Newbie - nebula filters and colour??

allsmiles (Ahmad) · #1 02-02-2009, 10:07 PM

Hi all,
I'm new to both this forum and amateur astronomy.
I recently bought an 8" inch skywatcher (collapsible) dob. however, other than bright objects i can't seem to find many DSOs. i'm guessing light pollution is the culprit.
I can, for example, see the m42 as a faint dust cloud without any hint of colour at all.
Would a UHC filter (i'm eyeing the baadar) allow me to see more detail and colour?? or is it the aperature on my scope??

Thanks

Alchemy (Clive) · #2 02-02-2009, 10:18 PM

a nebula filter will give more contrast, and the ability for more detail, particularly in the city areas.

the colors you see in pictures.... sorry its pretty much only for cameras, you may detect a blueish green tint but thats about it for me ... and i use a 12 inch scope.

erick (Eric) · #3 02-02-2009, 10:18 PM

For most people, it would seem that around 12" or more of aperture is required to pick up faint colour in M42.

If M42 is a "faint dust cloud" through your scope, you must be suffering from much light pollution. Try to get to somewhere 100km or more from the outskirts of Sydney and try again.

The DGM NPB filter gets good reviews for nebula
http://www.omegafiltersebuyer.com/se...ers/Categories
I have one and it is great on the Eta Carina nebula. Others say an OIII filter is good as well. But I only use it where there is no great light pollution. Perhaps something else would be better under your sky conditions.

renormalised (Carl) · #4 02-02-2009, 10:26 PM

You're not going to see much colour at all, in any scope except for maybe a dull grey-green, in any nebula. Your eyes just aren't sensitive enough to detect colours at such low light levels. However, people have seen some colour using rather large scopes...16" and larger. About the only thing you'll see good colour in is bright stars and the planets. One thing you will notice though, is that your UHC filter will help enormously by cutting out pretty much all the offensive wavelengths of light from the surrounds, namely streetlights and skyglow. That will allow you to see things you wouldn't normally see such as dimmer objects and details in nebula from an urban site. The Baader filter is a good one to choose. You also have Astronomik, Orion, Bintel, GSO, Astrodon and a raft of other companies that make LP filters.

Choice really depends on what you can afford and the quality you'll get for the price, but don't be put off by relatively cheaper filters. Many are of good quality and will serve you well. There are some exceptions. You'll pick up on what's best from listening to everyone here and asking for advice (also, when you can, join your local astronomical society...they're a wealth of info).

allsmiles (Ahmad) · #5 02-02-2009, 11:23 PM

Thanks a lot guys for the quick and informative replies.
I guess a filter would be good anyway
wow i really am i newbie then....i always thought you would be able to see colour under a dark sky...

How about galaxy viewing would that require a different filter altogether??

And why is it saturn appears almost as if it's too bright?...i cant seem to make out any detail (it seems pretty sharp, i can see the rings as a line and a couple of moons but that's about it)

sorry about the noob questions

Thanks again

renormalised (Carl) · #6 02-02-2009, 11:45 PM

Galaxy viewing...no....the UHC will allow you to see them as well, mainly by getting rid of the skyglow etc.

What size EP are you using to look at Saturn??. If it's a relatively low power (say a 26mm EP = x46 for an 8" dob), the reason why Saturn looks so bright is an effect of the FoV (field of view) and the intrinsic brightness of the planet. Bright planet against more dark sky in the FoV, the brighter the planet looks. It's an optical illusion. When you bump up the power (say a 9mm EP = x133), the brightness appears to decrease because you see more planet in the same FoV...less dark sky to see it against. The FoV and the brightness of the planet haven't changed, it's just the perspective in the eyepiece has.

allsmiles (Ahmad) · #7 03-02-2009, 12:01 AM

hey renormalised,

i have a 26mm, 10mm and a wide-angle 4mm on a scope of 1200mm focal length. i think i should get something in between, like a 6-7mm, will that help?? the 4mm makes it look a bit fuzzy so i think that should be reserved for darker skies right? (i did a star test and i think my collimation is fine)

thanks

renormalised (Carl) · #8 03-02-2009, 12:20 AM

Not necessarily....you have a reasonable spread of EP's there but if you were going to get another EP, I'd look at acquiring something in the 15-20mm mark....then you'd have EP's with x46 (26mm), x80 (15mm), x120 (10mm) and x300 (4mm). Since your max magnification (under ideal conditions) is x480, I wouldn't look at getting anything smaller than the 4mm you already have. Too much trouble when the exit pupil gets so small. You should only use the 4mm when it's ideal seeing as anything will ruin your night more than using high power when it's not warranted or not really ideal to do so. You have an f/6 system (FL/Ap...i.e. Focal length divided by apeture), so your scope will be somewhat forgiving of poor seeing with a reasonably high powered EP in average conditions, but it's still fast enough to cause problems if you bump the mag up too far for the conditions you're in. You start to see things like tube currents, wind shivers, atmospheric turbulence etc, when you get too ambitious with the mag'.

renormalised (Carl) · #9 03-02-2009, 12:22 AM

BTW....if you still want more choices of mag', get yourself a x2 barlow lens. That'll double the mag of each of your EP's. At x600 for the 4mm piece, that'll be almost unworkable unless you got that 1 in a million night of absolutely perfect seeing conditions. But your other EP's will be fine.

allsmiles (Ahmad) · #10 03-02-2009, 12:46 AM

Thanks for all the help renormalised.

renormalised (Carl) · #11 03-02-2009, 12:51 AM

Quote:

Originally Posted by allsmiles

Thanks for all the help renormalised.

No problem....that's what we're here for. To help

Geoff45 (Geoff) · #12 12-02-2009, 01:11 PM

Quote:

Originally Posted by Alchemy

the colors you see in pictures.... sorry its pretty much only for cameras, you may detect a blueish green tint but thats about it for me ... and i use a 12 inch scope.

A good way to demonstrate this is to go outside on a dark night and pick out a tree. You can't see any colour. However, the colour is still there. If you take a 15-30 sec photo, you'll see the green in the image.
Geoff