How to compare filters

[Mike_S]

Do you study the charts on what light it lets through quite extensively or do you mostly do trial and error? For example. You decide you want a OIII filter. How do you choose other than budget and what the manufacturer says it’s suitable for? Seen a few second hand around £85 but can get an Astronomik 1.25” for that brand new. How do you decide which to go for? Word of mouth? I bought a 2” skywatcher UHC filter that didn’t make any difference on Orion. Just bought cheap because didn’t know what to go for. Might try a cheap OIII filter just to see. Perhaps the cheaper ones let more light through for light polluted skies or small scopes? No idea really. 

Edited July 15, 2019 by MSammon

[John]

These days I tend to read reports and reviews then rely on the brand and it's reputation. I've owned Lumicon, Omega DGM and Astronomik in the UHC, O-III and H-Beta types and found them very good, as has been reported by others. Some less effective ones that I've owned included low cost UHC's (eg: Svbony, Optolong) which seemed rather marginally effective and the O-III's branded by Baader and Celestron (which are one and the same item) which are decent quality but, I now realise, have rather narrow band pass widths which results in a dimming effect on background stars that, for me, is too much.

To be fair all the narrowband (eg: UHC) and line (ie: O-III and H-Beta) filters that I have owned and used have had some effect but the Lumicon, Omega DGM and Astronomik had noticeably more impact. At a lower cost than those brands but still quite effective were UHC filters branded Telescope Services and the Orion (USA) Ultrablock, also a UHC type which I owned a few years back.

When looking at the band pass charts I look for a pass of 90% or higher in the permitted band widths, little or nothing outside of those band widths (or at least nothing that will impact visual observing), and sharp cut offs and a flattish top to the curve rather than a long thin taper. This profile seems to produce the most impact when observing suitable targets for such filters. The manufacturer ones can be a little mis-leading but there are independant band pass measurements that have been done on filters which are worth looking out for.

I don't pay much attention to manufacturer or vendor "blurb" to be honest. Forums such as this and the "Cloudynights" one where Don Pensack and David Knisely are well worth following for advice on filters, are my principle resources, plus my own experience now.

This might seem a little weary but I have to say that the more costly filter brands (ie: Lumicon, Astronomik and Omega DGM) do seem to have performed better for me than the less costly ones. From what I've experienced in lower cost filters, their band pass widths tend to be over-generous and the % max pass rates on the low side which results in less of a contrast enhancement when observing.

All filters take away light rather than add it of course but selective and effective admission of the right wavelengths darkens the background sky and permits the emissions from the target object to show up that much more.

 

 

[Ricochet]

1 hour ago, John said:

The manufacturer ones can be a little mis-leading but there are independant band pass measurements that have been done on filters which are worth looking out for.

This is a very good point. The graphs are only useful if they accurately reflect the performance of the filter (and you understand how to interpret the graph).

[Mark at Beaufort]

I have a Lumicon UHC and an Astronomics O-III. If you don't want to spend too much and want an O-III that does make a difference I would recommend this Castell filter from 365 Astronomy. I have the 2" version which gives me great views of the Veil in my 12" Dob.

https://www.365astronomy.com/Castell-OIII-Deepsky-Filter-for-1.25-Inch-Eyepieces.html

[Ben the Ignorant]

Teleskop Austria measures your filter's curve for 19€.

https://teleskop-austria.at/Filtertest_Filtertest-Wir-vermessen-die-Transmissionskurve-Ihres#m

[jetstream]

6 hours ago, MSammon said:

do you mostly do trial and error?

This is very costly- just ask me.

I would heed Johns advice for brands and maybe place the NEW Televue line in there too, which might be top line Astronomiks.

I am still leery of Lumicon eventhough I have 2 superb ones... BUT...the 2" Hb I purchased tainted me for the brand possibly forever. Same goes for Kniselys Orion Ultrablock recommendation, I got burned on that one too.

Knisely is a fantastic resource and honest- some of these brands can have extreme sample to sample variation though and brand quality can change as well ie Lumicon.

Our DGM NPB is very good, the older Astronomik OIII is VG but a bit wide, the older Astronomik Hb of mine is superb, and the older non laminated Lumicon UHC and OIII are the top of the top... these are my good filters.

If it were me I'd go Astronomik or NEW Televue. Eagerly waiting @John test results of the new Televue filters.

[Mike_S]

Can anybody confirm if Castell and Skywatcher are technically the same?

no worries if not. Looks like probably a Castell 2” or an Astronomik 1.25” for now. Not going to pay 2” Astronomik costs new. 

Edited July 15, 2019 by MSammon