How to observe thin lunar crescents

15.8.2008 (Martin Elsässer)

The main issues for daytime crescent observation are Safety and Contrast.

Safety

You must never look through your telescope, or any optics, when it is pointed at the sun, without using proper solar filters. Doing this wrong will immediately destroy your eye and you only have two of these. Cover any finder-scope with a solar filter or its cap and firmly fix these with duct tape. (Plastic caps tend to get soft and widen and fall off, when hot.) You can also simply remove the finder scope.

The best method for safety is to use a dedicated, long baffle, which prevents the light of the sun from reaching the optics, when the scope is pointed at the moon. For 10° elongation this baffle would need to be at least 6x longer then the diameter of your optics. You can do without such a baffle, but then you must MOST THOROUGLY CHECK the light path inside your optics, where the glare from the sun goes, when looking at the moon. I always hold my hand behind the optics first, when doing these observations and check for any bright light exiting the scope before very carefully looking through the optics. Really be careful here and if you are not sure about this, find an experienced solar observer for advice. The UK will have plenty of those.

You might use a cover with a smaller off-axis hole on your telescope, to reduce the diameter to abou 80mm and thus implify the baffeling task.

Always be carefull with any movement of the scope. Make sure that the sun never comes in the field of view. Use a slow and methodical approach. This is not the time to bring your kids.

Contrast

The lunar crescent is of inherently low contrast during the day and that is the main challenge. Your telescope should be able to retain the sky-contrast and then you can try to increase the contrast with other means so that you might detect the crescent. Alas a Schmidt-Cassegrain such as a LX200 is not really optimized for this task, due to its open optical construction. Simple, small refractors are better at this, but it should still work.

You can improve the contrast greatly, by using a red filter, which will block much of the scattered, blue light of the sky. You should use a low magnification, for the largest field of view with your scope. A piece of clear red plastic, mounted in the eyepiece, might work. THIS IS NOT A SOLAR FILTER, ONLY TO IMPROVE THE CONTRAST DURING OBSERVATION.

Operation

Align your telescope properly, so that your goto works well. Test your goto, by aligning at the sun (carefully, using a solar filter or the shadow-method!) and then letting the scope point at venus, mercury or bright stars. These should be well centered in the field of view. Of course, you need a drive for automatic tracking and this should be set to lunar-speed if your scope can do that.

Focus the telescope precisely for your eye, using all the filters, by looking at venus. You should focus from extrafocal towards intrafocal and stop as soon as the image appears sharp. This is vital for success, as your eyes will loose all reference when searching for the faint crescent, and the scope needs to be focussed for that situation, so that you have any chance of seing the crescent. Test this by moving away from the eyepiece, looking at infinity for some seconds and then looking through the scope again. Venus should immediately appear sharp.

If you have it use a binoviewer as that drastically increases the capabilities of your vision system. These are expensive and have limited field of view for your scope, though.

Use a black towel or hood, to cover your head when obsvering. Avoid all distracting stray light.

After aligning your scope and focussing it on venus, goto the scope to the position of the crescent. Check carefully for any dangerous glare. Then observe, using the black hood. Move the scope very slightly with the drives, as movement will make it easier to see the faint crescent.

Document your observation thoroughly: Location, Date, Time, Instrument, Filters, Magnification, Orientation of the crescent, Apparent angle of the horns and similar. Try different filters and maginifcations and compare the view. Look for known structures on the crescent, using a lunar map.

Its a good idea, to test all these steps on a easy crescent, some days before conjunction. Practice makes perfect.

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