Next entry: Sundog on the Alm

Previous entry: NGC 7510, a very distant open cluster in Cepheus

The Mini Coathanger
On the evening of Thursday August 23d, I had an hour of clear skies, so I quickly got out with my Sky-Window and the 15x80 Vixen binoculars and the 85mm Zeiss refractor equipped with the 20-60x-zoom eyepiece. I wanted to have a look at an asterism called the Mini Coathanger (STAR 22 from Phil Harrington’s Small Telescope Asterism Roster) in Ursa Minor. The Mini Coathanger, which looks very much like his big brother the Coathanger in Vulpecula, can be found at RA 16:29.0 and DEC +80.13. I always start the search at 16 Zeta Umi. About 2 degrees northeast of this bright magnitude 4 star, a diamond-shaped group of stars can be seen. At the Northern tip of this diamond shaped group STAR 22 can be found. The Mini Coathanger is made up off 11 almost equally bright stars, most of them of the 10th or 11th magnitude. The asterism is 15’ wide. In the Millennium Star Atlas, the whole asterism can be found on page 1046.

That night, I started my observing session at 21.00 hours UT. The seeing from my backyard was only 5 on a scale of 10. Most bright naked-eye stars were blinking like crazy. The transparency and sky darkness weren’t too good either. I couldn’t see all seven stars of Ursa Minor, so the limiting magnitude was well below 5. However, with the 15x80 I quickly located the diamond shaped asterism, but the Mini Coathanger stayed invisible. After observing for a few minutes, I started to see a little bar of four or five stars, that form a part of the Mini Coathanger. The stars were very faint, and I could not see the complete asterism, not even with averted vision. Then I switched to the 85mm Zeiss. At 20x the bar of stars already looked a bit brighter, and seemed easier to identify. Zooming up to 60 times, using averted vision, I could see the whole asterism, the bar and hook. I could not detect any colour in the stars. After observing for 20 minutes from under a black hood, I could see the whole asterism with direct vision. I made the sketch below to using the 85mm refractor and the eyepiece at maximum zoom. The field of view at 60x is about 1.25 degrees, but I only sketched the central part of the field of view. Before I finished my observing session I switched back once again to the 15x80 binoculars. This time, using the black hood and averted vision, I still could not identify the whole asterism with the 15x80.

image


At 22.00 hours UT I packed up and went inside (had to work the next day), enjoyed my cup of coffee and put down a few notes. Looking back at the short observing session I can definitely say that the refractor with its 85mm aperture shows stars of the 11th magnitude much easier than the 15x80 binoculars. With the big binoculars, you use two eyes, which should compensate for the 5mm difference with the 85mm Zeiss. But in the end, I think that the higher magnification (you start at 20x with the Zeiss) makes the difference. The slightly higher magnification gives a little more contrast. If you zoom in to 60x, the 15x80 can in no way keep up with the views the refractor shows. However, I will try to hunt down all the asterisms on Harrington’s STAR list using both instruments. They are ideal for short observing sessions. Both instruments are mounted on video tripods permanently, so I only have to take them into the garden and I’m ready to go!

Posted by Math on 09/02 at 02:47 AM
Deepsky log • (0) CommentsPermalink

Name:

Email:

Location:

URL:

Smileys

Remember my personal information

Notify me of follow-up comments?