the monsoons, it is difficult to make plans before September!

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