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John Noble
super member
Reged: 05/07/08
Posts: 179
Loc: Sandy Eggo, California
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Having played around with my various astrotoys for the past year and a half, I've decided to embark on more serious observing pursuits, starting with the Messier catalog. I'd like to do it "right", which means taking measurements. The Celestron/Baader astrometric eyepiece appears to be the only game in town (aside from $$$$$ Zeiss micrometers, etc.), and I'll be picking one up soon unless someone suggests a better alternative.
Just one problem: both of my scopes are on alt/az mounts (80mm refractor, 10" Dobsonian). An equatorial mount isn't in the cards for the near future, so I'd love to hear others' techniques for making accurate angle measurements.
It occurs to me that I could drift align the eyepiece's main micrometer scale and have a decent alignment with North for a few minutes' worth of observing near the alignment area, but is there a better way?
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"We had the sky up there, all speckled with stars, and we used to lay on our backs and look up at them, and discuss about whether they was made or only just happened."
--Huckleberry Finn
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gnowellsct
professor emeritus
Reged: 06/24/09
Posts: 733
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Angular measurements can be made by knowing the field of view of your scope with a given eyepiece and then estimating the amount of diameter the object takes up. Such estimates are really "for your pleasure" since it is almost always the case that photometry will show the object to be more extensive than what you see; and someone with a 30 inch dob will see more extension than you on most deep sky objects.
Estimating position angle in a dob, which is to say orientation to N and S, strikes me as very difficult.
Greg N
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"Aperture will get you through times of no money better than money will get you through times of no aperture."
featuring selected astrojunk:
bunch o' widefield eyepieces
bunch o' narrowfield eyepieces
couple o' Barlows
couple o' scopes
couple o' mounts
couple o' tripods
and a pier 'n' stuff
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John Noble
super member
Reged: 05/07/08
Posts: 179
Loc: Sandy Eggo, California
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Angular size measurements ought to be a cinch, yes.
Quote:
Estimating position angle in a dob, which is to say orientation to N and S, strikes me as very difficult.
Greg N
I thought so too until I ran across this:
Visual Double Star Measurements with an Alt-Azimuth Telescope (PDF file)
Seems I had it backwards: align the linear scale to the object, then allow the object's center to drift through the center of the linear scale and continue on to the outer protractor scale, noting the angle of contact.
It's an elegant approach. I have some doubts about accuracy at high declinations, not to mention the long drift times in the same areas, but it seems like a workable solution.
Has anyone else done this, or am I crazy?
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"We had the sky up there, all speckled with stars, and we used to lay on our backs and look up at them, and discuss about whether they was made or only just happened."
--Huckleberry Finn
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Moezilla
member
Reged: 07/16/09
Posts: 52
Loc: TX
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This is something that I too am looking to do when my scope comes back from Celestron. I'm going to check that PDF files that you linked to and see what it says. My interests are double measurements (SEP, PA) and using the eyepiece as John noted, to do observations "right". Let us know how your experiences with it is John.
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John Noble
super member
Reged: 05/07/08
Posts: 179
Loc: Sandy Eggo, California
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Full Moon and sick kids have slowed things down somewhat, but I should be in business by this weekend.
Greg's point about extension is interesting. I thought about it for a while, and I came to the conclusion that it's interesting information even if I don't get "textbook" answers. I agree that it's a "for your pleasure" thing, but the whole pursuit of visual astronomy is a pleasure trip: "serious" astronomy hasn't been done by visual observation in over a hundred years, aside from a few niche areas.
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"We had the sky up there, all speckled with stars, and we used to lay on our backs and look up at them, and discuss about whether they was made or only just happened."
--Huckleberry Finn
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John Noble
super member
Reged: 05/07/08
Posts: 179
Loc: Sandy Eggo, California
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OK, got my Micro Guide today and had a brief opportunity to play with it tonight before the clouds moved in.
I got consistent but incorrect position angles using Albireo as ay test target. I came up with 35-37 degrees over four or five runs, but the correct number is more like 55 degrees. I used my 80mm refractor on an alt/az mount, with no attempt to level anything. I don't know if this was a factor, and in my current DayQuil-reduced state of mind I'm not sure I could visualize what's going on with the respective angles. 
The eyepiece itself looks and feels like a quality piece of gear. Seeing was terrible tonight (Jupiter looked squirmy at 40X!) so I can't say much about optical quality. The scales are easy to read, though I do agree with reviewers who commented about the lowest illumination setting being a touch higher than optimum. This is the first eyepiece I've used with a "winged" eyecup, and I'm already turning into a big fan of this arrangement. It feels very comfortable and blocks almost all stray light.
More later when I have something to report.
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"We had the sky up there, all speckled with stars, and we used to lay on our backs and look up at them, and discuss about whether they was made or only just happened."
--Huckleberry Finn
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John Noble
super member
Reged: 05/07/08
Posts: 179
Loc: Sandy Eggo, California
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OK, I think NyQuil had the answer:
Albireo moves West (duh), which is 90 degrees away from North (duh, duh). Measured departure angle was 36-37 degrees, which subtracted from 90 degrees gives 54-53 degrees position angle. That's fairly close to the "book" answer of 55 degrees.
If anyone wants to correct me, please feel free.
I'll check some more targets tonight, clouds permitting, and see what happens.
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"We had the sky up there, all speckled with stars, and we used to lay on our backs and look up at them, and discuss about whether they was made or only just happened."
--Huckleberry Finn
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