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Old 11-08-2013, 05:03 AM
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nebulosity. (Jo)
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Ha With Camera Lenses

G'day

I've got a nice Nikkor 85mm 1.8 lens that I use for wide field imaging and as Im about to make a mono DSLR I was wanting to do a bit of NB with it as well. I thought I would start looking around for a Ha filter but am not sure how narrow to go, I have seen ones from 40nm to 3nm and am wondering what would be best.

Also, I generally use the lens at f4 because of CA but with the filter CA wouldn't be a problem? So I could use it at f1.8? (its still tack sharp right to the edges)

Now, I was thinking about a clip in filter (camera's a 350d) but wondering if it would work OK at f1.8? The other option would be to get a 2" filter and attach it to the front of the lens, what do you think would be best?

Thanks
Jo
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Old 11-08-2013, 08:33 AM
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Cool project. I was thinking of monoing a DSLR. An old 5D would make a great mono DSLR. They are cheap, they have large pixels (deep pixel wells) and probably easy to pull apart to get to the sensor and they are full frame.

I have Nikon 85mm F1.8 and have imaged with it. It needed around F2.8 to clean up the coma so its not just the chromatic aberration (CA) that needs to clean up its the coma. But F2.8 it should be fine as I recall.

As far as which HA filter to get that is not so clear. I think typically DSLR users go for a wider Ha filter than dedicated CCD users do. That lets in more wider band light to get a decent exposure. But then only 1 pixel out of 4 registers the Ha and even then it has to make it through a red filter as well on top of the already abysmally low QE of a typical DSLR sensor. But monoed hmm, QE is probably back up there with many CCDs.

The latest DSLRs have improved QE. The best QE is the Toshiba 24mp sensor in the Nikon D7100 at over 60%. The Nikon D800E is next best with its Sony Exmor 36.3mp chip and I think the D600 with its Sony Exmor 24mp chip.

But the older 5D is probably pretty good as it has quite large pixels for a DSLR so that gives it deeper wells which is also good for astro imaging.

So does your camera it into the poor QE, red filtered weak Ha signal so needs a wide Ha filter or the CCD every pixel counts with high Ha performance (QE)? Probably more the later so that would lead me to expect a medium width Ha filter of around 7nm. Baader makes a 2 inch circular 7nm Ha that is "fairly cheap". 3nm besides being extremely expensive are unlikely to be suited as they really require a fast scope (not too fast though) and high QE sensor.

That may be the go.

I'd love to see the results and get an idea of the sensitivity of the stripped sensor. It could be a cheap way of getting a mono full frame sensor that is currently around $6-10,000 all up cost in astro CCDs.

Greg.
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Old 01-09-2013, 04:34 PM
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Thank you Greg, very helpfull.

Jo
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Old 01-09-2013, 05:39 PM
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Slightly of topic, Jo I was considering getting one of those 85mm f1.8 Nikkor lenses so am interested in your comments re the CA and from Greg's comments coma issues they might have. Would you have any images then you could post that illustrated the problems (just test shots would do).

Roger
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