Jason,
Sorry, been busy - haven't had a chance to get online.
Hope the following helps...
Quote:
Originally Posted by jasonh
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?
|
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
- 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.
Quote:
Originally Posted by 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.
|
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
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/...lter/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.