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focuser?

Started by RobertHowe, 05/25/2009 09:26PM
Posted 05/25/2009 09:26PM | Edited 05/25/2009 09:27PM Opening Post
HI all

I am bringing a TV 85 and TV 76 to photograph the upcoming eclipse; the 85 with a 2x Powermate for the disc, the 76 with a TV 0.8x reducer for the entire corona. I intend to focus both scopes' cameras to infinity the night before using a focussing mask and digital camera.

Would I benefit from upgrading to the TV fine touch focusser? To spend another $400 on this project would annoy me, but I don't want to waste a LOT of work, time, trouble and expense by bringing home out of focus images.

The photo was taken with my 76, focussing by eye through my Canon. Other images that night were not so crisp..

Cheers

Robert

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Robert Howe
Wilbraham MA


TeleVue 85 f/7 // Astro-Physics Traveler 105 f/6 // Astro-Physics 130 f/8.35 // TEC 180 f/7
Coronado 60 DS
"Scopes, Brains and Wisdom--but no Beauty"
Posted 05/26/2009 06:23AM #1
While a fine focuser is an advantage, many a fine image have been taken through small refractors over the years without one. I wouldn't worry about using the stock focuser.

What I would suggest is that you practice daytime focusing instead of focusing on a star the night before. If the daytime temperatures are even a few degrees different than at night, your focus point is definitely going to differ--- in fact even if the air temperatures are about the same the fact that the scope will be in direct sunlight will warm up the metal tube and cause some expansion. It will be most noticeable with the TV76 and .8x reducer as that setup will have a shallower depth of focus.

During this year's eclipse Venus should make a good focusing target as it will be just over 40°W of the Sun. Sometimes aircraft or their contrails can also be used as a daytime focusing target.

---John
Posted 05/26/2009 06:32AM #2
Having a fine focuser helps, but not $400 worth if that is the only time you plan to use it.

I do have some questions, though, about your focusing procedure. If the temperature changes, the tube will change in length, and thus in focusing position. Therefore, it does not do much good to focus the night before. And, besides, what makes you think the focus you get the night before would be any better than the focus you get the day of the eclipse? You will be using a filter for the solar eclipse, and not for the previous night's focusing. And focusing on the edge of the solar disk prior to first contact is probably even easier than focusing on a star. Besides, you will have to refocus several times during partial phases before you get to totality.

If this is your first solar eclipse, do not worry about the photos.......Watch the eclipse. This will be a long one, so you have time to do both. But the important thing is to watch the eclipse.

You might get some benefit from looking at this http://www.alexastro.com/Alex%20Home%20Page/Articles/Ranger_20D%20Eclipse.pdf
It is my notes for the Egyptian Eclipse.


Alex


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