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D850 DSLR tests;
Crescent Moon & Earthshine

Posted: 20 March 2018

Friday, 16 March 2018, was cloudy. The sky cleared on Saturday morning, 17 March, with a forecast for a clear night. However, clouds returned mid-afternoon, defying the weather forecasts. Sunday, 18 March, was cloudy with some light rain (0.03"). Sunday evening I supported the Closing Reception for "An Inner Astronomy" art show at Triangle L Ranch in Oracle. The artist Catherine Nash had her paintings and sculptures on display and gave a talk. Following her talk the sky cleared and we had a star party. This photo from the Ranch shows the thin crescent Moon with Earthshine, Venus, and Mercury:


You can read my full report on the Oracle Dark Skies Committee web site. When I returned home clouds were coming back in. Monday, 19 March, was mostly clear during the daytime even though the forecasts called for mostly cloudy skies. However, as sunset approached, the cloud cover became more extensive, matching the nighttime forecast. I decided to open the observatory anyway to at least view the crescent Moon.

Open: Monday, 19 March 2018, 1816 MST
Temperature: 78°F
Session: 1213
Conditions: Partly cloudy

Equipment Used:
12" f/8 LX600 w/StarLock
2" 24mm UWA eyepiece
Focal Reducer


First, I SYNCed the observatory clock to WWV.

1823 MST: LX600 ON, StarLock OFF, High Precision OFF.

1824 MST: viewed the crescent Moon, 102X. Then viewed Venus, 102X. Venus is still showing a nearly full phase.

1827 MST: viewed Mercury, 102X. A crescent phase was visible.

1837 MST: sunset.

1838 MST: took this photo of the observatory and the western sky. The thin crescent Moon is at the upper left.


I then began preparing to do some vignetting test images with my new D850 DSLR camera. I had previously done vignetting tests with the D7200 DSLR. I mounted the camera at prime focus using the 2" visual back and 2" camera adapter. I pointed the 12" telescope at the still twilight lit sky and did exposures with and without the visual back extension and with and without the focal reducer. Here are the results:

Prime Focus (left: with extension; right: without extension)
photo photo

Prime Focus + Focal Reducer (left: with extension; right: without extension)
photo photo

Vignetting (darkening of the corners) is more evident when using the focal reducer. There is very little difference in the amount of vignetting when using the visual back extension vs without the extension. As it is more convenient to use the extension I will continue using it for imaging with the 12" telescope.

1902 MST: slewed back to the crescent Moon and took this photo, prime focus + focal reducer, 1/100sec, ISO 200, White Balance Auto, slightly cropped:


I increased the exposure to 1/10sec, ISO 400, for this photo of Earthshine (slightly cropped):


1911 MST: slewed to the star Sirius and did field-of-view measurements for the new D850 DSLR camera at prime focus with and without the focal reducer and with and without the star diagonal.

Due to the increasing clouds decided to end the session.

1954 MST: LX600 OFF.

Close: Monday, 19 March 2018, 2000 MST
Temperature: 55°F
Session Length: 1h 44m
Conditions: Partly cloudy

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