Do I need filters for nebulosity ?

[AlistairW]

Last night I randomly chose to have a go at the Pacman Nebula. I was actually honing my polar alignment, but had a bit of time spare so took 12 iso 800 5 minutes lights and the same for darks. Stacked in DSS and loaded in Photoshop. After a bit of a play on the curves, I could 'just about' see some nebulosity, but it was so faint that if you did not know it was there then you would miss it.

So is this where I should be using filters, Ha, O3, SII for example ?

I have a decent DSLR that has been astromodded and is peltier cooled and I am not ready yet to go CCD and narrowband, as I am still learning.

Is there a cheaper set of filters I can look at getting for my William Optics 81 and DSLR that isn't going to break the bank like a set of Astrodons. I know there are the Astronomik clip filters I could use, but maybe I just get the 'normal' glass filters. Do they even come up second hand ? - not sure I know the difference from a 7nm and a 12nm, apart from the wavelength of light they pass, - which is generally more useful ?

Anyway my Polar Alignment seems much better, as the PHD graph is smooth, and the stars I have no longer appear egg shaped, - so I am happy with what I set out to do :-)

Thanks

Alistair.

[wxsatuser]

You have an astromodded cam anyway, so I would up the exposure but don't over expose.

Where abouts was the histogram peak on the in camera histogram.?

[D4N]

You should see plenty with that setup.

The GT81 is f5.9 isn't it?

My scope is f6.78 and using 10 x 5 min exposures with a full spectrum modded DSLR I got this:

BUT, processing is also important, this is the exact same data that I processed differently:

I used a UHC-S filter during capture, I have to otherwise I get red bloat on the stars.

The only reason I could think of you not getting the nebula clearly is light pollution, either the moon or man made.

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[Dave In Vermont]

The short answer is: No - you don't need a filter to see these DSO's (deep-space objects), but a filter can be quite beneficial in teasing out a great deal of details. The thing to know is: Which filter would be best? You can experiment and try a few different filters, such as a UHC-S. And you can read about other's findings with different filters:

http://www.prairieastronomyclub.org/resources/by-dave-knisely/filter-performance-comparisons-for-some-common-nebulae/

This is written by David Knisely of the Prairie Astronomy Club, who is highly regarded in the world of astronomical-filters.

Clear & Dark Skies -

Dave

[AlistairW]

[Thanks for the replies, and yes I do have a bit of light pollution, but I would be interested in how bad you think it is. I live in the country in Somerset, and I can often see the mikyway band. However I have new neighbours that have yet to put curtains up, so light from their window does flood on to my garden (scope) at the moment. I was imaging last night whilst they had their light on, but that's just the way it is sometimes. I really don't know anything about filters, but if I was to use a Ha filter then I expect I should be able to get more detail. I just sent 10 mins in Photoshop and got the following :

You can vaguely make it out I think, but I don't know if that just not enough exposure, or bad processing or both. Anyway I am still happy with how my alignment went and getting Astrotortilla up and running.

[AlistairW]

Dave, that's a great article by the way seems the UHC filter get good reports.

[D4N]

If you use a Ha filter you will add quite a bit to your processing workflow as the Ha will need to be merged with the RGB to create a colour image, unless you are happy to go monochrome. Remember that the Ha won't actually speed up the image acquisition it will just allow you to take longer exposures.

UHC certainly helps in that respect too but I suspect something else is wrong as your stars are really dark, most of them are grey. How are you using levels and curves?

Do the raw light frames look bright or dark when viewed before you process them?

Nailing alignment and plate solving is great though, makes everything else so much easier.

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[Dave In Vermont]

Ha filters are one of the most expensive filters out there. And you should know - they really only work their magic with images - through astrophotography. As you are just starting out, and you are basically learning your way, I would forgo spending that kind of money on any filters. Unless you wish to spend, say, £200 on a filter and then - due to how much £ you spent on said filter - feel you must now go into astrophotography and are ready to plunk down a £1000+ on gear.

Own the filter - don't let the filter own YOU!

For future reference, most people would suggect one start out with either a UHC or a OIII filter - after doing your reading-up on these things and the objects they are good for elucidating more details out of.

You're quite welcome for the article. David has written a few more excellent ones as well. You can Google his name and likely find them that way. A member here asked him if he minded having his articles published on the internet in astronomy-forums. He said it was fine - as long as he is named and credit is given to him. Read: He's a nice guy!

Thank you David Knisely -

Dave