Gassendi crater rilles

[Nyctimene]

Learning never ends. When I was preparing for another lunar session yesterday, planning to revisit the splendid floor-fractured crater Gassendi at the N end of M. Humorum, I consulted the comprehensive three-volume "Luna Cognita" handbook by R.A. Garfinkle (a Christmas gift). For the first time, I learned, that there exists an official IAU nomenclature for the numerous rilles on the crater floor (see  picture below). I decided to spot as many rilles this evening, as would be possible with the 12" f/5 under rather good seeing conditions. The EQ platform tracked the scope quite nicely, so I had enough time to concentrate on the details, using a 6 mmf Ortho, giving 250x mag.

I could easily make out rilles I, II, and III (parallel to I and encompassing with I the small, but conspicuous crater Gassendi N; not named in the picture). VII was also visible quite well, whereas VI was more difficult, and only visible in moments of really good seeing. IV and V were out of reach, even with increased mag of 300x (10 mmf Ortho + 2x Barlow), as were the rilles VIII and IX (outside the crater, W of the conspicuous Rima Mersenius I).

I spent almost an hour with observing the rilles; and, somewhat exhausted, went over to craters Kepler and Encke. Again, a picture in "Luna Cognita" led me to a pretty three crater chain E-W, all in contact, that forms Encke M (see second picture below; also shown in the "21st Century Atlas of the Moon", 22, pg. 57; C 3). Well visible with 250x; and probably secondary craterlets, similar to those in the proximity of Copernicus.

Returning to the S, I gave my old nemesis, the concentric crater Marth, another try with 300x mag, and I think, that I could glimpse the inner crater ring for fractions of a second two or three times, but not with certitude. By contrast, Hesiodus A (in the same field of view!) showed it's concentricity plain as day. So I'll have to wait for even better seeing. Still, a really successful and pleasing observation, ending after 11/2 hours at 21.00 CET.

Learning never ends; and Luna never disappoints!

Thanks for reading

Stephan

Edited March 4, 2023 by Nyctimene

[CraigT82]

Fantastic 👍🏼 Going to have to add Luna Cognita to my wish list I think.

[Mr Spock]

Nice report. Gassendi has a lot to offer for a 12". It's a wonderful sight at high magnification. 

One thing I've been doing lately is using the orthos with a x2.5 Powermate. It seems to clear up any residual aberrations and make the orthos even clearer than they are already. The 18mm gives x211, the 12.5mm x304 and the 9mm x422 - a perfect spread.

[davidc135]

Nice write up.

A month on and with good seeing I too spent some time looking for rilles and craterlets within Gassendi using my 8.5'' f7.5 Dob. Magnification x 280.

Much of the system of eastern rilles extending north and south of the central  mountains showed at the best moments. i.e rilles I (south), II, III, IV and (IIRC) VII. But I coudn't spot V and VI.

The three craterlets, M, N and P given as 4, 3 and 2km were readily seen but a fourth 2km+ craterlet just south of Gassendi A was difficult and glimpsed just now and then.

David