O-III Filters For Visual Observing...

[bomberbaz]

On 09/12/2022 at 02:48, Louis D said:

It's so difficult to get above a 4mm exit pupil with an f/10 SCT or f/12 Mak, let alone an f/15 Mak.  You need a 40mm or longer eyepiece just to get close to 4mm.  50mm or more is better.  Then the problem becomes you're looking down a straw.

Agree with you Louis and to be fair I should have been more precise in my previous comment.  An average off the peg scope of much more than F6 is not going to provide a readily useable exit pupil for a line filters unless we are talking light buckets in my opinion. However when we get to light buckets, we get typically get to F ratios of F4 and below.

[Franklin]

My scopes are F/7.7 and the largest aperture is 103mm, so no light-buckets here. I have two nebula filters, a Baader UHC-S and a Badder OIII (visual 10nm) and even though the OIII has much more effect, ie. darkening of the sky background, I much prefer using the UHC-S. It helps with contrast against my light polluted skies but still allows plenty of field stars to be viewed.

[bomberbaz]

3 hours ago, Franklin said:

My scopes are F/7.7 and the largest aperture is 103mm, so no light-buckets here. I have two nebula filters, a Baader UHC-S and a Badder OIII (visual 10nm) and even though the OIII has much more effect, ie. darkening of the sky background, I much prefer using the UHC-S. It helps with contrast against my light polluted skies but still allows plenty of field stars to be viewed.

If you could get a use of an eyepiece around 36mm then this would give you an exit pupil of 4.7mm which would lighten things up and brighten the nebula too for the OIII, on the downside your only going to be around x20 so only decent sized nebula (eg M42, Crescent, NGC 7000) would work with this, hence my reference to larger apertures and lower focal lengths being more suitable for line filters.

However, I reckon any of your scopes would work really well with a UHC especially if you get into skies which are sub B5. I use UHC on my large bins (80mm) and it responds really well, had some cracking views of the lagoon nebula earlier this year as an example.

I too have a UHC-S filter and generally it is used on my fracs but it has had occasional use on the T350. Where it will come into it's own is when cranking up the power on planetary nebula although this is where you OIII should also stand out as previously mentioned.

[Franklin]

25 minutes ago, bomberbaz said:

If you could get a use of an eyepiece around 36mm then this would give you an exit pupil of 4.7mm

I have an LVW 42mm which is 2" and my UHC-S is 2" as well, but my OIII is 1.25" and the longest fl 1.25" eyepiece is LVW 22mm. They all have an effect but my skies here are around B7 so I prefer the UHC-S. I'm keeping hold of the OIII though for when I can get out to some darker skies.

[bomberbaz]

12 minutes ago, Franklin said:

I have an LVW 42mm which is 2" and my UHC-S is 2" as well, but my OIII is 1.25" and the longest fl 1.25" eyepiece is LVW 22mm. They all have an effect but my skies here are around B7 so I prefer the UHC-S. I'm keeping hold of the OIII though for when I can get out to some darker skies.

That should give some cracking widefield views using a frac. 

[Franklin]

3 hours ago, bomberbaz said:

That should give some cracking widefield views using a frac.

Sure does and without too much distortion at the edges in the 3" and 4".

Nearly 9 degrees in the little FL55SS, but that's got some curvature at the edges to begin with. Lovely little scope but more suitable for imaging with a flattener I think.

[bosun21]

Another 👍 for the Astronomik OIII and also the UHC. On starting out I originally bought the cheaper OVL/Skywatcher. Then the slightly more expensive SVbony filters before doing what I should have done in the beginning and bought the more expensive Astronomiks. Both these filters are invaluable in drawing out the details with deep sky targets. If you can’t stretch to the Astronomiks, then I can recommend the SVbony UHC.