Dew and perhaps cooling-induced Spherical Aberration

[Captain Scarlet]

I had a disappointing night last night for by no means uncommon reasons. The night promised and in fact was beautifully clear. Beehive was naked eye even through Skibbereen’s light glow.

The first problem was dew, and the second rapid temperature change. I’d brought my 12” newt out a couple of hours early from a 15-degree garage. When I got to it to observe, dew was already on the secondary and the paracorr. About an hour into the session to add insult, all the dew was frozen. I tried to persevere anyway but the stars too were terrible. I think the seeing was OK actually, but the rapidly still-cooling mirror  may well have been changing shape dramatically, as it was showing signs of really terrible SA, which normally it shouldn’t considering it’s a brand new OO 1/10 mirror.

M42 was amazing briefly, but only the 4 main A-D stars I’ve yet to see the E and F.

oh well, tonight’s forecast clear again so I’ll get the hairdryer out this time...

Magnus

[Captain Scarlet]

Well. Out again tonight and as forecast, the early part of the evening has been lovely and clear.

Filled with anxiety, that after my Star Test of the scope last night I'd paid big money for a dog from OO, imagine my relief when tonight the scope delivered a text-book and identical inside- and outside-focus star test. Thanks be to. Last night was, incidentally, First Light for the outside 3/4 inch of this new mirror. The inside part of the mirror got its FL a few weeks ago when I just plonked it into the SW cell replacing the extant mirror without regard to the different focal length, which required some surgery to the tube to place the secondary further away.

Unfortunately, as forecast, early cloud has now rolled in so the heavens are teasing me! "Yes you have a good mirror, we'll just about allow you enough time to discover that, but sorry no you can't play tonight."

Anyway, I'm pretty relieved. I've learned something fascinating though: a rapidly-cooling mirror, losing heat from its periphery well before its middle, thereby changes its shape and becomes "over-corrected": thinner at the edges than it should be, but still fat in the middle. I saw this first hand last night with that appalling star test. Temperature tonight was stable, and no dew (frightened away by my hair dryer close at hand no doubt).

Cheers, Magnus

Edited December 8, 2020 by Captain Magenta

[Paz]

That is interesting read, was the SA  issue  experienced with the use of a cooling fan on the primary or was the mirror cooling naturally.

[Captain Scarlet]

I’ve toyed with the idea of a cooling fan but don’t have one yet. The mirror was cooling naturally but I’d say the temperature had dropped from around 4-5 degs C to about -2 in the space of an hour.

I’d heard of the cooling effect on mirrors by reading some of Mike Lockwood’s writings.

M

Edited December 9, 2020 by Captain Magenta

[Jiggy 67]

On 09/12/2020 at 13:27, Captain Magenta said:

I’ve toyed with the idea of a cooling fan but don’t have one yet. The mirror was cooling naturally but I’d say the temperature had dropped from around 4-5 degs C to about -2 in the space of an hour.

I’d heard of the cooling effect on mirrors by reading some of Mike Lockwood’s writings.

M

That’s quite scary. Can permanent damage be done to the mirror??

[Captain Scarlet]

58 minutes ago, Jiggy 67 said:

That’s quite scary. Can permanent damage be done to the mirror??

No, no damage at all, just the figure changing in response to different cooling rates in different parts of the mirror. I was lucky to be able to observe two extremes on consecutive nights, first terrible, thinking I’d been supplied a dud; then with previous night still fresh-in-memory, a beautiful experience indicating a near-perfect mirror.

Just another thing to add to the list of unlikely things that need to be “just right” all together to be able to take full advantage of a good mirror: good seeing, good transparency and now as I’ve discovered, stable temperature.