10th May 2016 Mercury transit impressions.

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One could not have asked for clearer skies than yesterday for the Mercury transit. There were a few clouds after lunch but they never blocked the sun. After that the day remained cloudless until well after 9pm. It was also very warm at 72F which required a careful choice of clothing to avoid being cooked alive with sunburn over an 8 hour session out of doors. A dark, long sleeved, cotton shirt provided protection without too much warmth. A pair of shorts helped me to stay reasonably cool while still exposed to wind and sun. Later on the deliberate choice of situation provided vital shelter thanks to our high garden hedge.

 

I had to drag the massive pier along the drive past the house to place it outside the garden to have any hope of an unobstructed view. Otherwise thermal currents rising from our own roof would have dominated the proceedings from beginning to end. Then I had to carry the 8' long, 45 lb refractor out to the mounting and lift it up into place. The transit started at about 13.10 CET with the sun at about 53 degrees altitude but I had already set everything up well over half an hour earlier to be ready. 

The computer for capturing AVI files from the Neximage camera rested on the footplate of a folding workbench. This was placed just inside the gates to provide some shelter from the sun while remaining close enough to the telescope. Focusing and centering an object on the screen required a clear view but still teased at intervals. The small monitor sat on a heavy piece of kitchen working surface to provide greater area for the mouse and keyboard without any risk of it lifting in the gusty wind. In fact the easterly wind proved to be a nuisance at times while capturing videos. Though one cannot complain if the sky is clear when many others had cloud. 

Plans to use multiple instruments were shelved so that I could concentrate only on the 7" on the MkIV mounting. Though this meant removing and refitting the Neximage 5 camera at intervals. I wanted to practice taking afocal 'snaps' in between capturing AVI files. The refractor was stopped own to 6" @ f14.4 nominal and fitted with a Baader AstroSolar film filter.

iStar claim a 35% extension of effective focal length with their R35 lenses to bring a 6" to f/19. False colour was indeed very low with hardly a fringe at focus when used visually. I worked my way up through my secondhand eyepiece collection and reached 175x without Mercury losing sharpness! There was no sign of increased CA due to magnification. Turbulence was another matter altogether despite the large lawns and miles of fields surrounding my observation site. The nearest building was a hundred yards way in the direction of the sun.

The image scale through the Neximage 5 Camera was far more than was strictly necessary or even desirable for solar work. When used in the 5MP mode with 4x binning for centering it all looked well. When binning was disabled the camera stared at only a tiny area of the Sun. The central sunspot group or Mercury each filled the field of view. Capturing both together was completely impossible in the Y800 640 x 480 capture mode. The sunspots and Mercury seemed to be floating on a complex, seething jelly. Even when I had temporarily lost Mercury outside the Neximage 5's field of view the surface detail was easily enough to be seen passing across the computer screen.

As previously discovered, the Canon Ixus 117 camera has a mind of its own when taking afocal images of the sun. It seems quite unable to relax and take an image regardless of settings. The sun's image was often completely out of focus or heavily shadowed with dirt and dust shadows. I cleaned the eyepieces but that wasn't the problem. I removed the star diagonal and fitted extension tubes but the star diagonal wasn't the problem either. Still the debris shadows persisted projected onto the sun's image. I cleaned the camera lens repeatedly but still the shadows spoilt the images. From previous experience with the TZ7 I now believe the debris is on the sensor or in the Canon lenses. I have dismantled the TZ7 several times to clean debris from its sensor. The camera's zoom action seems to suck dirt into the optics where it lodges permanently in the field of view. I really ought to do a close examination of the unwanted shadows when rotating the camera and the eyepiece separately. Hindsight is always so perfect!

In retrospect I could have used the 5MP still image capture option for recording Mercury's progress on the Neximage 5 but the scale would still have been very large. My first Sony compact digital camera always took fine solar images without any effort at all. I had deliberately chosen the Ixus for afocal astro snaps because of its small clear aperture and short zoom better matches ordinary eyepieces.

Initial images show the absolutely tiny size of Mercury relative to the Sun. Trying to learn how to use a brand new camera during a rare transit may not be wise but it provided plenty of forced practice. The MkIV showed it age and lack of regular maintenance in lagging badly between reaction to its control paddle button pressings. A quick adjustment with a hex key solved some slack in the worm drive housings. Interestingly, the view through the Neximage 5 camera was extremely rapid to the slightest touch of a control button when in capture mode setting. Which only goes to emphasize the high magnifications involved.

The bottom line is: Do not expect to become a world famous astronomical imager on day one! Particularly if you are relaxed [slapdash?] about any aspect of your instrumental set-up. It take precise adjustments to focus, collimation and mounting alignment. Now add an intimate familiarity with the camera, its capture software and image processing software like Registax.

Mercury, the Sun's limb and its spot groups provided plenty of hands-on, eyeball practice yesterday. I gained a little more useful experience with the camera and iCap but climbed perhaps only one rung on the steep learning curve in Registax.

Not least, solar imaging needs a reasonable, non-reflective computer monitor. One within easy reach of the telescope focusing knobs. Or preferably a motorized focuser to allow one to peer closely at the screen from the shelter of a decent solar screen. Mercury was a tiny black dot when seen visually through the eyepiece. But became a bouncing, gigantic, gyrating ball of fluff above a blazing bonfire seen across a shimmering desert through the Neximage 5! I kid you not. Perhaps I should have hoped for cloudier conditions to keep thermal issues at bay?

Perhaps it is easier with shorter telescopes? But then you'd only have to add a Barlow to make the image huge enough to actually see anything! It was a fun, interesting and rewarding experience despite all the apparent negativity mentioned above. I only write it all down to remind myself what I will probably have forgotten by next time I go out there and try and capture what is happening up in the sky. Familiarity will come with practice. It just takes longer at my age. I have taken my first faltering steps on what is often claimed to be the long road to financial ruin that is astro imaging. Those who have already proved their imaging competence have gained a lot of respect from me.

Click on any image for an enlargement.

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