Astrodon 3nm vs 5 nm Filters

[JohnC64]

I am looking at getting some Astrodon narrow band filters (OIII and possibly SII) to complement my Ha Filter.  I currently have a 3nm Ha filter and it is absolutely fabulous. Very small stars and with a nice contrast.

I was wondering if I went for the 5nm filters would they match the 3 nm HA that I already had.  I didn't want to  end up with odd colour star halos when i combined the narrowband images.  The reason for the question is that the 5nm are £330 each compared to the £500 each. I can afford one 3nm filter (0III) or both at 5nm.

The other thing that I am wondering if the exposure times for the 5nm would be less than the 3nm.  So more photons would be getting through so my reasoning is that more signal would be getting through.

Decisions decisions .  Any thoughts would be gratefully received.

Cheers John

[Tim]

£500 seems a lot for a filter (Sii) that blocks out most of the sky!!

I don't imagine exposure times would be noticeably different between the two wavelengths John.  I find Sii imaging to be singularly boring and unrewarding personally, with such weak signal on the majority of targets you need loads of exposures to get the noise anywhere near acceptable levels. For this reason I prefer to use my red filter and shorter exposures, along with short Ha exposures to gather a false Sii layer. When you compare the results from a long Sii exposure, and a short Ha exposure, they are very similar for a lot of objects. Depends how sciencey you want to be I guess?

If it was my £££ I'd be going for the 3nm Oiii filter, and doing bi-colour or HST as above.

FWIW, Hubble narrowband images have different star sizes with haloes 

Tim

[Gina]

I agree.  I would also recommend the 3nm OIII filter.  In fact it's more important to have a 3nm OIII than a 3nm Ha though I have to say I found the 3nm Ha a vast improvement over the 5nm.  Lovely tight stars and much better having matching bandwidth so that the stars matched.  I also bought a 3nm SII but I can get very little out of it and rather regret buying it - I think an NII 3nm filter might have been a better buy but NII is only present in a few DSOs rather like SII.  TBH I'm finding I only use Ha and OIII filters - SII seems a waste of otherwise good imaging time that would be better spent capturing Ha.

As for different B/W and the amount of wanted light... I find them the same. What differs is the amount of unwanted light which is more with the 5nm B/W.  In some cases the 5nm Ha filter seems to give more image but this is due to the NII component which the 5nm filter passes and the 3nm doesn't.  There would appear to nothing gained by a wider OIII filter from the image point of view.

[Sp@ce_d]

When I ordered mine with the QSI Ian King talked me out of getting the 3nm Ha & SII, so I went for the 5nm. OIII was the only one I ended up with a 3nm.

[Davey-T]

Been pondering on this for a while, bought the Astrodon 3nm HA so ATM I've got the 3nm HA and the 5nm HA in the wheel but haven't got round to a side by side comparison due to the weather.

Seems to be conflicting advice on whether the 3nm O111 is better than the 5nm O111 but will probably get one anyway and see for myself at least as far as my camera /  scope is concerned.

Dave

[JohnC64]

Many thanks for all the replies.

I think that I will go for the 3nm OIII. It was always my original  intention, its just when you get to the opening the wallet stage, you begin to have doubts :-)

I will probably go for HOO imaging initially and try the false SII technique that Tim mentions as it looks easy to gather the red channels at the same time. 

Many thanks once again

John 

 

[ollypenrice]

It's alright for you, John. This size of Astrodon a free compared with the 2 inch... If Ibought a 3nm OIII I'd have to have myself arrested.

Olly

[JohnC64]

Hi Olly,

Yes the prices at 2" get heart-stopping. One thing to be said for the QSI camera is that although it is expensive to start off with,  you can use the 1.25" filters. (Mentally rationalising the purchase of more astro equipment) :-)

JOhn

[Filroden]

This might be completely dumb, but if you own a 3nm Ha filter and a 5 or 7nm Ha filter, could you take equal total exposures from both, subtract them and effectively get the NII signal? So you could map 3nm Ha to one colour and the difference between 5 or 7nm and the 3nm to another colour?

[Gina]

I wondered that myself.  I shall be trying it but I don't know when.  I have 5nm and 3nm Ha Astrodon filters and could easily have both in my EFW mini.  Have to say, I shall be very interested in trying this and it would be nice to make use of the 5nm filter.  Of course the stars in the difference image will be little rings rather than dots   But I could get over that by removing the stars from the difference and substituting the 3nm Ha ones.

Forecast is dreadful but not always right.  BBC forecast agrees

[JohnC64]

There is an interesting graph on page 4 the following pdf from the Astrodon website. http://www.astrodon.com/uploads/3/4/9/0/34905502/astrodonnarrowbandfaq.pdf

It shows that the 3nm filter does indeed remove most of the effects of NII and a 6nm filter would capture it.  I think Gina has hit the nail on the head and pointed out that the stars would be troublesome to correct. It would require star mask to remove the stars from one image and replace then with the starts from the 3nm.  It would require ninja level PixInsight skills :-)

It is worth an experiment though, especially before paying out for a dedicated NII filter.  The Experiment would also show what NII signal was available in the objects we image too.

A very interesting idea to try out though
John

[Sp@ce_d]

2 hours ago, JohnC64 said:

There is an interesting graph on page 4 the following pdf from the Astrodon website. http://www.astrodon.com/uploads/3/4/9/0/34905502/astrodonnarrowbandfaq.pdf

It shows that the 3nm filter does indeed remove most of the effects of NII and a 6nm filter would capture it.  I think Gina has hit the nail on the head and pointed out that the stars would be troublesome to correct. It would require star mask to remove the stars from one image and replace then with the starts from the 3nm.  It would require ninja level PixInsight skills :-)

It is worth an experiment though, especially before paying out for a dedicated NII filter.  The Experiment would also show what NII signal was available in the objects we image too.

A very interesting idea to try out though
John

Yeah.. thats what Ian K 's reasoning for going 5nm was really.. Any NII you'd actually want to capture anyway. It's hard enough getting Ha & OIII time on a target let alone SII. Adding NII into the mix.. I dunno about you but unless there's some miracle longevity pill soon the road ahead of me is shorter than the one behind by some distance!!