Moonlight and a 7nm Ha v's 3nm Ha filter comparison

[swag72]

You may have seen that I did a comparison between my Baader 7nm Ha filter and my Astrodon 3nm filter http://stargazerslounge.com/topic/187535-comparison-between-7nm-ha-filter-and-3nm-ha-filter/. In this thread came up the suggestion about a comparison in moonlight. As it was clear on either side of the full moon and I didn't have a target in mind, I decided to oblige!

Here's the details so that you can make of it what you will.

My target was the soap bubble (a rather faint planetary nebula near NGC6888).

On the night before the full moon, I took 5x1800s subs with the 3nm Astrodon Ha filter. It was calibrated with flats. Last night (1 night after full moon) I took 5x1800s subs with the 7nm Baader Ha filter of the same target. Again, flats were used.

Each subsequent stack was given a histogram stretch in PI so that the peak of the histogram was in the same place and there was no black point clipping at all.

The kit used to capture the data was the same in each case, the only change being the Ha filter and also the position of the rather large, almost full moon!

Firstly, I am pleased that even with just 5 subs under an almost full moon I was even able to capture the soap bubble. That gives me some hope for some other faint targets that I have my eye on! Also, it elimates my sofa time on full moon periods as I didn't feel happy with the data I was getting under those circumstances. That will please my husband as he rather enjoys sole command of the remote control andultimate control over the TV programme choices!

Hope you feel that this is a fair comparison - I really couldn't do it on the same night as I rather like my sleep!! If you can think of a way I can make it a better comparison, then do say.

[michael.h.f.wilkinson]

Interesting comparison. The 3nm really has it under these conditions

[fwm891]

Hi Sara, 3nm has certainly pulled out more of the bubble, just seems that some of the stars with the 7nm seem tighter (to me)?

[Gina]

Hi Sara, 3nm has certainly pulled out more of the bubble, just seems that some of the stars with the 7nm seem tighter (to me)?

Yes, the 3nm is certainly better But I agree, it's rather odd that the 3nm hasn't produced tighter stars - maybe the seeing or transparency wasn't as good on that night. I take it the focus was alright.

Once I get my DIY FW done I can compare 7nm Ha with my 5nm Astrodon in the same conditions. Of course, I don't expect as much difference as with the 3nm Astrodon.

[Ant]

I am surprised, but there is no doubting the evidence!

Being two nights apart the position of the moon will be quite different - what were the angular distances from target to moon on the two nights. This will make a difference, but irrespective of that it still shows that the 3nm filter has picked a huge amount more data than the 7nm.

The stars are also quite a bit brighter on the 3nm - whats the cause of the flaring towards the bottom of the stars? I had something similar and eventually worked out that it was focuser slop (not a lot, but enough). I then preceded to break the focuser while trying to solve it!

Ant

[swag72]

Regarding star sizes etc, it could easily be due to transparency. This is a pretty massive crop into the whole frame, so I'm not too worried about potential issues as I have taken enough images now with this setup to know that at normal sizes there are no issues. I take on board your point about angular distances with the moon etc, but short of staying up all night and doing one image with each filter, then refocusing and moving to the other, this was about as good as I could get it - I'm rather partial to sleep

I was specifically wanting to see if the 3nm really does pull out more detail than the 7nm in the original thread, then this time more specifically under moonlit conditions.

I know that this is not massively scientific, but I decided that it may be a little bit of evidence if anyone is looking at the difference in detail that can be obtained between a 7nm and a 3nm filter. My work now is done on this - It's been sufficient for me to convince hubby that I need the rest of the 3nm filters!!

[Jessun]

Thanks for taking the time to do the comparison Sara. Helpful for us all, and especially one like me on the verge of pressing that order button...

/Jesper

[johnrt]

Hmmm, what a turn up. I note the inconsistencies, - transparency, luna position relative to the target, but this speaks volumes. I think you are quite correct the 3nm filter is a class above the 7nm.

Now I feel very sorry for myself with a 12nm Ha filter in my wheel

In fact I am pulling the same face as Frankie in my avatar.

[davew]

Those images are so far apart they could come from two different cameras !

Trying to match collection from two separate nights is a no win situation. Those are convincing results.

The small stars in the 3nm aren't disappearing as they did before and it all looks smoother too.

Dave.

[swag72]

I agree Dave, I think that there is a marked difference between the two, despite the variations in data capture. It's for the individual to decide whether this additional data (as I see it) is worth the money. The Baaders do perform well, and the Astrodons are mighty expensive.

All I hope I have done here is presented the difference in as controlled and methodological way as I can.

[ollypenrice]

Excellent post, Sara. While under perfect skies I didn't find an overwhelming difference between filters the story in moonlight is totally different. I've also seen Per using an Astrodon here in the bright moon recently and, again, I'm satisfied that it is a class above.

Damn.

Olly

[opticalpath]

..... Now I feel very sorry for myself with a 12nm Ha filter in my wheel

Don't despair ..... even the 12nm Ha filter can capture the elusive Soap Bubble. This is 18 x 600 seconds, binned 2x2, using the Astronomik 12nm Ha filter from a suburban site. I had to stretch the heck out of it to see it though, and the star halos are pretty ugly!

Adrian