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Dobs and Light Pollution

Started by splathrop, 07/20/2003 01:46AM
Posted 07/20/2003 01:46AM Opening Post
I am thinking about supplementing my 4-inch APO with some kind of Dob, to better observe galaxies. At my location the best conditions just barely disclose the milky way. A casual observer would never notice it.

Although I can locate quite a few galaxies with the 4-inch, I can't really observe most of them. Based on what I read here, I assume a dob of some size will make a big difference on those. Is a 12.5 Starmaster, or similar, going to be big enough to make a major difference on these targets under these conditions?

Taking it the next step: I can't see M101 at all unless I go to a dark site. Is that likely because my sky background is brighter than anything in M101, and therefore a dob wouldn't see it either, or might a 12.5-inch dob actually reveal galaxies that are totally invisible to my 4-inch because of light pollution? (I know the dob would show galaxies that are too faint for the 4-inch under dark skies, but this is a different question, focusing specifically on faint objects being totally washed out by light pollution).

Finally, please notice that these are not questions about whether aperture wins, or wins under light polluted skies, or wins when you can stand an egg on end during the equinox. I stipulate to all that. I just hope some of you can maybe provide specific answers to the questions above.

Thanks for any help,
Steve
Posted 07/20/2003 02:17AM #1
I have a 5" AP and once complimented it with a Discovery 12.5" DOB primarily for deep sky. The Milky way is barely visible from where I live, but there still was was a huge differece in the brightness of deep sky objects for clusters and nebula. To be honest, galaxies where not that impressive from my point of view.. and your skies (by your description) appear to be slightly more light polluted than mine. There's an interesting article in the August Sky and Telescope called Dark Skies Rule explaining aperture relative to contrast in both urban and dark sky settings.


Rob
Posted 07/20/2003 03:00AM #2
Hi Steve,

Your skies sounds exactly like mine, and I wondered the same thing. We just purchased (about 6 weeks ago) a 10 inch dob and it did make a big difference. Although I can't see detail in galaxies unless were at a dark site, I can see the cores and they are very noticable. I'm waiting for the Sombero to be up because we did see the dark lane in our six inch in our backyard so I'm sure the 10 will show it really well. Hope that info. helps and clear dark skies to ya!
Ron
Posted 07/20/2003 03:24AM #3
You will get significant DSO improvement with a Starmaster 12.5 ELT compared to the 4" apo under mag 4 skies. A dark site is always desirable, and you will still be challenged with finding the objects with light pollution while star hopping. Digital setting circles can ease the burden (Sky Commander, Argo Navis, NGC, etc.).

The SM 12.5 isn't bothered as much by reduced light transmission from broadband filters, so they can help. This scope is a quick setup or roll out and keeps your feet on the ground.

You didn't ask about GoTo, but there are other 10" - 12" options that can take the pain out of finding DSOs for the price of a new 12.5 ELT. But they won't have a CZ mirror.

Posted 07/20/2003 03:51AM #4
I would suggest you buy as big an aperature as you can afford and move with your car and body. There are a lot of inexpensive nice scope to choose from today, both new and used. Don't buy the biggest one for sale unless you can carry it around with what you have. Don't buy the cheapest one because of its price. Choose quality and a size (Aperture and dimensions) that you can deal with and something that you like the look it provides.
My humble opinion.
Floyd

[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue grin
Amateur Imager
Posted 07/20/2003 04:07AM #5
Something else to consider: if you have a big dob, you can get further with a light pollution filter. Since such a filter works by filtering out certain wavelengths, it will necessarily dim galaxies too (since those objects gan their brightness from the full visible spectrum), but if you're sucking in more light with a big dob then you have more light to 'waste' in the filtering process.
Posted 07/20/2003 04:19AM #6
Hello there Steve,

Well, though I'm a newbie/tyro, I have spent a great deal of time with galaxies through the same light cup as yours, the TV-102 ;-). My sky sounds like it's darker than yours (between LM=5.5 - 6.0). A better metric to determine how dark (or bright :-() your sky is, is to see what's your naked eye limiting magnitude near the object you want to see, rather using the Milky Way. There are widely varying degree of light pollutions ;-).

Having said this, I can see M101 just fine from my backyard during moonless nights when it's near the zenith. But I don't see any spiral of course :-(. M101 is very large and thus have a low surface brightness. Thus, surface brightness is one of the key factor whether you'll see the galaxy or not. Galaxies that have bright core and/or high surface brightness is much easier to detect and/or see.

Having said this, it all depends on the galaxy and the type of galaxy (core vs coreless like a dwarf). I don't have experience viewing from light polluted city (except M51 ;-) but I would estimate that aperture will definitely help under this condition. However, with coreless galaxy like the dwarf having very low surface brightness like Barnard's Galaxy (NGC 6822), I'm guessing that even very large aperture will have problem with this one in the middle of the city. In order to see this type of galaxies, you need *contrast*. Because the sky background looks gray (some says orange ;-) under polluted sky and the galaxy itself looks gray, there may not be enough contrast in the city regardless of aperture! Incidentally, I couldn't see this galaxy from my backyard either, but it was quite easy under a really dark mountain sky (LM=6.5 that night) with my 4" TV-102 Light Cup at low power. Perhaps, the guys with the 12.5", 15", 16"-20" light bucket can provide objective inputs as to the visibility of the coreless and/or low surface brightness galaxies from the city.

One last example, a fellow I know could never see M51 through his TV-85 under his LM=2-3 city sky, horrible I know :-(. After he got his 12.5" Portaball, he had no problem seeing it ;-), albeit not spectacular by any mean.

Oh, I like to add that you may wish to prose your question on SAA as well as Tony Flanders participates there. He has a great deal of experience observing DSOs from the city with various apertures. Here's his web site also.
http://mysite.verizon.net/vze55p46/index.html

Good luck on your quest. Personally for me, it's good to have one small scope like our TV-102 and one large scope. One thing you might want to think about. Galaxies always look better under dark skies; so portability may still be a factor if you want to astro-commute with it.

Ron B[ee]