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Old 03-03-2018, 09:45 PM
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LewisM
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7nm H-a with an OSC

A mate traded me a 7nm H-a filter (2"/M48 Optolong) for one of those silly eyepiece things

ANYHOW, I know using an H-a filter OSC is entirely possible (1/4 the pixels at least get something - dithering to the rescue! ), but approx. how much longer an exposure compared to RGB OSC? I currently do 10 minute subs with the FSQ-85 and the LP filter, so what is a good ball-park on acquiring H-a data commensurate to the RGb? 20 min? More? Is it a linear relationship?

And lastly, is it necessary or advisable to use the UV/IR cut filter with the H-a filter? (I stack my filters in a special filter-adaptor I had custom made - see below - the end screws off to reveal internal M48 female threads to mount the filters)
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Last edited by LewisM; 03-03-2018 at 10:16 PM.
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Old 03-03-2018, 10:36 PM
kens (Ken)
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RGB filters have a bandwidth of around 100nm so you could expect a 7nm filter to receive about 1/14th the number of photons in a given time from a broad band target or sky glow.
That would suggest exposures 14 time as long are needed.
My RGB subs are usually 30s long and my NB 7nm subs 300s. But for the NB I increase the gain to reduce read noise and therefore the required exposure but its still 10 times. Thats with an ASI1600 but the same principles apply.
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Old 04-03-2018, 11:58 AM
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Thanks Ken...the thought of 6000+ sec subs for H-a don't particularly appeal - I may just try lots of 20 min instead (I have tested the mount/guiding, and individual 30 min subs are no issue - probably longer!)
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Old 04-03-2018, 01:37 PM
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Camelopardalis (Dunk)
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This may be a little off piste, but since you’re using an astro ccd with no UV/IR cut like a DSLR would suffer with, surely you’d expose your red about the same? Well, maybe a bit longer... depending on the QE of your sensor...

The Ha is just a more stringent LP filter... of course you’d be best to experiment to ensure you’re taking sky limited subs, and I know old fashioned CCDs have horrendous read noise compared to a 1600, but surely it’d be worth a shot?
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Old 04-03-2018, 02:19 PM
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If you're going to be using a 7nm Ha filter the UV/IR is going to be redundant. It will however increase the likelihood of getting internal reflections

And you're using an old clunky camera Lewis, you're going to need 5+ hour subs to get over read noise
In reality 20-30 minutes will be fine. You're using an OSC so getting bogged down in getting the absolute most out of the sensor isn't worth the effort
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Old 04-03-2018, 02:44 PM
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old clunky ... Lewis
Yup, describes me well.
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Old 04-03-2018, 03:23 PM
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Yup, describes me well.
That quote was taken completely out of context!
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Old 04-03-2018, 04:25 PM
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You expected less?
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Old 04-03-2018, 04:50 PM
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Yo ex Les?
I've never dated a Les...
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Old 04-03-2018, 07:46 PM
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Sharpiel's name is Les - I can arrange a meeting if you like?

Just bring him some bird seed for his pets.
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Old 05-03-2018, 11:21 PM
sharpiel (Les)
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I've never dated a Les...
Yoohooo.....over here boys.

Waves hands and batts eyelashes alluringly...
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