3 replies to this topic

[jeffry7]

I went out early on the morning of November 25 and went to work. Where I work there is a 5 story parking garage with a clear view of the sky. I parked on the upper most floor and went to the east side and looked for the moon. I almost missed it because it was so thin and there was a good deal of light pollution in that direction. (It was a thin light brown crescent on a dark brown background.)

 

I had brought my Canon 550d, my 800mm mirror lens, and my Skywatcher GTi goto mount. I also had a 2x tele-extender which meant I was at 1600mm.

 

I tried to take a movie, but the display would not show anything. When I set up a still I saw why. To get an underexposed shot I had to run up the ISO to 6400 and the exposure time to 1/6th of a second.

 

Still, I am glad I tried it. I have a handful of stills which I will try to process later. But I am also planning another try Christmas day. I am wanting to back off to a 300mm lens which should be faster. (I need to work out the light budget and see if I can get down to an exposure that will allow 30 frames/second.) I also have a cheap 70mm f=400mm telescope I could try. At this size I can get raw movies via magic lantern.

 

The image is an unprocessed still only modified for size and resolution for the forums.

 

Any suggestions on how I might improve this shot or do better for my Christmas expedition is appreciated.

Attached Thumbnails

• 

[james7ca]

Very early or very late crescent moon shots are pretty difficult if you want to image while the sun is below your horizon (because the moon will be very low in the sky). That said, you might get a slightly better image if you realigned the color channels to help remove the colors caused by atmospheric refraction (or dispersion). An Atmospheric Dispersion Corrector (ADC) might also help but those devices work much better at slow f-ratios which would be somewhat difficult if you want to image the entire moon in a single frame (because you'd need a pretty large sensor to image the entire moon at a long f-ratio).

 

It's also possible to image the crescent moon during daylight hours so that the moon is higher in the sky. The problem then becomes one of contrast between the bright blue sky and the thin crescent moon. You also need to be very careful that you don't get the sun within your field of view while either looking for or imaging the moon. If you image during the day then you'll probably want to use an IR-pass or deep red filter to darken the sky.

 

All that said, I think you got a pretty good image.

 

By some coincidence I was out imaging on the afternoon of Nov 24 and I decided to capture images of the moon and planets after I had finished my session on the sun. I was using a Celestron C6 with an 807nm IR-pass filter (for the moon and planet shots, I used a solar filter for the sun) and by the time I got to the moon it was already fairly low in the sky (below the sun in the western sky). Thus, it would have been much better if I had imaged when the sun and moon where both higher in the sky (maybe 10 or 11AM for the moon).

 

Anyway, below is a very poor image of the limb of the moon that I took at 2:49PM on Nov. 24 of this year. I would have gotten a better image if I had been using a shorter focal length, since with the camera I was using (a ZWO ASI178MM) and at a focal length of 1500mm I only got a fraction of the entire moon. But, I think it shows the potential of capturing a thin crescent during the daylight hours.

Attached Thumbnails

• 

Edited by james7ca, 04 December 2019 - 02:12 AM.

[jeffry7]

...

 

It's also possible to image the crescent moon during daylight hours so that the moon is higher in the sky. The problem then becomes one of contrast between the bright blue sky and the thin crescent moon. You also need to be very careful that you don't get the sun within your field of view while either looking for or imaging the moon. If you image during the day then you'll probably want to use an IR-pass or deep red filter to darken the sky.

 

...

Other then the obvious "be careful" how does one avoid getting an eyeful or sensor full of sunlight?

[james7ca]

Other then the obvious "be careful" how does one avoid getting an eyeful or sensor full of sunlight?

If you have a reasonable polar alignment and a GOTO mount with known (by you) accuracy or lack thereof you can generally be safe as long as the object is at least 10 degrees from the sun. However, when I slew to a new object I block the aperture of the scope (with my body, or a card, or with a dew cap) until the mount has moved to a safe position (i.e. don't try to slew past the sun). Also, if you have a solar filter (thin film, full aperture is probably the best) then you can use that to sync to the sun and then slew to your target and then remove the solar filter (if it seems a safe distance from the sun).

 

You just need to be super careful if you are working anywhere near to the sun.