7" f/12 iStar refractor 24: Early results

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The Moon was only 20 degrees high when I first set up. By 10pm it had risen to 27 degrees altitude.

Early on the thermal effects were as obvious as last night. Towards the end of my observing session it had calmed considerably though not completely. Frequent cloud also obscured the Moon, on and off, throughout the evening. I took some handheld 'snaps' at intervals simply by holding the camera to the eyepiece. A 20mm Meade 4000 in this case for a nominal 108x visual.

It is interesting how the camera recorded some false colour at the limb and as an overall wash but made it a different colour to my own vision. I don't think this should be considered a true test of the objective's CA as the Moon is still very low and my photographic methods were very crude. Even tilting the camera could make matters much worse or possibly better.

The Cheshire eyepiece showed that the objective had "auto" collimated itself slightly better than last night simply due to the OTA being removed, moved to storage and then remounted again today.

In the interest of greater comfort I swapped the straight extensions for the 2" star diagonal as the Moon rose higher. This needed slightly more re-balancing than the sliding weight had to offer. Though the balance was still well within reason for observing. The view through the eyepiece  sharpened throughout the evening.

These 'snaps' are quite interesting but I have managed  better quality with other telescopes. Though that was when the Moon was much higher. I really need my smooth barreled 20mm [no-name] eyepiece and its DIY tubular adapter for the camera lens nose to do better. The adapter ensures concentricity and squareness of the camera to the eyepiece. Not using the adapter makes centering of the image in the camera purely a matter of luck. The optical characteristics of the cheap, 20mm Plossl eyepiece just seem to be ideal for taking such simple afocal snaps. The problem is that I cannot find it! The Meade doesn't have the same size of eye lens and it does seem to make a real difference. A short zoom digital compact camera also helps thanks to its limited aperture.

Just before I packed up last night the Moon was traveling along, just above the ridge of the house. So, as an experiment, I moved the entire instrument across the lawn to place the Moon clear of the roof. The difference was only very slight or undetectable. The house was not being heated tonight because of the thick insulation in the roof. The sun had been shining brightly all afternoon which may well have warmed the southern face of the roof. Sunshine usually helps to warm the indoors by radiation through the insulation. The roof cools off after sunset but the lower grade heat indoors cannot easily emerge again due to the thick insulation. This may help to reduce thermal effects until the the wood stove is lit and the chimney begins to spill its heat above and over the roof. Wind direction will dictate which way the heat moves.

Now that I have proved [to my own satisfaction] that the lens is not a lemon I have posted on the Cloudy Nights ATM and refractors forums/fora.

It has suddenly occurred to me that the unusual CA correction of the iStar Rx objectives will probably respond very differently to minus-violet filters. Which are designed to reduce the usual achromat's purple haze.[Red-blue.]  I had temporarily mislaid my Baader Fringe Killer filter. So have not had a chance to try it on the 7" R35 yet.

These four images are from the full moon using 20mm and 25mm Plossls, Baader Fringe Killer filter and my DIY, [detergent bottle top] camera centering and spacing adapter. I applied several stops of minus Gamma and one step increase in Contrast to bring out the Lunar mare.

False colour has been very noticeable to my aging eyes with an equivalent focal ratio of f/16 [Rx amended] for a CA-ratio of 2.3. I did try my No8 yellow filter on the moon but it had little useful effect and no noticeable change to the blue-green fringe on the lunar limb. I am not fixated on the limb colour as atmospheric refraction could well have played a part at such low viewing altitudes. Nor should it be forgotten that the telescope is still not perfectly collimated. I will have to order some socket head screws online. The cross-head collimation screws I used are far too soft even with a quality screwdriver.


Third consecutive evening testing on the low Moon and the worst conditions so far. The sky was misty so that the near full moon lit up a huge area to the west. Vega, high overhead, was the only reliably seen star. Others were only visible at intervals. Had a look at intra and extra focal image of Vega at 144x. Lots of rings seen with an expanded white center, cerise ring and blue outer halo. The camera did not capture the rings with the 20mm Plossl.

  
Click on any image for an enlargement.

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