RGB filters and light pollution.

[jasonh]

Firstly, hello all, I just joined up today. I have been a lurker off and on for a little while.

I live in Auckland, NZ, and see there are a couple of guys that I know - many I don't but that may change if I ever get over to your south pacific starparty :)

I have posted this on another forum but I see there are a bunch of experienced photographers over here as well so lets give it a go..

I have been using a OSC camera and a dslr before that. I just ordered a STL11000M and I have to do something about filters..

I am trying to get some idea of a preferred vendor. I know of three worth looking at. Astrodon, custom scientific and astronomik.

What I am not sure about is how these different vendors might perform under light polluted skies and if they are different does it really matter?

I ask because although it seems everyone has astrodon on their cameras I have also read that they may not be as good in light polluted areas. This is because they dont just let RG and B in to each filter, there is some crossover intentionally. Apparently custom scientific doesn't do this. What the article said was that this reduces contrast and lets more light pollution in being widerband then necessary for the colour.

I live in auckland so the skies are not ideal. 4.x mag towards the city and down to 4.8 just past zenith away from the city.

So when you hear guys in europe and the states talk about light pollution its probably worse a lot of the time..

Filters are not cheap so its playing on my mind a little bit. Any comments would be much appreciated. If the comments I have seen re astrodon are not an issue in mag 4.5 skies then perhaps I should just get what everyone seems to get..

No try before you buy in NZ, I suspect its the same in Aus..

Thanks guys, any help appreciated.
Jason

[Zuts]

Hi,

Can you look at the transmission curves from the various vendors and see if there are any differences?

Also, since RGB is not narrowband then regardless of crossover wont they all suffer from LP as opposed to imaging in Ha, SII, OIII.

Hopefully Jase or Peter Ward can jump in on this one :)

Paul

[jase]

Jason,

Welcome. No need to lurk around here.

You’ll find that regardless of the filter manufacturer, light pollution will always compound the performance of a given RGB filter set. If light pollution is a real issue, consider narrowband imaging or escaping to a dark sky site to collect the RGB data – depends on how keen or serious you are.

I’ve had the pleasure of using both Astrodon and Custom Scientific filter sets. The three manufacturers you mention all produce excellent filters for the amateur. It would appear Astrodon is becoming the de-facto standard. The “tru-colour” design is achieved through colour cross over techniques between the individual R,G,B wavelengths. This technically allows an RGB balance of 1:1:1. In reality, the balance is never 1:1:1, but is close. I’d still recommend performing a G2V star calibration. The Custom Scientific filter set also produces nice results – I currently use these.

The nice attribute of the Astrodon filters is that the colour cross over allows for OIII teal to be successfully acquired. I found this is only really prevalent for certain targets, namely the Dumbbell nebulae. You can of course still obtain similar results with other filters by blending specific data in to the G and B channels. Many believe the Astrodon filters make imaging easier. I certainly haven’t found using them any easier than other filters. There is still the opportunity for colour gradients to occur on each channel, balance issues, reflections etc. Just about all manufacture par-focal filters so refocusing between filters isn’t an issue. Though I typically refocus anyway. Keep in mind that an LRGB set may not be par-focal with narrowband filter set. The Astrodon’s are, but the narrowband Custom Scientific narrowband filters aren’t. Again, this isn’t a big issue if you refocus between filter changes, or even better configure a filter focus offset so that when the filter is made active, the motorized focus compensates for this. Plenty of ways to solve the par-focal problem.

I’d recommend getting some unmounted filters for the SBIG carousel to minimize vignetting (very minor anyway). Something that I don’t like with the Astrodon’s is that there is and I and E series of filters based on the chip you are using. From my perspective, this is too limiting. What if I wanted to change from the STL11k to an STL6303? You guessed it, I need to purchase a different set of Astrodon filters to match the transmission/chip sensitivity to maintain balance. I do however know of a few people that use the I series with an ST10XME with no ill effect, but you’ve got to wonder…

StrongmanMike (check deep sky section) has punched out some magnificent images with an Astronomik filter set riding the ProLine. Its difficult to go wrong with either of these three (CS, Astronomik or Astrodon) filter set manufacturers for LRGB sets. In comparison, narrowband filters require additional consideration. Only a few manufacturers offer small bandpass window such as 4.5nm Ha filter. You need to shop around and calculate your optimal imaging train configuration. A larger bandpass window will let more continuum energy through (more stars etc), thus will reduce the required exposure time, but may upset the Ha purist who want the cleanest possible transmission or looking to use a companion Ha continuum filter to remove the remaining impurities (typically faint stars). A wider bandpass can also result in star bloat problems if not careful (usually manageable through image processing). If you're doing self-guiding and the guide chip is also behind the Ha filter, you may find guide star selection will become difficult - you may need to bin the guide chip boost the sensitivity (compensate for the lack of light the filter is permitting through). Plenty of other things to consider. The narrowband filter selection is probably another discussion.

The above is probably not a great help, but will try to answer further questions you may have.

[gbeal]

There ya go Harry, thought this would happen, good old Jase.

[jasonh]

Thanks Jase, good to see you jump in. I have read some good information from you while 'lurking' :)

From your comments it doesnt sound like the LP comment I saw would be a significant, if any, problem for the astrodon. It is interesting about the I series being for specific types of chips. I don't know when I will change but it will at some point so this is something I will consider.

1:1:1 - to be honest I am not sure what is harder? Calculating how long for each chanel or selecting the percentage when combining? Perhaps there is more to it that I don't know. Either way you seem to spend so much time processing whats another 5 mins?

I will be doing narrowband and have to consider what filters to go with. On my current camera I use a baader 7nm and am happy to change it for the new camera/scope. I would like to think about oiii and sii as well. I have made quite an investment in my equipment lately and want every opportunity to use it, full moon and all! :)

The stl will be going on the back of a 12.5" RC which I am currently building with optics from star instruments.. So the final product will be a stl11k on the back of a 12.5" f6.6. A much better fov then my current 110mm WO for the smaller stuff :)

Only concern with the stl is exposure times, pretty awful qe at Ha but I hope a good balance camera all around.

Thanks,
Jason

[jasonh]

Hi Jase, I would appeciate if you could expand a little on the optimal imaging train configuration.

I know that my stl isnt the most sensitive ha device (far from it!) and that the 12.5" isnt the fastest at f6.6. Other than this I know that the narrower the band the longer the exposure but better detail/contrast in return.

I don't know what to take in to consideration given my other gear other than a 3nm filter wouldnt be a good choice with my 32% qe STL.

Without a lot of experience with them it would be easy to fall for advertising which says things like 'better transmission for low qe devices like stl11k' (astrodon, not exact quote but close). Is 6nm too narrow? not sure. My current camera is 50% qe at Ha (qhy8) so I don't know what this would translate to on my new rig in extended exposure times and return on quality.

Sigh, more money than brains hehe :)

Same for sii and oiii - now these three filters are expensive.!

[Peter Ward]

Jason,

Dr Alan Holmes at SBIG did a nice piece which is available for download at their website here:

http://www.sbig.com/pdffiles/SBIGFilterSet.pdf

I've used with CS and AstroDon sets, and ended up going back to the CS variety, as they give better colour saturation. Unfortunately llight pollution, particularly with Oz's penchant for High Pressure Mercury lights, tends to be spread across the spectrum., so the only real fix is to get good at removing gradients (particularly if you are about to get an STL11k) or shoot from dark skies.

Cheers
Peter

[jase]

Jason,
Sorry, been busy - haven't had a chance to get online.

Hope the following helps...



Calculating the RGB set for G2V star calibration isn't difficult. I posted information on these forums regarding this process a while back (shown below for reference; Here's how I do it, others may have another process they wish to share.
Firstly, look up a table of G2V stars and select one close to zenith. You want to minimise as much extinction factor as possible. Also choose a star that isn’t going to saturate easily. There is a great reference table here - http://www.gemini.edu/sciops/instrum.../g2vstars.html (http://www.gemini.edu/sciops/instruments/niri/standards/g2vstars.html)
Once you’ve selected your target star, begin to focus the star as best you can through your optical system. Proceed to take 10 or more equal exposures through each individual filter (R, G, B). I generally go for 30 second exposures, but its up to you.
Register and average (combine) the frames.
In MaximDL or other astro imaging software, I then measure the intensity of the G2V star in each frame (R, G, B). Make sure you’ve selected the right star!!! This sounds stupid, but when you’ve got a huge FOV you may need to reference your astro planning software or star charts. You need to ensure the measurement encompasses the entire star, not just the core.
Take note of what the intensity values are; As an example:
R = 600,000
G = 700,000
B = 500,000

So based on these (example – don’t use these!) figures, you can see the example optical system is most sensitive in the green wavelengths. What ever is the most sensitive channel, make that ratio value of 1. So now that we have established G = 1, we can compute the R and B weights. If you divide the green number (700,000) by the red number (600,000) you get 1.16. Divide the green number by the blue number you get 1.4.
So your calibrated RGB weightings are R:1.6, G:1,B:1.4 for that specific optical system/filter/camera combination.From this point on, you can take equal exposure lengths as you image certain objects and use the correct weightings for each channel to obtain the right balance.

You should perform this even if you’re using true-balance filters such as those made by Astrodon. You’ll find the spectral response of your entire optical train will not deliver a RGB of 1:1:1. There are likely to be subtle variations.



Indeed the STL11k doesn't have the best sensitivity around the 656.3nm wavelength (Ha). But you can make this up with longer subs. Don't be afraid to go long with 15 or 20min subs. Selecting a narrowband filter isn't something I would recommend deciding on quickly. You need to consider the optimal bandpass for your optical system and environment. Indeed, the narrower the bandpass (say 6nm or less) will require greater exposure lengths compared to a 7nm and higher bandpass filter. A narrower bandpass can also be more effective a blocking unwanted light such as moon/sky glow, however narrower isn't always better when considering fast focal ratios in particular camera lenses.

"All dielectric interference filters shift predictably short with increasing angle. The combination of fast F/cone and wide field angles limits the use of narrow band filters to slower systems. Filters narrower than 13nm will be significantly degraded by the very fast F/cones where the sum of the F/cone and half field angle are greater than about 13°. Again, they are great for slower systems, where slow begins at F/4." Quoted from http://www.hbastro.com/Telescopes/Fa...derations.html (http://www.hbastro.com/Telescopes/FastAstrographConsiderations.html)
I would recommend reading the above link amongst many other narrowband emission filter web info.

I'm in agreement with Peter. The CS set really delivers some great colour saturation. As there is no colour crossover between G and R filters, its great for red channel luminance work. I've done a few RRGB images like this an was impressed. Unfortunately, my website is still down so can't show any examples. As I previously said, you can't go wrong with the three RGB sets you mention. Bert (avandonk) alerted me to another contender - Hutech/IDAS - http://www.sciencecenter.net/hutech/idas/rsfilter/index.htm
They seem to have a special coating to reduce internal reflections without reducing the filter efficiency.

Let me know if I can assist further.