Monday, January 29, 2018

Preparing for the Blue-Blood Moon Lunar Eclipse...

This Wednesday's full moon is unusual:

First, it is the SECOND full moon in the month of January, 2018. As such it is commonly referred to as a "blue moon", where we get the expression "once in a blue moon" because it is a fairly rare event to have two full moons in a given month.

Secondly, this full moon is graced with being a total lunar eclipse, which is when the Moon passes behind the Earth and enters its shadow cast by the Sun.

Here in Oregon it will occur early Wednesday morning from about 4 am until 6 am, due west. You will need to have a fairly clear view of the horizon to see it, though, as it will begin about 30 degrees above the horizon and drop during totality all the way down to about 5 degrees above the horizon.

Thirdly, and here is the "joke":
Question: When is a "blue moon" NOT a "blue moon"?
Answer:   When it is a "blood moon"!
In other words, when the Moon dips into the shadow of the Earth, the color of the light bending will shift slowly from yellow, orange, brownish and even "blood" red.


In preparation for this Wednesday morning's total lunar eclipse I have been testing various lens and telescopes to get a better idea as to how large the Moon will appear in the images I take with a Canon T3i DSLR camera body.

The good news is that the totality of a total lunar eclipse lasts a very long time (up to several hours) compared to a typical total solar eclipse (only about 2 to 6 minutes). This allows one to be able to change the lens/telescope at will once totality occurs.

And it is not always desirable to image the Moon as large as possible in the image. If one includes some foreground objects (trees, buildings, statues, clouds, etc) one may want to use a lens of much lower power to capture the context of the eclipse, as these images (taken by others, not myself) show:

(I would give photo credits but these were posted on the web without them)

Since the Canon T3i is a "cropped sensor" camera, the typical focal length measurements do not line up with what one could expect with an old fashioned 35mm camera lens chart. Hence the need to test my various lenses and telescopes to get a better idea of what size the Moon will appear in the final images.

Here are the 5 lenses and telescopes that I expect to use, weather-permitting, this Wednesday:

250mm focal length: Canon 55-250mm zoom lens (f/4.5-5.6)

350mm focal length: Tele-Lentar film camera telephoto (f/5.6)

400mm focal length: Celestron Short Tube (70mm f/5.7) OTA

750mm focal length: Celestron C-5 telephoto (127mm f/6 Cassegrain) OTA

1,350mm focal length: Celestron NexStar 4 SE (102mm f/13 Maksutov-Cassegrain) OTA

So I tested these in poor weather conditions last night (lots of clouds, but some thin enough to see the Moon) with the camera and lenses/telescope on a stationary tripod: Here are the results without cropping the images:





(the clouds really got thick when I took this shot!)

As you can see, the NexStar 4SE (1,325mm) telescope did not quite take in all the Moon, which may not be useful except in the earlier pre-totality phases of the eclipse.

The 400mm and 750mm images point to the Celestron C5 and the Celestron Short Tube as being the best overall for as telephoto as possible with what I have.

There is one more scope I may use which will be about 500mm, a Daystar telescope used in the CATE project, that I hope to test tonight and update this page afterwards.
Daystar 500mm telescope

However the 350mm, though not as telephoto seemed to create a sharper image with more control over the f/stop by use of its diaphragm settings.

The much wider view that the 250mm telephoto zoom lens offers will likely be the one I use to put the eclipse "in context" of the foreground mountains, trees, etc.

Now I just need clear skies in the area of the Moon on Wednesday from 3 am until 6 pm, though the present weather forecast is foreboding...
Tuesday over night forecast:

Wednesday morning forecast:

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