FILTERS.

Hi all,

Just wonderin if anyone could enlighten me on the use of filters.

I have yellow, green red blue orange and moon filters and have really no idea what they are for..(apart from the moon filter... obviously!!:laugh:)

what filters improve what targets? what are H-ALPHA and OIII.

Are any of the filters i have any use in astrophotography?

Just one other question... What does the term "binning" refer to in astrophotography? And wat does LRGB mean?

PHEW!!!!


THANKS ALL

Filters, generally, cut out or prevent the transmission of certain parts of the light spectrum.

For visual use, this would tend to either dim bright objects (e.g. the moon) to allow for prolonged visual observing. In the case of planets, certain filters enhance surface details (such as on Mars). For nebulas, some filters enhance nebulosity.

For photographic use, filters are typically used to either cut out light pollution or enhance parts of the wavelength to which your imager responds (eg OIII, SIII, Ha etc). Other filters are used to standardise photometry (UBVRI Cousins/Bessell filters). Another technique is that with a monochrome camera and the use of an RGB filter set, you can recreate a colour image.

The filters you would choose would really depend on what you want to do. Judging solely on your post, I'd imagine the filters you have are for visual use. They *might* be OK for your first foray into RGB astrophotography but there are others here that could probably give you a more definitive answer on that point.

I think I've explained RGB - the additional L stands for luminance - you can either take an image with a luminance filter or just take an image with no filter. Pretty much the same thing.

Binning refers to a technique whereby the CCD imager you are using reads the pixels. 1 x 1 binning means each pixel is reported as single value. If you had 400 pixels, you'd have 400 values. Binning 2 * 2 means four pixels in a 2 by 2 square are averaged out and only one reading is reported. For 400 pixels, you'd have 100 values reported. Why and when you would use binning depends on what you're trying to achieve.

Apologies if my definitions are clumsy and I'm prepared to be corrected by far more technical folks than I. But I think I've given you a reasonable overview.

If you're interested in imaging, there's plenty of folks here that will help out. But I'd suggest getting an astrophotography book (just google it) to get a better sense of what you might be letting yourself in for...

Hope that helps.

Dave

For visual astronomy at night, there are two main types of filters, ones for planets and ones for deep sky objects. Filters for planets are generally just coloured glass. The colours help to improve contrast on some planets. A guide to which colours work well on which planets is here:
www.telescope-service.com/filters/start/...ersstart.html#colour

As for deep sky, these filters are much more complex. Deep sky objects are broken into three categories:
1. Clusters
2. Galaxies
3. Nebulae (clouds of gas)

Clusters are groups of stars which shine light across a broad range of the visual spectrum. Filters don't really help here.
Ditto with galaxies.
Nebulae are different. They shine a significant proportion of their light at certain wavelengths. This is due to the excitation of molecules in the gas. Examples are H-Alpha, OIII (pronounced "Oxygen 3" ), etc. Deep sky filters are ones which are designed to maximise the throughput of light to the eye at these specific wavelengths but block other light. As a result, they are much harder to design and manufacturer, hence they are much more expensive than simple coloured glass filters. While the total amount of light entering the eye is reduced slightly with these deep sky filters, the contrast is significantly improved.
The wavelength of H-Alpha = 6562.8 Angstroms
H-Beta = 4861.3 angstroms
There are two Oxygen III lines: 5006.9 Angstrom and 4958.9 angstrom
Three SII (pronounced "Sulphur 2) lines 4072.3, 6718.3 and 6732.7 angstroms.

I should also mention that there are also Light Pollution filters. As the name suggest, these filter out the wavelengths of light from street lamps Sounds good in theory but I have never read rave reviews of them and few people I know actually use them.

In terms of a deep sky filter to buy for visual astronomy: the best all-round deep sky filter to get IMHO is the UHC filter. Why? - it works well on nearly all nebulae as it lets in light for both H-Alpha and OIII and blocks the rest. More about them can be read here. www.gerdneumann.net/v2/english/eng_astronomik_uhc-e.html

Hope this helps.

Michael.