HA filter assistance please NM??

[BlackWidow (Mardy)]

OK I did post about filters a few nights ago and think I have made a choice that I need to purchase a Hydrogen Alpha CCD filter like the Baader of Astronomik.

My question is about nm ratings on the filter. The Baader is a 35nm and the Astronomik is around 7 nm. I have also seen other values.

I have a 10"LX200 at f:6 and am not sure what rating would suit my scope the best. Does the higher number give me shorter exposure times or longer?

I am using a QHY8 colour so I will have longer exposures to start with. I want to be able to mix HA shots with RGB and then Colour Balance stars etc to get better Contrast and detail in some of my shots...


Regards
Mardy

[RickS (Rick)]

Hydrogen Alpha emission occurs at a very specific wavelength (656 nanometres.) The bandwidth of a filter tells you how narrow a section of the spectrum it will pass. If you had a 12nm Ha filter then it would pass light with a wavelength between 650nm and 662nm. As well as the Ha, it will pass light which is not from Ha emission but which is close in wavelength. By comparison, a 6nm filter will pass light with a wavelength between 653nm and 659nm. You'll still get some non-Ha light, but less than with the wider filter.

Generally, the narrower the better. Unfortunately, narrower filters are usually more expensive. The one caveat is that for very fast optical systems narrow filters may cause problems due to bandwidth shift, but this won't be an issue for you at f/6.

The bandwidth of the filter won't affect exposure time appreciably. If you get a shorter exposure time with a wider filter it's because you're collecting light that's not part of the signal you want.

Cheers,
Rick.

[Merlin66 (Ken)]

The only other benefit a wider bandwidth gives, is that all that non-Ha light contributes to making stars a bit more visible! helps when trying to focus.

[Poita (Peter)]

I can tell you from experience that the 35nm is a total waste of time on the QHY8.
The 7nm is the minimum you can get away with, and the 3nm is the ducks nuts, but exposure times get very long.
Baader also make a 7nm filter
http://www.ebay.com.au/itm/Baader-2-...item19ccbb70c7

[BlackWidow (Mardy)]

Thanks all for your help. I am going to go for the Baader 7nm as per the link.. All you have told me adds up and it has saved me from purchasing the wrong filter. I was going to go for a UHC and then I thought the HA 35nm.. This stuff is expensive, and it's good to get it right the first time. I have lots of spare gear allready LOL

Thank you!
Mardy

[Poita (Peter)]

There is a really good article on filters on the mallincam group, I'll dig out the link.
http://tech.groups.yahoo.com/group/m...cal%20Filters/

It is for video and visual work, but still well worth a read.

You may have to sign up to read them, I'm not sure.

[Poita (Peter)]

Remember that you are also effectively reducing the resolution considerably when imaging with a Ha filter and a colour bayer CCD.

It still gets great results, but something to be aware of.

[BlackWidow (Mardy)]

Thanks Peter for the link. I am a yahoo member, but I am not able to get into this link. Been going around in loops for the last hour LOL.. Never mind. I do thank you for your assistance.


Regards
Martin

[BlackWidow (Mardy)]

Update! I got into the site... Thanks