My first Ha session

[RobertI]

Encouraged by the nice results that others have been getting with Ha I thought I would finally try my 7mm Baader Ha filter with my Altair Astro 60mm finder/guider scope. I chose this scope because of the F3.75 ratio and wide field of 1.57 degrees.

I'm pleased that I managed to get some results and most of the objects I have never seen before which is a bonus. The images are rather noisy (despite using darks) and I assume this is because I had to stretch them a fair amount. Any comments or suggestions on how to get better results are welcome.

All images captured with a nearly full moon.

The Pacman Nebula (Cassiopeia), definitely the best of the bunch:

The Soul Nebula (Cassiopeia):

IC59 (just above centre in image below ) & IC63 (below centre in image below) were very faint with IC63 being the brighter. It seems that this is because IC59 is mostly reflection and not picked out very well with this filter. Perhaps not the best objects to have chosen on my first night out with Ha! The bright star is a strange shape - I don't if anyone has any ideas as to why this might be? I don't expect the optics in this finder/guider to be much good but it is a doublet.

The Cave Nebula (Cepheus). Again, very faint even after 60 seconds.

Next time out with Ha I shall try my Megrez 72mm with a reducer and see how the quality compares.

Thanks for looking.

Rob

[laudropb]

Some nice captures. Thanks for posting.

[Spacedout]

Looking good Robert, you are having far too much fun 

[xtreemchaos]

great set Rob,well done.  thanks for sharing   charl.

[johnno]

Nice one's Rob,

Question, how come some are clear and some are grainy, have you forgot to set something?

[RobertI]

Nice one's Rob,

Question, how come some are clear and some are grainy, have you forgot to set something?

Good question Johnno. I think it's because I had to stretch the fainter ones more with the result they appeared more grainy. The bright Pacman neb didn't need much stretching as it was so bright and was not too grainy. I don't know if darks had something to do with it too, i used a master darks taken from a previous session in the main. The master darks matched the expsure lengths except the last one of 60 seconds, which used darks of 45 seconds. i'm not really sure what difference darks make to the noise reduction or whether its just hot pixel removal. Perhaps someone can enlighten me?

Overall the shots are more grainy than I expected.

Rob

[RobertI]

Looking good Robert, you are having far too much fun

Yes I am! The late nights do take their toll a bit, but worth every second.

[Chris]

That's a very productive night you've had Robert, I'm loving the Pacman!

 Like you say it will be good to see how the Megrez compares when you get a chance

[Martin Meredith]

Hi Rob

Great shots with your finder/guider. I think many EAA-style Ha shots tend to be grainy for short exposures. Mine certainly are, apart from the ones that are really good Ha sources like M16 and M17. Once they're blended in with some O-III for instance the graininess tends to reduce I find.

I was wondering if the diagonal lines you're getting in IC1848 are due to non-fresh darks? I've also seen those kinds of artefacts when stacking lots of short subs (which I put down to read noise) but in your case that seems unlikely given the sub length.

Martin

[RobertI]

Hi Rob

Great shots with your finder/guider. I think many EAA-style Ha shots tend to be grainy for short exposures. Mine certainly are, apart from the ones that are really good Ha sources like M16 and M17. Once they're blended in with some O-III for instance the graininess tends to reduce I find.

I was wondering if the diagonal lines you're getting in IC1848 are due to non-fresh darks? I've also seen those kinds of artefacts when stacking lots of short subs (which I put down to read noise) but in your case that seems unlikely given the sub length.

Martin

Thanks for the information Martin, very helpful. I'll try some of the brighter nebs as a comparison, although my viewing location is so appalling I can only get down to a Dec of +40 which is somewhat restrictive.

My darks were from a previous session I think and it was much warmer at that time. I think I did take some fresh darks in the end but only about 5 of them which may not be enough?

Do you always use fresh darks? If so roughly how many do you take?

Thanks

Rob

[orion25]

Awesome work, Rob 

Reggie

[Martin Meredith]

I tend to use fresh darks every time and if I'm going for shortish (10-15s) exposures I might take 20+ on the basis that I could end up observing for the equivalent length of time.

The idea is that what we're trying to measure is the true dark current (which we tend to think of as noise, but isn't), but the measurement itself is noisy, so we need lots of darks to reduce the noise in the estimate of the dark current. If our estimate is not a good one, dark subtraction runs the risk of introducing more noise into the image. My understanding is that the noise contribution coming from the darks (i.e. from the master dark) should be smaller than the noise sources in the lights, so as not to contribute anything significant. What this means is that in very dark skies we need more darks (in the same way that in such skies we should avoid using too short subs as it allows the read noise to show through). Conversely, in a light-polluted skies you can get away with fewer. If you can get hold of a copy, chapter 4 of Berry and Burnell's Handbook of Astronomical Image Processing is really useful on image calibration, but Craig Stark's series of article on SNR also do a great job (look under 2009-2010 here).

I use the time while taking darks to prepare my online notes, take a SQM reading,  fire up some John Coltrane or Bill Evans, or just to chill out anticipating the first object… I don't begrudge the odd 5-10 minutes or so unless it looks like clouds are expected.

Occasionally I top up the darks during the session but often forget. On nights like these with fast temperature drops I imagine it is more important to do so. 

Martin

[HiloDon]

Excellent results, Rob. You picked some tough subjects for first time. If M8 is still visible, try that one. There's no question that dimmer objects will get a bit grainy when stretched. You'll have some great objects to get once Orion comes back earlier, or you can stay up later. You should be able to get a nice HH/Flame combo. The Rosette is a good one, too.

The artifact you got on Navi (I think that's Navi) could be from something in the light path. Check out the scope closely and see if there's anything in the mounting of the objective or on the baffles in the tube. My Orion mini had a burr on one of the baffles that caused a similar problem. I sanded it down and solve the issue.

Don

[RobertI]

Martin - many thanks for the useful information on darks. I think it would help me if I understood the thoery a bit more So I shall read up on those articles. In the meantime I shall take fresh darks each time and make sure I get a good number. All part of the learning curve I guess.....

Don - thanks for the suggestions; to get some of those nice winter nebulae I shall have to have a session in a friend's garden where I can get a nice view south - I think the results will be worth the effort though. Regarding the artefact on the star, i was using a homemade dewshield which may be the cause you were describing. i shall investigate further, although I may ditch the finder guider for the 66mm or 72mm semi-apos plus reducer, which gives almost same focal length and F ratio as the finder guider, but theortically much better optics. It will be an interesting comparison.

thanks agin.

Rob

[Astrojedi]

Rob,

This is a great first start. You managed to do better than me when I first started viewing using a narrowband Ha filter. Most of the graininess in your images is due to the stretching of the histogram. I would also try viewing once without darks to see the difference.

Are you using the Lodestar or the Lodestar X2? The X2 has such low dark current that I just sometimes just view without darks.

If using darks I think what Martin said makes sense. The 'noisiness' of the sensor changes with temp and exposure time so fresh darks make a difference. I find that usually 10 are sufficient as after that given the inverse square law the incremental benefit starts to diminish.

Hiten

[RobertI]

Rob,

This is a great first start. You managed to do better than me when I first started viewing using a narrowband Ha filter. Most of the graininess in your images is due to the stretching of the histogram. I would also try viewing once without darks to see the difference.

Are you using the Lodestar or the Lodestar X2? The X2 has such low dark current that I just sometimes just view without darks.

If using darks I think what Martin said makes sense. The 'noisiness' of the sensor changes with temp and exposure time so fresh darks make a difference. I find that usually 10 are sufficient as after that given the inverse square law the incremental benefit starts to diminish.

Hiten

Hi Hiten,

Thanks for the kind comments and the advice. I am using the standard Lodestar so I guess that's the noisy one!

I like the idea of trying with and without darks - I shall try that next time out.

Rob