Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

Sign in to follow this  
GlassWalker

Narrower narrowband filters

Recommended Posts

Does anyone here use very narrowband filters with very fast systems?

Currently I have a set of 12nm bandpass Ha, SII, OIII filters from Astronomik. Even with my existing kit, that isn't narrow enough for the length of exposures I want to do, and I'm thinking about getting much narrower ones. But I also tend to work at very fast optics.

E.g. I historically tended to use f/2 at 2 minute exposures, which most of the time was already light pollution limited. Since I got an EQ6 I've dabbbled up to 8 minutes, and I might get a little more depth but the background is definitely the limiting factor there.

So, any tips on very narrowband filters and fast systems? Being realistic, I need to slow the optics down anyway as I move to longer focal lengths, so I guess how narrow can I realistically go at say around f/2.8?

I notice that Astrodon suggest their 5nm filters are workable at f/2, and the 3nm is good for f/3.5. But their data is based on a very large 50x50mm target area, so the light ray angle variation in the middle of that would be reduced further.

Share this post


Link to post
Share on other sites

Sounds like one of those things that only experimention with your own scope/camera might resolve (an expensive test though!). As I'm heading down this route I'd be interested too.

My understanding of how narrow band, multi layer coatings work is that the acceptance angle of the incident light is likley to be more critical with the tighter coating specs. In other words a 3nm filter will work better with more collimated incoming light than say a 7nm or 12nm filter and therefore is less angle tolerant. Personally I only have a set of Baader 7nm filters which I haven't used much on fast (F4) optics. s

Share this post


Link to post
Share on other sites

f/4? Too slow :D Well, I'm slowing down from f/2 to f/2.8 since it isn't economic as I get to longer focal lengths.

Your understanding matches mine. As the light rays go off angle, the effective filter passband shifts so you're not getting what you want any more. But balancing the optical speed and filter bandwidth is going to be interesting... 5nm is a 2.4x improvement over the 12nm I'm using now. 3nm... gets more interesting at 4x, but the increased off angle rays would negate that theoretical advantage so I might as well be using slow optics anyway.

I have been playing with low-fi raytracing in a spreadsheet. I came up with a surprising conclusion, although not hugely useful. The light spread due to faster focal ratios as the aperture size changes (and by definition the focal length also) only holds if the sensor size changes to give the same field of view. If you have a fixed sensor size, and fixed focal ratio, "going large" actually reduces the light ray angles at the extremity.

Share this post


Link to post
Share on other sites

I'm sure you'd find Astrodon results from Hyperstars which would go some way to answering your question. I only have the Baaders at 7 Nm and they are perfect at F3.9 in the Tak but you want to go faster than that.

Could you ask Starizona if the Astrodons worked in their scopes?

Olly

Share this post


Link to post
Share on other sites

I am finding that the 8nm Ha filter I have has no real advantages on the F2.8 compared to the F7 or F5.6 scopes I used to use it on.

Share this post


Link to post
Share on other sites

For what it's worth I have seen no problems using the Astrodon 5nm filters at f2.8.

( This bit really needs a diagram so bear with me ! ) If you imagine the diagrams that are published by the filter makers you'll see that the emission line usually lands in the middle of the band width. The Astrodon's are reasonably efficient. He claims 95% in the 5nm filters.

What happens as the speed increases is the band shifts. It's not a lot but measurable. What seems to happen is that instead of being 95%, the efficiency drops because the top of the band no longer passes in the middle of the emission line. This may drop to, say, 90%. Still more efficient than some filters running at f4.

From what I've seen a 7nm filter copes with f1.8. I doubt you will have a problem.

Other than Starizona you could also ask Ian King. He gives a straight answer to a straight question. ( Which is why I use 5nm filters )

Dave.

Share this post


Link to post
Share on other sites

mmm, so would an 8nm Ha filter be ok at F2.8 assuming it is a good make ?

Share this post


Link to post
Share on other sites

I can tell that the bit I wrote is nonsensical. Sorry for that.

5nm is good for f2.8 so 8nm certainly will be.

Dave.

Share this post


Link to post
Share on other sites

Looking again as the Astrodon website, they do say "3 nm filters can be used on systems to f/3" and separately "There are a few ultra-fast optical system that operate at f/2. In this extreme case, our 5 nm filters is probably a better choice."

Thinking more, given I'm currently limited by the background level, I think I have the headroom to go longer to compensate for slower optics, which will also help relieve the rather critical nature of focusing and other optical aberrations.

I had forgotten about Ian King even though I have spent a lot through him in the past. Having seen the price of 3nm filters... let's just say I'm not jumping into this quickly! If I go that route, I think I'd be looking to buy filters for life, and need to consider future sensor and optical upgrades too.

Share this post


Link to post
Share on other sites

One way to investigate this would be put one of the narrowband filters into a spectrophotometer and measure precisely how the transmission changes if you vary the angle of the filter. I have access to Perkin Elmer machine at work so I could try it out.

Another thing we have found with optical coatings and particularly finely tuned band pass coatings is that unless they are made using particular techniques they are very prone to wavelength shift with temperature fluctuations. Thinner substrates also made the temperature drift effect worse. If the coatings are IAD (ion assisted deposition) or IBS (ion beam sputtered) then they should be stable.

Share this post


Link to post
Share on other sites

Using Astrodon again as an example, they say they're sputtered. I haven't really shopped around as it were yet what options there are but I feel like going for the 3nm!

As for the performance off axis, they give an example of that on their page as I linked previously. The thought is, any "lost" rays would also cost exposure and may even pass unwanted light instead, so there will remain an optimisation with slower ratios if that might be the case.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

  • Recently Browsing   0 members

    No registered users viewing this page.

  • Similar Content

    • By Corpze
      Hi, I just published a video of how I am cleaning my filters - I thought I might do this while my telescope is back at service due to some kind of astigmatism.
      How do you guys clean your filters? it seems that every dust particle finds it way to the filters just as you are done cleaning them, and to sit in a damp bathroom seems kind of a mess...

      Anyway, here is the video
      / Daniel
       
       
    • By Anthony RS
      Hello,
      I'm selling these 2 filters since I'm getting a mono astro cam. The filters are barely used and in perfect condition, no scratches, no fingerprints, not even dust. They are both amazing filters, probably the best investment I've made. I've attached some images
      taken with these filters, using a 100$ celestron newtonian, a 250$ Canon 500D and the infamous AVX. Also attached are images of the filters showing their perfect condition.
      Astronomik CLS-CCD Canon Clip in Filter: https://www.astronomik.com/en/filter-gegen-lichtverschmtzung-filters-against-lightpollution-lpr/cls-ccd-filter/clip-filter-eos-mit-astronomik-cls-ccd.html
      Original price including VAT is 155 Euros (around USD 182). I got it for around USD230 including VAT, shipping and custom taxes.
      Selling for USD 100.
      Astronomik Ha 12nm Canon Clip in Filter: https://www.astronomik.com/en/clip-filter/clip-filter-canon-aps-c/clip-filter-eos-mit-astronomik-h-alpha-ccd-12nm.html
      Original price including VAT 194 Euros (around USD 228). Got it for around USD 270 including VAT, shipping and custom taxes.
      Selling for USD 150
      I am willing to ship them on my own expense using LibanPost (from Lebanon). Shipping might take time; if you would like to use some other shipment method please contact me to discuss the price. 
      Feel free to buy one or both together.
      Let me know if you have any questions. 
      You can also contact me on <private email address removed>
      Cheers,
      Anthony









    • By melsmore
      First the disclaimer. This is my first attempt to sketch Mars, or indeed any planet. Also, it started to rain and I had to abandon it with some urgency (hence no orientation marker). I used a HB pencil and a blending stump. But I think I can see a couple of features that match Ade Ashford's app.
      The Wratten 21 filter improved the view enormously (although you may find that hard to believe looking at the picture) helping show the surface detail and improving the seeing. I tried it the day before in my 80mm refractor, but that just made the image too dim, but on the 115mm it was very good, so I recommend it to anyone with 115mm or larger.
      I tried sketching the moon (Plato) several years ago, but SWMBO pronounced it (I'll paraphrase her here) more Feline Anatomical than Selenographical. While this one may look more like a two year's old attempt at drawing a rabbit, at least no one can confuse it with the rear end of the cat. So regardless of the criticism I receive here, I'll give it another go tonight (weather permitting). 🧐

    • By A_D05
      Hi everyone, 
      I am thinking about buying a Celestron Nexstar 6se to use for astrophotography at a very beginner level. I am thinking about using a ZWO ASI224MC as it seems a good option. I have a few questions about how to setup/use it - here they are:
      1.) Do you just screw it on where the eyepiece is usually at?
      2.) Do you just have the cable going to a computer where you have stacking spect where that stacks the photos it takes?
      3.) Does it automatically take photos of planets and DSO’s?
      4.) Does it need any filters to get color on nebulae as those are they primary things I want to photograph
      Thats all, as you can see I am a beginner to astrophotography and just want to know how to use the camera to take decent pictures.
      Thanks all 
    • By GiL Young
      Hello all. Thank you for taking a moment to read this & offer your suggestions/ advice. I'm sure my basic question has been asked many times: "In your opinion or experience, what are the first, most important, necessary accessories I should add in order to maximize the the use and ease of a newly acquired Celestron Nexstar 127 SLT ? I have a power supply adapter, 9mm,10mm, 25mm eyepieces, 2x Barlow and 90 degree diagonal mirror adaptor. With a modest budget and a 2 week deadline, I've researched numerous reviews and narrowed some choices to additional Plossl eyepieces, an assortment of filters or a dew shield. All of and these can probably be purchased within my budget but I'm even willing to take the plunge on an upgrade to a better mount, which I have no idea where to start. Any and all suggestions, advice and opinions are gladly appreciated. Thank you all, from across the pond in northeastern US. 
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.