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Convert sky forecasts into max magnification?

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#1 MRascal

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Posted 25 November 2012 - 12:14 PM

Hello all. After many years of hoping Santa Claus would bring me a telescope, I've decided to be a little more proactive! I've done a lot of research over the last week or so on the many aspects of telescope aided sky gazing. To this end, the various forums of Cloudy Nights have provided invaluable information. Thanks!

At first I thought the hardest part of this would be choosing a telescope. Then I realized that choosing a mount might be harder, and now I see that choosing eyepieces is even harder still.

OK, on to my question. I'm currently planning on purchasing a C11 EdgeHD OTA. I've been trying to frame the upper magnification limit that this scope could support under normal viewing conditions, so that I can make wise choices when buying EPs. From reading various posts it appears that the atmosphere will place the limit on this scope for most nights (also the viewing target plays a role).

I've found the website cleardarksky which provides seeing forecasts and I was wondering can one turn their various colored squares into a rough max supported magnification?

Barring that, does anyone from suburbian Denver have a number they'd be willing to throw out?

Once again thanks for all of the useful information contained within this site.

#2 frito

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Posted 25 November 2012 - 12:36 PM

you mean the clear sky chart like in my sig? its more of a guide really because its a prediction. in my experience its best to just go out and check out the seeing for yourself with a telescope on the moon or jupiter etc. as a general rule of thumb if you want to use high mag you need to wait for the object to get high up in the sky. the lower it is and closer to the horizon the less it matters what the overhead seeing conditions are because your looking through so much atmosphere that the seeing will never get very good.

just last night we were predicted to have average seeing. when jupiter was lower on the horizon, about 20 to 30 degrees up the detail was poor i could just barely make out the dip in the band the GRS is in.
i semi packed up but left the scope out and covered to prevent dew. watched a 2 hour movie then went back out when jupiter was closer to 40 degrees up now and the seeing was better there. i knew it would be because the moon was in the same area before it and the seeing was pretty good on the moon earlier that night. with planets like jupiter you still need to wait for those moments of clarity esp if you run fairly high power on it but when they come they can be breathtaking in detail seen.

oh and its worth noting that only one time has the clear sky chart said i was going to have 5/5 seeing and it was actually pretty close to right. it said we were going to have 5/5 a few times in the past 3 days and its really been more like 3 or 4 out of 5 IMO. its a prediction not reality.

#3 MRascal

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Posted 25 November 2012 - 01:53 PM

Frito, thanks for the quick response. Yes I mean exactly like the clear sky chart in your sig. I realize that these are forecasts, and I didnt expect them to be better than normal weather forecasts, but I was hoping that they were good on the average.

I've seen several "rule of thumb" formulas for computing the max magnification a scope can handle (eg. 50xaperature rule on Chuck Hawks website). For an 11" scope, this would be 550x magnification. But then I see many postings in the forums by people saying that they only get Y magnification in practice due to the atmosphere.

I've noticed a few things about these statements:
a. Y is almost always significantly less than 550x
b. Y varies with location

So my plan to answer this question for my locality was to average many nights (months perhaps) of clear sky chart data together and then convert that to a magnification limit imposed by the atmosphere. And that in turn should provide a lower limit on EP focal lengths that I should consider. But I have not seen a way to convert the seeing scale to a limit.

In your case, when you get a night that you consider 3/5, what magnifications can you use? Similarly when you believe you've got a 4/5 or a 5/5 night, what magnifications are useable?

#4 Eric63

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Posted 25 November 2012 - 02:18 PM

What I have found so far is that with average seeing a planet like Jupiter can only take around 150X with my 127Mak. With good seeing I can push 170X to 190X. I have had better than good seeing once (but not excellent) and took my scope to 225X. It was actually quite nice. I am still wating for that evening of perfect seeing to see how it will be at 250X or more.

As for the moon, even with average seeing I take it to 200X or more and it holds up well. I also found earlier in the year that Saturn can take a bit more magnification than Jupiter.

I hope this helps.

Eric

#5 MRascal

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Posted 25 November 2012 - 02:40 PM

All responses help. Thanks.

#6 MRascal

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Posted 25 November 2012 - 02:44 PM

Eric,

Are you able to relate your magnifications to the clear sky forecast for your area?

#7 Eric63

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Posted 25 November 2012 - 02:54 PM

Actually in my area I have found the forecast to be quite accurate. Keep in mind that the forecast is aimed at larger scopes (11 inch or so) and so if you have a smaller scope (like me) you may find the seeing a bit better. Often it shows a 2/5 and I find that it more like a 3/5, and some 3 are more like a 4. When I see a 4 I know that I will have a bit better than that, but not a 5. Also, keep in mind that you may have pockets of better seeing. A few weeks ago I went out when it showed a 3/5 and for 20 or so minutes I had 4+.

Another good indication for me is how much the stars are twinkling. If they are barely twinkling I know it will be a good night of seeing.

Eric

#8 Eric63

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Posted 25 November 2012 - 03:02 PM

This link may also help

http://www.weatherof...o/seeing_e.html

Eric

#9 Seldom

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Posted 25 November 2012 - 03:04 PM

For Jupiter and the moon, I see more detail if I dim down the objects by masking the end of my scope. I drape a piece of sheet metal flashing over my 6" Newt to cut the moon down by half, and put a towel over the dob, to do the same thing for Jupiter. Re twinkling, I saw Sirius low on the horizin the other night flashing red and blue.

#10 frito

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Posted 25 November 2012 - 04:18 PM

yeah star twinkle is a good indicator of really bad seeing. if i see stars up at zenith twinkling like crazy and i have before i know its going to be a bad night for high magnification :) ones down by the horizon pretty much always will at least in my area from what i've seen and that just goes back to whole your looking through many times more atmosphere at that angle thing.

to the OP with my 8" on average nights say 3/5 seeing prediction nights i can usually push my scope to 160x. on better than average nights 190x and on the best nights i've got so far it in the 240x area. my highest mags i can get are using a barlow and that would be 320x and 380x but i've never had seeing good enough to get clean images at those high magnifications and don't expect i probably will anytime soon either.

#11 Tony Flanders

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Posted 25 November 2012 - 04:32 PM

I will admit that I hardly ever look at the seeing forecast. Being primarily a deep-sky observer, seeing is invariably secondary to transparency for my purposes. In practice, I just go out and use the highest magnification that I find useful.

Anywhere can have superb seeing and anywhere can have terrible seeing. The only thing that differs is the frequency.

You might as well have an eyepiece -- or more likely an eyepiece-Barlow combination -- that delivers 50X per inch of aperture or higher for your telescope. Sooner or later you will want to use it.

But as far as I'm concerned, magnifications over 40X per inch are pretty specialized -- really useful only for splitting double stars. For planets I generally prefer 30X to 40X per inch of aperture when the seeing is excellent. Most (but not all) serious planetary observers seem to agree.

Magnification is a very personal thing. Remember, the only point of magnification is to overcome the limitations of your eyes. All the information in the image is there at every magnification. If our vision was perfect, we wouldn't need high magnifications. However, nobody has anything close to perfect vision.

Looked at that way, when people boast of using ultrahigh magnifications, what they're really stating is that their vision is unusually poor.

On my 12.5-inch Dob, my standard eyepieces deliver 59X, 114X, 151X, 227X, and 305X. Most often, I end up using 227X both for deep-sky objects and planets, due to the limitations of the atmosphere. (227X is ideal for many DSOs even in perfect seeing.)

On good nights, I often use 305X. And on really good nights, I use my Barlow for higher magnifications. Such good seeing doesn't happen nearly often enough to have a dedicated eyepiece devoted to it. But a Barlow is always useful, and its image degradation is minimal. So it's tailor-made for those rare occasions when I want outrageously high magnifications.

This is for the northern half of the U.S., where the jet stream is usually overhead. In the southern U.S., good seeing is much more common.

Hope that helps.

#12 panhard

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Posted 25 November 2012 - 04:41 PM

Tony you covered the bases quite well. :waytogo:

#13 frito

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Posted 25 November 2012 - 04:48 PM

great explanation Tony, explains very well why i usually end up using a little less mag than i can on Jupiter because my vision is pretty good still being a young 29 yo that has yet to need glasses. i always want to try to push the mag for some reason though hoping it brings out more detail.

#14 Jon Isaacs

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Posted 25 November 2012 - 09:04 PM

Looked at that way, when people boast of using ultrahigh magnifications, what they're really stating is that their vision is unusually poor.



I agree with what Tony has said, don't plan on using it every night but it's smart to have a combination that provide very high magnifications, if it's only one night in the year when the seeing is under 1 arc-second, that's still the best night of the year and you don't want to miss out for the lack of an eyepiece.

I think there are particular objects that support very high magnifications but in general, lower is better than higher... even when the seeing is quite good, for most stuff, 200-250 is reasonable. But some small but bright planetary nebulae reveal additional details at high (50X/in and greater) magnifications as do double-stars that are near the theoretical limits of a particular aperture.

Jon

#15 TexasRed

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Posted 26 November 2012 - 02:22 AM

That 50X/inch rule is mostly just for splitting very close double stars on perfect nights. I think you might do better planning for eyepieces that will give you an exit pupil of about 0.5mm when used with your Barlow and one that will give you an exit pupil of about 2mm.






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