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ATM, Optics and DIY Forum / Re: Optical Formulas
« Last post by Michael Timmons on Today at 05:11:22 AM »
Quote
Are there simple formula for the following that factor in the central obstruction (if any)?

Dawes Limit
Rayleigh Limit
Limiting Magnitude
Light Grasp over the naked eye

Thanks for any help!

A normally-sized secondary does not reduce the resolution of the primary mirror, merely cause additional diffraction and reduce contrast.
So the Dawes limit of 4.56"/D where D is diameter of objective holds true in reflectors and refractors, or 116"/D in mm
The Rayleigh limit is about 138"/D in mm
and there is a tighter double star limit called the Sparrow limit at 108"/D

Limiting magnitude has many many variables.
Here is a good calculator based on the work of Bradley Schaeffer:http://www.scopecity...-calculator.cfm
To see the detailed html code behind the page, examine the source code by hitting CTRL + U
You can actually tailor the calculator for YOUR scope by entering in the reflectivity of your optics.
Most of the "meat" of the calculator html code starts about line 1050.

As for light grasp over the naked eye, it's a simple relationship between the diameter of the pupil and the diameter of the scope in the form R²/r², where R = radius of scope aperture and r= radius of dark-adapted pupil.
One magnitude is 2.512x, so 2.512 to the X power = R²/r² [in magnitudes, assuming perfection transmission (not possible)] where X is the number of magnitudes gained.
The problem with a simple calculation is that it doesn't take into account that stars are points, and reducing the size of the telescope's exit pupil makes fainter stars visible by dimming the background sky.
However it's a reasonable way to figure out how much deeper you will see with one scope over another where the R²/r² figures are for different scope sizes.
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Beginners Forum / Re: $ eye pieces
« Last post by loraderclot on Today at 05:11:19 AM »
Don't worry too much about "getting it exactly right" the first time. Your tastes and your needs will change over time, depending on the scopes you use, your preferences for viewing, comfort level with glasses, or without. And your budget, of course.

The best advice I ever got was to start with eyepieces in the 65-70 degree FOV range that provide magnification of about 70x, 140x, and 210x.  I started with only three, as above, and added a few more as time passed. Very high magnification (over 250x) is rarely needed, due to light pollution and atmospheric conditions.

Edit: others already mentioned eye relief, or ER. I also strongly prefer ER in the 15-20mm range. Far easier on my eyes...

c/s,
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Light Pollution Topics / Re: How to darken my backyard
« Last post by Jason Pederes on Today at 05:11:03 AM »
Cool Observeratory!!
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It wasn't a test question. It's just that which side you were on determines whether the outer area is high or low.

Dave
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Quote
Make sure the tripod legs are tightly attached to the head. I'm betting it's a tripod problem. That narrow leg spread just doesn't look solid.

It very well may be a tripod problem. I have tried the tripod with and without the pier extension and extended and retracted legs. The legs don't make much difference but taking off the extension did help maybe 20%. But I need to use the extension.

Even Sky-Watcher support suspects the tripod. But this tripod is the only option with the mount.

Going to try it on an AZ-EQ6 tripod on Saturday. Very curious to see if that cures the ills.

Still can't get past that other longer refractor users don't have a problem. One guy with an ES 127 says he's very pleased with it. However a couple users in Europe reported that they had to exchange it and the replacements worked fine. Yet for another exchanging didn't help. Arggh.

TK
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Back when...I used to hang black shower curtains in a pvc bi-fold frame I made. Worked well placed between me and the street light or neighbors security light that failed to recognize my presence!Mike
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Except for the 2.8x Klee I've never been impressed with the shorty triplet barlows. They all added noticeably more scatter to the image than a good doublet barlow like the 2x Televue or the Dakin. The Klee is a great performer, but causes vignetting with many eyepieces.

The Takahashi 2x 1.25" barlow is designed to fit ahead of the 1.25" diagonal. I had one and it was very good, much better than the common shorties, though I thought my Televue and Baader barlows offered a slightly cleaner image.

The Baader VIP 2x barlow, which is really excellent, comes in an adjustable-length tube and might be able to work either ahead of or in the diagonal. I use mine screwed directly into the nosepiece of my Baader prism diagonal, where it operates beautifully at about 2.5x, though that combo requires quite a bit of focuser out travel.
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This is a lithium ion rechargeable battery that powers my Nexstar GT mount.
http://www.amazon.co...0?ie=UTF8&psc=1

CN Thread on above battery:
http://www.cloudynig...-battery-power/

Link to 114GT Manual:
http://www.celestron...anuals-software
Search for "1228334587_nexstargt.pdf"

Nexstar GT tech support nexstarsite:
http://www.nexstarsite.com/

I have a Nexstar GT (mount/tripod only) purchased for 160usd thru a vendor when they closed out the supply. By no means am I familiar with all the different versions. The plus handset will not work on some of the earlier mounts. If you need too, there were (do not know about current supply) Nexstar Plus GT mounts (no tripod) and handsets on ebay/etc. at very modest prices. Replace the old mount/handset on the tripod, or just a new handset, with the Plus version. I bought one as a back up to my older GT version, the Plus handset does not work with my version mount, but works with the newer version GT mount it came with. If my old mount or handset fries, I will just put the backup Plus version on the tripod.

Good viewing,

Dave
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Beginners Forum / Re: Kid in a candy store
« Last post by contreleri on Today at 05:10:22 AM »
Quote
Does anyone else feel like a kid in a candy store when you get a perfectly clear evening at an unobstructed site with low horizons? I have a local field I go to where there's periodically community astronomy events. It's about two levels darker than my house with much clearer horizons, and I can never follow an observing plan. Every time I've been so far I've hit up all the planets in the sky (Venus, Mars, Uranus), and all my current favorite objects (M42, M45, a handful of carbon stars, Eta Cass, Iota Cass, Beta Monoceros) before I can begin to settle down and look for something new.

In some ways, my backyard is a blessing. Between the house and neighbor pine trees I have a strip of sky from about 25 degrees to zenith on either side that's about 30 degrees wide. With such a restricted view, it's easier to settle down an observe an object for a while before being tempted away. I was able to spend about ten minutes with NGC 1023 the other day in Perseus, which certainly isn't a fabulous target for an 8" under heavy light pollution, but I was fascinated that I was even able to find it at all.

I'll usually go first for the neat stuff that's heading for the horizon. After that, not so much pressure.
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Joe:

You might consider the "standard" off-axis Newtonian that Dan McShane of DGM builds. It uses an an off axis section of a parabolic mirror, essentially like an aperture mask, to achieve a Reflector with no central obstruction.. The about F/11.

Jon
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