Ha filter question

[RobertI]

Having seen the excellent results that Don and others in this forum are getting with Ha filters, I am thinking of getting a 7nm Ha filter myself for use with my Lodestar. I am thinking this would also reduce the light pollution from nearby Sodium lights.

The problem is that these Sodium lights are of the high-pressure variety and I believe they have a broader spectrum - they certainly look 'whiter' than the standard low-pressure street lamp, although still orange. If I hold a standard Skywatcher light pollution filter up to a standard low pressure light it is significantly reduced in brightness but if I do the same with the nearby high-pressure light it makes very little difference. 

My question is would an Ha filter be effective in reducing the broader spectrum of high pressure Sodium Lights? Also do they also work for Mercury lights?

Any comments appreciated. Thanks.

Rob

[nightvision]

Useful info that may help:

http://www.asdi.com/getmedia/05be9959-ebac-4251-a1ce-1537b5e7b743/SPECTRAL-SIGNATURES-OF-NIGHTTIME-LIGHTS.pdf.aspx

https://en.wikipedia.org/wiki/Sodium-vapor_lamp#/media/File:High_Pressure_Sodium_Lamp_Spectrum.jpg

[HiloDon]

Hi Rob,

The NB Ha filters block all light not in that narrow band around the 656.3nm line. Based on Tony's reference above (thanks, Tony), HP Sodium Vapor lighting does not generate light in this band, so you should be good to go. The typical light pollution filters have much broader transmission and aren't nearly as effective as a NB filter. Even most moonlight is cut with the NB Ha filter. You also see a great reduction in number and size of stars. One downside on the NB Ha filter is that they are expensive, but well worth the cost in my opinion. Remember, they only work for emission nebulae that generate light at that Ha line. There are a lot of them though and they are quite something to see.

Don

[nightvision]

Using filters:

http://karmalimbo.com/aro/filters/filters-useD.htm

Match this excellent paper and its charts (which cover video astronomy and various target types) with the street light data above and you should be able to work out what you need.  

http://karmalimbo.com/aro/reports/paper_MethodForEvaluatingFilters-part1.pdf

http://karmalimbo.com/aro/reports/paper_MethodForEvaluatingFilters-part2.pdf

And here for even more stuff on filters and spectral related stuff:

http://karmalimbo.com/aro/

Many hours of reading! 

[RobertI]

Thanks for the excellent links Tony and the information Don. I think its now clear that Ha will block my troublesome lights so I shall be planning a purchase soon! Thanks again.

[ronin]

Is a Ha filter applicable to video work?

It will block sodium and mercury and everything except the 656nm wavelength and a small bit either side.

Not sure what the intended use is, but not sure why you would want use what is a narrow band filter on a video setup.

[HiloDon]

Is a Ha filter applicable to video work?

It will block sodium and mercury and everything except the 656nm wavelength and a small bit either side.

Not sure what the intended use is, but not sure why you would want use what is a narrow band filter on a video setup.

If the video cam is used for near real time viewing of DSO's because of it's low light capability, then it would be applicable. It will only work for emission nebulae like M16, 17 and 8. M20 is a good example of one with part emission and part reflection. Here's an example of captures of M16 without and with a NB Ha filter. These were taken with a SX Lodestar X2, but you would get similar results with a high sensitivity video cam with long exposure capability.

Don

[RobertI]

My next question is the best way to fit the filter? I cannot see a way to fit the filter inside the 1.25" draw-tube , but there is i believe a 'C thread' at the sensor end of the lodestar, do i need a C thread to 1.25" converter of some sort?

Thanks

Rob

[Paul81]

Yep you need one of these

http://www.firstlightoptics.com/all-accessories/low-profile-125-nosepiece-c-threads.html

Screws onto the Lodestar and at the other end you can add a standard 1.25" filter.

[RobertI]

Great stuff cheers Paul.

[Dom543]

An additional benefit of H-alpha or other narrowband filters is that they reduce or eliminate star bloat. This is particularly important for those using inexpensive achromats or old photo lenses. That type of optics cannot focus all colors of white light to the same point. As a result, even after careful focusing, images of the stars on the sensor are disks rather than dots. They then get overexposed, while we are waiting to collect enough photons from the faint DSO's, adding to their bloat.

Star bloat is particularly disturbing in the case of nebulas that are in the plane of the Milky Way. Less of a problem for galaxies that we often see in directions perpendicular to the plane of the Milky Way and that can be isolated from star-rich areas.

Clear Skies!

--Dom

[Macavity]

Just pushed the button on a Baader 7nm H-alpha filter. Was considering the merit of the 35nm version

(not least the cheaper price!) but decided to "go for it". Looking at transmission curves, the wider one

has an H-alpha profile very like my UHC-S filter. I had considered stacking the latter with a long-pass

RED filter to get a rough preview, but no longer have a general set of (Wratten) colour filters (doh!)

But something one might TRY, if one does? Idem combining a UHC-S filter with green/blue for O(III).

One day (night), I'll get round to testing this sort of thing. But looks like rain until mid-August here...

Aside: The idea of bi-colour: H-Alpha + O(III) mapped to red/blue (+synth green?) seems interesting.

(Feeling I need a bit more colour in my monochrome astronomical life!)

[Dom543]

Chris / Macavity,

You made the right choice. Before I bought my first NB filter, I asked around and got useful input on this thread http://stargazerslounge.com/gallery/member/26379-dom543/. The short summary is that the narrower bandwidth has several advantages, especially to fight light pollution and moon glow.

Multi-color using filter swapping does work with color cameras. I have some examples on this thread http://stargazerslounge.com/topic/239385-multi-colour-narrowband-in-lodestarlive/ and most multi-color images in my gallery http://stargazerslounge.com/gallery/member/26379-dom543/ were made using this technique.

An added advantage of the Baader filters is that they are parfocal (within the same type family). This eliminates the chore of refocusing after each filter change.

By the way, if you switch from one filter to two filters at the same time, you double the length of the path the light travels in glass and consequently, you have to refocus.

Clear Skies!

--Dom

[RobertI]

Dom, your first link seems to be to your gallery rather than the thread you mention, looks like an interesting link, can you post it again? Thanks.

Very nice gallery by the way, and a useful resource for seeing what's achievable.

Rob

[Dom543]

Thank you Rob!

Sorry for the bad link. Here is the correct link to the CN thread about the NB filters. http://www.cloudynights.com/topic/485764-h-alpha-filter-bandwidth-for-fast-optics/ I found the input by Samir Karushi particularly informative, as it is based on experience with both narrow and mid-band filters.

Cheers!

--Dom

[HiloDon]

Just pushed the button on a Baader 7nm H-alpha filter. Was considering the merit of the 35nm version

(not least the cheaper price!) but decided to "go for it". Looking at transmission curves, the wider one

has an H-alpha profile very like my UHC-S filter. I had considered stacking the latter with a long-pass

RED filter to get a rough preview, but no longer have a general set of (Wratten) colour filters (doh!)

But something one might TRY, if one does? Idem combining a UHC-S filter with green/blue for O(III).

One day (night), I'll get round to testing this sort of thing. But looks like rain until mid-August here...

Aside: The idea of bi-colour: H-Alpha + O(III) mapped to red/blue (+synth green?) seems interesting.

(Feeling I need a bit more colour in my monochrome astronomical life!)

Chris,

I agree with Dom, you made the right choice. I looked at creating a poor man's (or woman's) NB Ha filer with a Baader IR cut filter and a Wratten #25 red filter. But the resultant bandwidth was about 80nm, hardly narrow. But, maybe someone will give it a try.

Your mono results are great, and I still prefer it over color. I think you'll find much more detail with the mono without the Bayer mask blocking some of the channels with the Ha filter. But, the color does have a certain appeal. Can't do without it in outreach. A lot more oohs and aahs.

Please post some results if your skies ever clear.

Don

[Martin Meredith]

Yep you need one of these

http://www.firstlightoptics.com/all-accessories/low-profile-125-nosepiece-c-threads.html

Screws onto the Lodestar and at the other end you can add a standard 1.25" filter.

Hi Paul & Rob (& others)

H-alpha beckons here too…. I just to wanted to check: I'm assuming that with this nosepiece in place the Lodestar is still the same diameter all along its length (i.e. no problems sliding it up and down the focuser)?

I'm wondering too about other solutions for multiple filters with the Lodestar. Has anyone used a manual filter wheel (like the first one on this page) with the Lodestar? The only issue I can see is possibly losing the handy property of reaching focus easily on my Newt via sliding town the focuser tube. I guess its going to need an adaptor at each end of the filter wheel to achieve the same flexibility.

Any advice appreciated!

Martin

[Paul81]

Hi Martin,

Yep the gauge of the adapter is in-line with the lodestar camera, and with the filter attached you can slide the lodestar up and down the focuser (i.e. nothing sticks out). I have been using a CLS filter on mine no issues.

I can't quite remember if mine is from FLO or modern astronomy - but I think they are all the same.

With regards to filter wheels, I am planning on getting a filter wheel too - especially as I am currently working modifying LL for multi spectrum stacking (mainly for NB filters) - its the natural next step for EAA (especially as I use multiple filters when visual observing).

Paul

[PeterW]

Definitely my favourite filer, though I have a 12nm one. I stack it with a deep red filter to help reduce the streetlights further... The more attenuation the better.

Especially at this time of year when there are loads of nebulae to go after!

Cheers

Peter

[Macavity]

I suppose it depends on what you want to do? Since I only plan on changing

filters manually (randomly? lol) I decided to go for a TS filter drawer system:

http://www.teleskop-express.de/shop/product_info.php/info/p2165_Filter-Quick-Changer-incl--1x-1-25--filter-drawer---low-profile.html

Compact? T2 (M/F) threads... adds only 15mm to the optical path etc. etc. 

On the scope side, I have a (posh) Baader 2" barrel, to fit all my focussers:

http://www.teleskop-express.de/shop/product_info.php/info/p481_Baader--16-Adapter-from-2--to-T2---2--focal-adapter.html