H alpha

[bmitchell82]

Is there anything special about focusing in H alpha?

[Hagar]

Nothing special about focusing with a Ha filter fitted. The same as using standard RGB only a little more dificult because you cannot see quite as much with the filter. I still use at Bahntinof mask and bahntinof grabber to achieve the best focus possible on one of the very bright stars then move to my target then shoot off a 2 minute test image to check focus.
Depending on how narrow the bandwidth of the filter the harder it gets and the less stars you can see to achieve focus.

Good luck Brendan.

[bmitchell82]

thanks doug, the 13nm Astronomik filter arrived today so time dependant i might give it a burl tonight maybe tomorrow.! ahhh fun times 365 day a year astro imaging (minus the windy, the stormy weather. :D)

[Hagar]

13 nm is a good choice particularly with a DSLR. It does allow the light frrom a lot more stars through than the 7nm that I use.
Ha imaging is a little diferent with capture and processing but I am sure you will come to terms with it very fast. Just lots more exposure time but the end result is worth it and it also provides a sharp detail when blended with your RGB images.
Rob Gendler has some great tutorials and notes on Ha blending which are well worth a look if you intend to follow this path.

Have fun Mate.

[Merlin66]

I keep seeing this comment:
13 nm is a good choice particularly with a DSLR. It does allow the light frrom a lot more stars through than the 7nm

and for the life of me can't understand why this would ever be the case with Ha light????
The emission from Ha is a discrete line at 6563A.

Other than possibly some SII close by it doesn't matter one iota whether you view this single emission line with a 13nm or a 7nm filter. Other than the transmission efficiency of each filter, they will still pass Ha light??????

[AlexN]

Stars still pass through Ha filters due to their inherent brightness, I've imaged using a 13nm Astronomic, a 7nm Baader and a 3nm Astrodon, I can tell you right now, the 3nm is a pain in the backside to focus, the 7nm is doable on anything brighter than say, Mag 5, the 13nm I would rarely bother to slew to a brighter star to focus, I could almost always find a star bright enough to evaluate focus within the field of view of a given target..

A 7 and 13nm Ha filter also let in NII, Close SII and a few other emission lines, a 3nm lets only Ha pass. they are almost infuriating to focus... I use up to 10sec focus exposures on a mag 2 star at 3nm, otherwise maximDL's autofocus routine will fail... in any uniform star field, it can be up to 30 seconds before you actually start to see stars...

[Hagar]

Perhaps you keep seeing comments like this because they are FACT, put down the theory and take a look, you might be surprised.

[Merlin66]

Guys,
It's not the theory that worries me - I have that well understood!
What is being said is that imaging through various bandwidths ALLOWS additional light ( other than Ha) to get through from the star's continumm spectrum. This additional "stray" light has nothing what so ever to do with Ha; it may be SII or NII or background light....
I've used many narrow band filters for imaging and understand the challenges of focusing in the red... but it's NOT the Ha emissions as much as some other light to help either visualise or image with.
Enough from me! I'll go back to my spectroscopes and let you guys do the imaging!!;)

[Hagar]

Spot on, Nobody said it had anything to do with Ha. The only comment made was that the wider bandwidth filters allow more light through making focus easier. The resultant image unlike a spectroscopic analysis will still give stunning Ha detail while maintaining some asthetic stars in the image.

[bmitchell82]

wow ;D i was just asking if focusing in Halpha if there was anything special i should know about :) I have also found that because of the DSLR's sensitivity at these wavelengths that i just have to take longer subs.

I plan to image my heart out with the 10" at 45-60 min subs on a target i will leave untill its out but it will be in the range of 20hrs of data which is easy to do now that i can do it at home! :) happy days people! and thankyou for all of your advice!

[AlexN]

45 to 60min subs with all your ear on the EQ6 might be a bit of a stretch, and to be honest, shouldn't be needed. I think you'll find 30min subs to be heaps, and 15 or so of them should do the trick for a nice smooth image provided you take good calibration frames.. Temperature is going to be your killer.. ICNR is perhaps the only way to ensure your darks are matched to your lights, but who wants to take 1 hour to collect 30 minutes of data...

Im ranting because im bored... sigh..

[multiweb]

You're better off stacking a lot of 10min subs. Anything can go wrong in 45-60min subs. I never went over 30min but you're bound to throw a lot of subs unless your tracking is rock solid.

[multiweb]

The incidence angle of the light cone seems to matter. I have a 7nm but I need to get a 13nm: http://www.hbastro.com/Telescopes/FastAstrographConsiderations.html

[bmitchell82]

I know my setup is good for 20-30 min subs with the new meade tripod and the muchly upgraded saddle/puck arrangement as long as theres very little wind i can hold guiding for a fair while.

Marc you are right though even if i do throw a few shots away im not going to go and slash myself :) this imaging will be taking place in my back yard when the moon is at its worst.! now if i was out in the country and took a few hours to get there, and i chucked 30% of my images in the bin ide be destroyed. :S :)

as it is i have the ability to leave my scope fully setup in the back yard and take images for most of the night. as long as i set my mount limits i will never have any problems! set the scope up change the battery at about midnight and leave it plugging away :) get up at 5 or 6am before the pidgeons think its a great sitting spot!

[multiweb]

You have to realise that if you do 10x10min or 5x20min you'll capture the same amount of photons and when you stack the result might not be that different but for the camera read noise. That's what I have observed doing testing anyway. Nowadays I don't bother going past the 20min mark if I'm impatient that is ;) Shooting more subs gives you the flexibility of having more data to choose from. The hyperstar is great for that because I collect so many subs. I can dump half of them and be choosey. Not the same when you have your 5x20min hard earned subs.

[AlexN]

Indeed... too much can happen in a 45min to 1hour sub.. I've ran a few 1hr subs in Ha from home, they've come out fine, the issue was the amount of satellite trails and planes that can destroy the image.. every sub had at least one satellite trail. 20 min subs are obviously still affected, but not to the same level. and losing one 20min sub is a lot less painful than losing one 60min sub. Whats more, if you were to set up and have the scope running 10 minute subs, and a large gust of wind came for arguments sake, every 50 minutes, you'd still get 300 minutes of data in 6 hours, and one sub per hour trashed due to the wind gust shaking it about. If on the other hand you were doing 1 hour subs. you would wake up in the morning to find 6 trashed subs, 0 useful data for a whole nights imaging.. Despite the fact that as you say, its not as painful as losing subs out at dark skies, one full clear nights worth of data down the drain is painful, regardless of whether its from dark skies, home, during the new moon cycle.. its still time that could have been used to collect useful data.. I'd definitely go with Marc's thoughts.. shorter subs, but lots of them... 20 min should be plenty deep enough through 13nm Ha... Just, Plenty of them.. 18 of them should do the trick... 6hrs data.. obviously, the more the merrier, you can then be a bit more aggressive with sigma rejection stacking etc.. You'll end up with better images if you take more subs in any case...

[allan gould]

Brendan, Ive attached a single 15 min H-alpha sub taken of Orion but not with your 13mm filters. This was with a UV filter and the Lumicon Night Sky H alpha filter. It probably gives a 35-40mm bandwidth which considering last night I was shooting just to the side of a full moon is not too shabby. I sometimes feel that you can get mired in the minutia of things when there can be easier and quicker solutions.
Scope used was a 5" ED Apo and the camera a QHY8 OSC with 2x2 binning. Single 15min sub with flats, bias and flat-dark sub in DSS then levels and curves in PS CS3. This shot is a part of the evaluation Im doing of my QHY8 for imaging as well as its general utility.
Just need a 2" OIII filter now. Used a Bahtinov mask and Nebulosity for focus/capture.

[bmitchell82]

so im pretty sure then i should be right doing 10 min subs thanks for the comparison. I guess the 10" will suck down enough light to make a composition that is pleasing enough. I might start on it tonight so it should be fun.

[AlexN]

Good luck with it Brendan!! Looking forward to the results! :D

Alex.

[Bassnut]

I found you can focus with live view if you 1st slew to the brightest star you can find, the likes of Formalhault or Achemar, easy as.

[bmitchell82]

happy days! i just have to wait for all this high level cloud to go away! and then get some time when im not studying or going to dinner for the mr's bday! grrr.... theres always something to stop me!