Narrowband exposure lengths and focusing

[earth titan]

After what seems like an age, I have time at home

I bought an Ha filter the other month, but have yet to use it and have a few questions.

How much longer do exposures need to be and what is the best way of getting focus?

[Advertise]

No expert here at all and still learning myself but I can atleast say what I do.

For exposure lenght I usally go with 600s

and for focusing I use a babhtinov mask and after I finetune with Neulosity's fine focus.

[AndyUK]

How much longer do exposures need to be and what is the best way of getting focus?

It really depends on what the object is, what you're trying to achieve, how good your guiding/tracking/PA is...  and how long you can manage / are willing to spend on a DSO! 

I 'd agree that 10 mins is fine as a starting point, but 15 mins will get you deeper... and 20 mins deeper still.  If you look at some DSO imaging posts, you'll see some who are quite happy using 30 mins exposures (but I'll admit, with my skies and niggling differential flexure, that's a step too far for me!).  The 314L has a sensitive chip so I would suggest you start with 10 mins, and once your happy with that, try going longer. 

As I said, for me the limitation is my DF (I have no idea where it's coming from as I use an OAG!), but you may find with some DSO's with high Ha content that if you push the exposures up, whilst you may get more outer nebulosity, some of the nebula may begin to "blow out".  Some of that can be dealt with in processing, or you can choose to take 2 different exposures (say 30 mins and 10 mins?) and then blend the two together...

[ChrisLX200]

It may seem everything is a lot dimmer through the filter but of course it is transparent to the Ha light so the same length of exposure will get you just the isolated Ha signal. In practice though I use a longer sub - around double I would use for a normal luminance (so anywhere between 15 to 30 mins  subs). Focusing I don't know, I use autofocus with 4 second exposures @ f5.2 and I already know the absolute focus position with that filter so it's a case of fine tweaking only. Stars will appear dimmer - as will the Bahtinov spikes if focusing manually, so again you'll need to increase the exposure value to brighten it up a bit.

ChrisH

[MartinB]

The easy answer is a long as you can manage, but there is a more complicated answer along the lines of "it depends".

If you want to make things more complicated have a look here 

http://www.ccdware.com/resources/subexposure.cfm  and  here http://www.stark-labs.com/craig/articles/articles.html  go down to 2009 and look at the articles on noise and SNR.

My camera has a very sensitive chip with quite low read noise and I mostly use fast optics.  I have significant LP but still haven't reached the ideal sub exposure length after 30mins using a Baader 7nm filter.  I could go beyond 30mins but if you loose a 30+min sub due to a passing cloud you really feel the pain! I like to have a minimum of 10 subs for a statistical rejection stack so I compromise and limit exposures to 30mins.

[RikM]

I am in the same boat as the other guys. I usually go for 15 - 20 min subs and aim for 8 - 12 sub as a minimum. I lost 2x 20min subs to clouds the other night and that was bad enough, I would hate to loose a 30min or even 60min as some people do.

[earth titan]

Thanks folks. I'd supposed 10 min subs would be required as a minimum.

Focusing is my main interest. The Ha gives a much dimmer picture (obviously) and finding it difficult to get much to focus on. I generally use the Atik software to check focus - perhaps I need to use a mask.

[RikM]

Forgot to say about focusing. I normally use a bright star with a Bahtinov mask and Bahtinov grabber software. Depending on the star 6sec exposures at f/5 are typical but I have had to use up to 20sec if the star is not very bright.

[lensman57]

After what seems like an age, I have time at home

I bought an Ha filter the other month, but have yet to use it and have a few questions.

How much longer do exposures need to be and what is the best way of getting focus?

I can easily go up to 1200s per sub with an F6 scope and a 7nm Ha filter. Under a true dark sky you can even go deeper if your guiding is up to it.

A.G