7nm Ha imaging - min separation from Moon?

Omaroo (Chris Malikoff) · #1 03-08-2009, 10:59 AM

Arrghh.. ruddy Moon!

With the great weather we've been having, I've been trying to pick nebulous areas of the sky to image because the moon is up. Problem is that without goto, star hopping is problematic to say the least given that I can't easily see ones that I need to locate target nebulae. Last night the Moon was so bright that the sky looked almost devoid of stars, and I could and did actually read a book by its light.

There' a pile in Sagittarius that I really want to image in Ha, but the Moon is doggedly following Sagittarius at the moment and it's pretty damn bright - see the attached map.

Question - with a good scope (4" Tak with standard dewshield) - i.e. a refractor - what separation should I need to provide from the Moon before I might be able to image an object near it? With the Moon just off Sigma Sagittarii (Nunki), should I be able to image in 7nm Ha as close as say M20 or M8 without being blasted? I notice that Marc (Multiweb) was in the area a couple of days ago and still produced good results.

renormalised (Carl) · #2 03-08-2009, 11:17 AM

I don't think you'll have too much trouble getting some good Ha images of the nebs there, Chris. The closest of them is at least 11 degrees away... that's 22 Full Moons. I wouldn't try if they were closer than that otherwise the glow would get too overwhelming. I'd have a go and see what results I got, then based on that, I would know for future reference just how close I could get without washing everything out.

[1ponders] (Paul) · #3 03-08-2009, 11:48 AM

I'm surprised that you have any problem at all. If you are imaging Ha only, then I would have thought that reflected moonlight would contain minimal if any Ha reflection.

Please explain. (in high pitch nasally fishshop owner voice

)

Omaroo (Chris Malikoff) · #4 03-08-2009, 12:00 PM

Not having had an Ha filter before Paul, I guess I'm wondering just how much light the Moon reflects in the area - on all wavelengths - i.e. it's white light. Would this not include insanly high levels of light at 656nm as well? If the saturation levels were high enough Id've thought that no matter how narrow the bandpass figure is, there'd be so much light it wouldn't matter. Not so then? I could just go on unabated?

[1ponders] (Paul) · #5 03-08-2009, 12:03 PM

I'm like you Chris, very newbie when it comes to Ha, but I can't imagine it being too bright.

But then again I've been wrong before.....not often, but it has happened.

dugnsuz (Doug) · #6 03-08-2009, 01:18 PM

Hi Chris,
I thought I would get some more 12nm Ha images of M8/M20 last night too but I felt that the moon was too close to the region to get adequate results - so didn't try! The moon was actually in the PHD window when the Goto had slewed to the correct area!!! that's some guide star/object!!
Tried getting some shots of Eta instead (well away from moon) sinking into the west.

Did some Eta shots a while ago as the Moon was beginning to encroach - shots became increasing bleached of any detail so I've given the moon a wide berth for my Ha imaging ever since. 7nm might give you a narrower window so to speak, but I would suggest avoiding the moon!

Doug

renormalised (Carl) · #7 03-08-2009, 01:22 PM

So even from that far off (Moon-Eta separation) there was bleaching...

Quote:

The moon was actually in the PHD window when the Goto had slewed to the correct area!!! that's some guide star/object!!

Well, at least you had no trouble acquiring something to guide on!!!

Omaroo (Chris Malikoff) · #8 03-08-2009, 01:37 PM

Quote:

Originally Posted by dugnsuz

Hi Chris,
I thought I would get some more 12nm Ha images of M8/M20 last night too but I felt that the moon was too close to the region to get adequate results - so didn't try! The moon was actually in the PHD window when the Goto had slewed to the correct area!!! that's some guide star/object!!
Tried getting some shots of Eta instead (well away from moon) sinking into the west.

Did some Eta shots a while ago as the Moon was beginning to encroach - shots became increasing bleached of any detail so I've given the moon a wide berth for my Ha imaging ever since. 7nm might give you a narrower window so to speak, but I would suggest avoiding the moon!

Doug

Spot on Doug. That's the other problem of course - guiding. Even at 11 degrees sep as Carl suggests, the QHY5 is going to be, I'd assume, totally saturated. Could be wrong though.

If only the Eta Carinae region wasn't so far over in the west by the time it gets dark.... oh well - it'll be cloudy tonight here in Sydney anyway.

Time to get that ArgoNavis me thinks. At least then I don't have to rely on star hopping based on...uhmm... invisible stars.

dugnsuz (Doug) · #9 03-08-2009, 01:44 PM

Quote:

Originally Posted by Omaroo

Spot on Doug. That's the other problem of course - guiding. Even at 11 degrees sep as Carl suggests, the QHY5 is going to be, I'd assume, totally saturated. Could be wrong though.

If only the Eta Carinae region wasn't so far over in the west by the time it gets dark.... oh well - it'll be cloudy tonight here in Sydney anyway.

Sorry for confusion...Last night decided to image Eta instead of M8/20 due to proximity of moon...turned out pretty well for 12x5mins as it was sinking fast!

The Eta bleaching story comes from a session about 2 months ago when I tried to image Eta under a full moon which was very close to the target.

Tandum (Robin) · #10 03-08-2009, 03:23 PM

I was pointing at M8 with filters on the week-end and had no trouble guiding. 1st one is QHY8 on the vixen @ F9, 2nd one is starlight on the tak at F6. I used a 2" 7nm Baader Ha on the imaging camera and guided with a qhy5 on the other scope. No cal files, just playing around and trying to colliminate the vixen so ignore the triangular stars in the first one