Apparent Magnitude question

umbralumina · Jul 23, 2007

Hi to you all I have been wondering for a time now if there is some kind of equation or something like it to calculate the maximum apparent magnitude that can be viewed through a telescope or binoculars, based on the diameter of the lens. I wanted to know this to use as a reference for the next telescope/binoculars I plan to buy. I have been trying to figure it out but I cant get certain results, if anyone can help me with this I would really appreciate it.

MeteorWayne · Jul 23, 2007

Welcome to SDC! Apparent magnitude is not the term you are looking for. That refers to the brightness of a star at a standard distance (I believe it's 1 parsec, ~3.26 light years) What you should be asking for is the actual magnitude of stars or planets. Naked eye, under dark skies (which really don't exist in many places anymore) you can see to about mag +6.5. Binoculars can get you to mag 9 to 11 depending on the size. Telescopes can get you to mag 11-20 depending on the size. What do you intend to look for? In other words, what are you hoping to see, and how much are you willing to spend? <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

3488 · Jul 23, 2007

Hi MeteorWayne, I would be very surprised if it is possible in any largely inhabited area to see down to magnitude +6.5. i would expect, the Sahara Desert, The Gobi, Siberian Taiga, The Southwest deserts of the USA, Antarctica, Greenland, etc places like those, it is probably still possible. Mind you, we still have some dark places here in the UK. Hard to believe I know, but central Wales is not bad as are some parts of Scotland & the Pennines. Dartmoor on Devon aint too bad either. Andrew Brown. <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

nexium · Jul 23, 2007

If you can afford to launch the equivelent of the Hubble space telescope, 30 magnetude is possible with very long exposures, but it would take a lifetime to image a millionth part of the sky at 30 magnetude. Light polution may mean that surface of Earth telescopes will never achieve 30 magnetude. Please refute, comment or embellish. Neil

nexium · Jul 23, 2007

The formula is something like: maximum magnetude = lens diameter times length of exposure divided by square arc seconds falling on the ccd (charge coupled device). A constant is needed depending on the units, such as diameter in meters, exposure in seconds. I think the square arc seconds takes care of focal length, magnification, depth of field, but perhaps not the dimentions of the ccd, nor the temperature of the ccd. Sorry it is complex. Eye ball at the eye peice is limited to about 17 magnetude even with very large telescopes. Computers can cancel the light polution, but produce stars that arn't there = artifacts at extreme cancelation. Neil

nexium · Jul 23, 2007

Perhaps that is lens diameter squared? Neil

umbralumina · Jul 23, 2007

Well in my backyard I have rather high light pollution, on a good night I can see with naked eye up to 4.5 without squinting, maybe even 5 if its a perfect night. With the binoculars I got (7x) I have seen up to mag 7 I think, I can see ptolemy's cluster barely, but the clusters around cygnus look really nice. The telescope I got is really bad (60mm), Its rather old and the tripod is a nightmare, its never stable. So I wanted to get something like a 5 inch telescope, I have my eye on Celestron PowerSeeker 127EQ. As for binoculars I would like either the Orion Explorer 25x100 or the Celestron SkyMaster 25X100. So I was wondering up to what magnitude I could see with either of these. I would like to see nebulas and maybe even some galaxies, and maybe neptune as well. With what i got, the andromeda galaxy seems like a lighter smudge on the sky, i cant really see anything, just because I know its in there is why i can notice the smudge hehe.

MeteorWayne · Jul 24, 2007

That is about right Andrew, unfortunately the standard conditions used to calculate a ZHR (LM +6.5) are very rare on the planet today. However, on a very clear night even here in NJ I can reach +6.0 occasionally. And my friend Lew, who has better eyes, might come very close to +6.5 under the same conditions. So much of it depend on the quality of the Electromagnetic receiver <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

MeteorWayne · Jul 24, 2007

Just thought I'd mention, Neptune is within reach of even the binoculars you have, if you know where to look. <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

3488 · Jul 24, 2007

I've nabbed Neptune in 10 x 50s before. Even last year, when Neptune is still pretty low as from my latitude. As MeteorWayne says, you MUST know exactly where to look & somehow prop your arms so that the binos are not wobbling around. I've also nabbed 1 Ceres & was able to track its movement with said binos. Below: general finder chaert for said outermost planet. Andrew Brown. <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

Saiph · Jul 29, 2007

actually meteor, Apparent magnitude is the correct term. You're thinking of absolute magnitude, which is a measure of the star's real luminosity. Anyway, I've done some digging about and found a few versions of the equation for "limiting magnitude" of various telescopes. There are several versions because there are lots of complicating factors (such as telescope design, and the user's pupil diameter). Here's one: ML= 3.7 + 2.5 * Log10(D2) from here <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

MeteorWayne · Jul 29, 2007

You are correct Saiph, I guess I had part of my brain switched off that day <img src="/images/icons/smile.gif" /> This month's S&T had a chart addressing this issue showing Mag +11 for 10x50 Binocs, and Mag +15.5 for a 10 inch (25 cm) scope, just to give some ballpark numbers. Wayne <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

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