Is it worth getting another visual filter?

[RobertI]

Not sure where to post this - hopefully this section is ok. I recently treated myself to a ZWO manual 5 position filter wheel to allow me to switch between filters when viewing DSOs. It's actually quite a lot of fun switching between the filters for certain objects. Currently I have installed the following filters (see also graph screenshots):

• Astronomik UHC - Good all purpose narrowband filter for emission nebulae
• Astronomik OIII - 'Line' filter, great for certain emission nebulae and for "blinking" to find those elusive planetaries 
• Astronomik UHCE - Designed for smaller apertures, higher transmission, getting closer to a 'broadband' LPR filter, good for comet ion tails due to carbon line (apparently)

I'm primarily using it with the 102ED at the moment but could easily switch to the C8.

I leave one slot empty for normal viewing, so the question is what filter could I put in the fifth slot? I have thought about a blue filter for planets or a neutral density filter for the moon, but most of my lunar/planetary is done with bino-viewers and I'm pretty happy with the unfiltered views (plus wouldn't reach focus with filter wheel), so I'm thinking of possibly a really good light pollution / skyglow filter for DSOs. Most of my light pollution is LED these days so not sure what would work. Is there anything that might make a difference for galaxies and reflection nebs? I suspect not. Would a Neodymium do anything more than my existing filters? 

Any thoughts/advice appreciated. 

EDIT: I tried the UHCE on M33 the other day under fairly bright skies - it darkened the background and made the view a bit more appealing, but didn't reveal anything extra, but also didn't seem to lose anything. 

Edited October 30, 2022 by RobertI

[Graham Darke]

I’m not convinced that anything is to be gained from broadband light pollution filters for visual observing in these days of LED streetlights. Maybe in the past when things were predominantly low pressure sodium they were more useful but less so now. I think you have the most useful filters already in your UHC and O III. The only other one to consider is the H Beta but that is only effective on a couple of objects. I have one of Baader’s Moon and Skyglow filters and it makes no visible difference to my eyes. 

[RobertI]

Yes I think you’re right about the LED lighting. The only other thing is that I believe sky glow emits in a fairly narrow band, which might be filterable, but I don’t know how much of my intermittent bright skies is due to sky glow and how much is just high level mist/murk. Since posting the above I have been thinking that perhaps the Hb might be worth a go…..

Edited October 30, 2022 by RobertI

[Stardust1]

Rob, this is very interesting thread. I'm thinking of getting a filter wheel as well for visual. Are you using 2" or 1.25" filters? 

[RobertI]

1 minute ago, Stardust1 said:

Rob, this is very interesting thread. I'm thinking of getting a filter wheel as well for visual. Are you using 2" or 1.25" filters? 

I’m using 1.25” filters. So far it’s working well, it’s nice to have the filters ready to try and the filter wheel sits permanently on the diagonal, but gets removed when I use BVs for planets/ lunar. The filter wheel is very well made. 

[Don Pensack]

You're missing an H-ß filter, and the Astronomik is a good one.

For a light touch on enhancement for non-nebular targets, try the CLS filter instead of the UHC-E.

[Louis D]

You can pick up a Neodymium filter for cheap these days as one of the cheap moon & skyglow filters out there on ebay or other sites.  They are basically the same as the more expensive ones, but with a tad less transmission.  That way, you could see for yourself if they enhance any objects.

You would just need to make sure you're getting a low profile one.  I just noticed a bunch of them are high profile now that won't fit in your filter wheel:

[bosun21]

I have found that filters are of little to no use when galaxies are concerned and prefer viewing them without. On the odd occasion a Baader neodymium skyglow can help 

[RobertI]

8 hours ago, bosun21 said:

I have found that filters are of little to no use when galaxies are concerned and prefer viewing them without. On the odd occasion a Baader neodymium skyglow can help 

Yes it makes sense, I know that there is no logical reason that a filter should help with galaxies unless there is filterable sky glow, and in theory a filter would actually remove the fainter galaxy detail. I was viewing M33 with various filters last night to try and find NGC952 (I couldn’t). It did darken the sky and improve the aesthetics of the Galaxy as a whole, but I cannot say it made a difference to the discernible detail, which is what you would expect I guess. The UHCE was the best and it did make me think that perhaps a CLS would be worth a try, but also the Skyglow might help on certain nights. I think there is a bit of psychology going on here too, when you have experienced how well narrowband filters work with some emission targets you can’t help thinking you are seeing the same result on all faint fuzzies,  but clearly you don’t. All of this reinforced that the best way to reduce sky brightness without affecting the target is to go to a dark site. 🙂

[pavel_s]

Have a look at Baader UHC-S which is known as a good broadband filter.

[Louis D]

If it ever arrives from China, I'm going to give the Svbony UHC a try.  For only $19, I figured it was worth a go.  Based on Star Hunter's light pollution filter test, it appears to be about the same (with a bit less blue) as the Baader UHC-S for a fraction of the price.

No filter:

 

Baader UHC-S 2″:

 

Svbony UHC 1.25″:

Spectra:

Edited December 18, 2022 by Louis D

[Paz]

I'm  late to this bit I would recommend two... a Hb filter and a clear focusing filter that is parfocal with all the others.

I find with uhc filters etc it is hard to get a good focus as everything is dimmer. I like to focus on something bright enough to make it easy. Getting the focus pinned with using a parfocal clear filter and then switching to a dim view that is already in focus is great.

[Franklin]

Another tip, if using eyepieces with enough eye-relief, is to hold the filter between finger and thumb and just swing it in and out of your view. That way the effect, if any, becomes immediately apparent.

[Don Pensack]

1 hour ago, Paz said:

I'm  late to this bit I would recommend two... a Hb filter and a clear focusing filter that is parfocal with all the others.

I find with uhc filters etc it is hard to get a good focus as everything is dimmer. I like to focus on something bright enough to make it easy. Getting the focus pinned with using a parfocal clear filter and then switching to a dim view that is already in focus is great.

You don't focus on the nebula, you focus on the stars in the field (and there is always a star to focus on).  And unless the filters are perfectly parfocal, which they realistically are not unless extremely expensive, like Astrodon filters, having the clear filter is a waste of time.

Because you'll have to refocus on the next filter anyway.  The H-ß filter will be the darkest of all, so having it follow another filter with a wider bandwidth might be a good idea.

[Don Pensack]

34 minutes ago, Franklin said:

Another tip, if using eyepieces with enough eye-relief, is to hold the filter between finger and thumb and just swing it in and out of your view. That way the effect, if any, becomes immediately apparent.

This process, called "blinking", doesn't work well in light polluted environments because the ambient light reflecting off the nebula filter is brighter than what can be seen through it.

It works OK in really dark skies, but, as a technique, its true value is in planetary use, where you can quickly see what filter you want to use to yield the detail you want to concentrate on.

[Louis D]

16 hours ago, Don Pensack said:

It works OK in really dark skies, but, as a technique, its true value is in planetary use, where you can quickly see what filter you want to use to yield the detail you want to concentrate on.

I can vouch for that.  I have been able to quickly try multiple filters in this manner and home in on the best 2 or 3 for a given planetary object.  Then, with two hands, I can move two filters in and out of the light cone in quick succession, and sometimes even stacked, to tease out details.  If one seems to be working especially well, I'll go ahead and screw it into the eyepiece yielding the best view for that night's conditions.  In this manner, I can rapidly try out multiple combinations of eyepieces and filters, wasting less precious observing time futzing about with equipment.

Edited December 28, 2022 by Louis D

[Paz]

14 hours ago, Don Pensack said:

You don't focus on the nebula, you focus on the stars in the field (and there is always a star to focus on).  And unless the filters are perfectly parfocal, which they realistically are not unless extremely expensive, like Astrodon filters, having the clear filter is a waste of time.

Because you'll have to refocus on the next filter anyway.  The H-ß filter will be the darkest of all, so having it follow another filter with a wider bandwidth might be a good idea.

Yes I use stars in the field to focus as they are brighter and higher contrast than extended objects but the brighter they are the better. I star hop manually and if a nebulous target is in the middle of nowhere surrounded by dim stars made dimmer by a filter I will struggle focusing, and moving away to something brighter in order to focus can be a lot of hassle.

I use parfocal baader filters when I'm using a filter changer and it works well for me. I admit I don't know if other manufacturers make their filters parfocal.

[Louis D]

1 hour ago, Paz said:

I use parfocal baader filters when I'm using a filter changer and it works well for me. I admit I don't know if other manufacturers make their filters parfocal.

Especially if, like me, the observer uses filters from multiple manufacturers and even multiple generations from a manufacturer (looking at you, Lumicon).  In that circumstance, there's little hope of parfocality across filters.

I have been known to swing over to a nearby star to achieve best focus and then return to the nebula.  This is best done in one axis if using an alt-az mount to make getting back more repeatable.

[Louis D]

I've gotten a couple of new filters (Lumicon's second most recent OIII that they're clearing out for $30 with their MOVINGSALE2023 code and the Svbony UHC), but no time at night to try them out yet.  I imaged their spectra through my spectrograph and came up with the following:

The Lumicon UHC and OIII Old both date to the late 90s.

The Zhumell barely does anything for OIII being too far right of the lines, but might work as a comet filter and the C lines.

The new Lumicon leaks an unnoticeable bit of red, so a major improvement over my vintage filter.  In fact, the vintage Lumicon OIII is more of UHC with no H-beta line.

The Svbony UHC is more of a light pollution filter than a true UHC filter.  It might also make a decent magenta filter on Mars.

I hope to try them all out on the Orion nebula in the near future.

[Louis D]

On 15/01/2023 at 21:08, Louis D said:

The Zhumell barely does anything for OIII being too far right of the lines, but might work as a comet filter and the C lines.

I verified tonight that my old Zhumell OIII does indeed work quite well as comet filter when viewing Comet C/2022 E3 (ZTF) tonight in my GSO 6" f/5 Newt under Bortle 6/7 skies.  I was able to see the greatest extend of its coma with this filter in place.  The Lumicon OIII just made things worse by blocking the comet's light, so it's definitely the right shifted nature of the Zhumell bassband working to let through the C lines.  Too bad they aren't sold anymore.  I wonder if any currently available cheap OIII filters have this same off-OIII band performance.  I'm glad I kept it around.

The Svbony UHC was second best, helping to increase the amount of coma visible by blocking light pollution.  The Zhumell Urban Sky (Moon & Skyglow) filter helped a tiny bit, but not enough to want to go back to it a second or third time.

On the Orion Nebula, the Lumicon OIII was best with the Svbony UHC very close behind by also passing the H-Beta line.  The UHC provided a more pleasing view by showing stars better.  The Zhumell OIII made it somewhat more difficult to see the nebula because I think it is passing only one of the two OIII lines, and then just barely.

All of this was done early before the full moon completely washed out my skies.

[RobertI]

Interesting read, thanks Louis. I also tried some filters on the comet over the weekend with a bright moon washing everything out. I didn’t have time to do a proper assessment and the bright sky was a pain. The Astronomik UHCE has been sold as helping view ion trails but sadly not a hope! The best view was no filter with a higher mag to darken the sky, but I’ll try again when the moon has gone, assuming the comet is still around! 

[Don Pensack]

12 hours ago, Louis D said:

I verified tonight that my old Zhumell OIII does indeed work quite well as comet filter when viewing Comet C/2022 E3 (ZTF) tonight in my GSO 6" f/5 Newt under Bortle 6/7 skies.  I was able to see the greatest extend of its coma with this filter in place.  The Lumicon OIII just made things worse by blocking the comet's light, so it's definitely the right shifted nature of the Zhumell bassband working to let through the C lines.  Too bad they aren't sold anymore.  I wonder if any currently available cheap OIII filters have this same off-OIII band performance.  I'm glad I kept it around.

The Svbony UHC was second best, helping to increase the amount of coma visible by blocking light pollution.  The Zhumell Urban Sky (Moon & Skyglow) filter helped a tiny bit, but not enough to want to go back to it a second or third time.

On the Orion Nebula, the Lumicon OIII was best with the Svbony UHC very close behind by also passing the H-Beta line.  The UHC provided a more pleasing view by showing stars better.  The Zhumell OIII made it somewhat more difficult to see the nebula because I think it is passing only one of the two OIII lines, and then just barely.

All of this was done early before the full moon completely washed out my skies.

IIRC, the Explore Scientific O-III filter had a wide enough bandwidth to reach the 511 and 514nm C2 lines in the spectrum.

And it wasn't the only one.  I'll have to go back through my notes.

[Louis D]

My two OIIIs are both narrow enough to not cover both sets of lines, thus I found myself having to swap between them based on the object.  Stacked, they pass barely any light and view darkly on all objects.

At least I finally found a good use for my decade old Zhumell OIII filter.  It was never very good for its intended purpose.  It was only once I saw where its passband fell in my spectrograph imagery that I had my aha moment it might make for a good C2 line filter.  Last night I confirmed it.

I don't know if I'd pay $100+ for a Lumicon Comet filter just to get the views I was getting last night of Comet C/2022 E3 (ZTF), but if Svbony would market one for under $40, I'd probably recommend it.  They could add it to their UHC, CLS, and M&SG filter package.  I have no idea if their OIII filter is any good.  Hopefully it is on-band like my Lumicon and not off-band like my Zhumell.  Basically, any OIII rejects that have a too far right passband could find a new home this way.

[Don Pensack]

54 minutes ago, Louis D said:

My two OIIIs are both narrow enough to not cover both sets of lines, thus I found myself having to swap between them based on the object.  Stacked, they pass barely any light and view darkly on all objects.

At least I finally found a good use for my decade old Zhumell OIII filter.  It was never very good for its intended purpose.  It was only once I saw where its passband fell in my spectrograph imagery that I had my aha moment it might make for a good C2 line filter.  Last night I confirmed it.

I don't know if I'd pay $100+ for a Lumicon Comet filter just to get the views I was getting last night of Comet C/2022 E3 (ZTF), but if Svbony would market one for under $40, I'd probably recommend it.  They could add it to their UHC, CLS, and M&SG filter package.  I have no idea if their OIII filter is any good.  Hopefully it is on-band like my Lumicon and not off-band like my Zhumell.  Basically, any OIII rejects that have a too far right passband could find a new home this way.

Though a number of the wider UHC filters pick up the C2 lines, only one of the O-III filters I had went up that high, and it was the Explore Scientific.

My Optolong O-III did, too, but I read they've changed its bandwidth to a narrower bandwidth.

[Louis D]

I just checked my spectrograph test images, and the Svbony UHC does indeed go far enough right in blue-green to completely encompass the Zhumell OIII's passband.  I even have a stacked image showing little to no change in the latter's passband when stacked with the former.  This actually agrees with what I observed last night with the comet.  The Svbony UHC made it easier to see the comet's coma against the sky background while the Zhumell OIII improved contrast even further.  The Lumicon OIII just made everything darker when observing the comet.  Thus, the Svbony UHC and Zhumell OIII were showing the C2 lines while the Lumicon OIII was blocking them.