Will filters to improve visual observations of DSO on a small telescope? Any recommendations

[Giles_B]

It was a wonderful clear night in Bristol last night. I got some great - really great - views of Jupiter and Saturn - the atmosphere seemed crystal clear and they were probably the best I've managed since getting my scope, a 130mm Newtonian (Celestron starsense explorer 130AZ ).

Once again I was also draw to deep sky objects which I find fascinating (despite the light pollution of a big city). You can't see much from my garden, but I spent a good time looking at the Andromeda and the Hercules cluster. The view is very faint, and it is difficult to make out structure. I understand, thanks to discussions on SGL, that this is normal for visual astronomy and not helped by the limitations of my location and equipment.

While I'm content to persevere, I wondered if it would be worth getting any (more) filters for my set up. I have a moonlight/city light filter, which doesn't seem to do much to the view when I combine them with my 25mm BST explorer eyepiece, but I have read (I think) that some coloured filters can improve the view of DSOs.

Is it a pointless waste of my money to add filters when I could get a much better effect by driving 20 miles outside town to somewhere with less light, or would filters be worth it?

If so, can anyone recommend 1.25" filters that won't break the bank?

[RT65CB-SWL]

Hi @Giles_B and welcome to SGL. 

I find that using a UHC and/or OIII do improve viewing for DSO's. That said, I also use a Neodymium, (my 'Swiss-Army' knife filter), is my most used filter and I live in Bortle 6 skies.

[vlaiv]

Color filters won't work on DSOs so don't waste your money on that.

Only filter that stand a chance of making significant difference is UHC type filter (OIII as well - but is more expensive and more "specialist" filter - usually requiring larger aperture). However, it is only useful on emission nebulae type targets - like Ha regions and planetary nebulae.

Galaxies and clusters won't benefit from it.

[Pixies]

Dark skies trump everything for visual DSO observing.

IMHO

Edited September 7, 2021 by Pixies

[Stu]

56 minutes ago, Giles_B said:

Is it a pointless waste of my money to add filters when I could get a much better effect by driving 20 miles outside town to somewhere with less light, or would filters be worth it?

Yes! Certainly coloured filters are not worth it for DSOs.

For galaxies and clusters, dark skies are the most important thing, so the drive would be worthwhile. For emission nebulae, a UHC or OIII filter is well worthwhile. It is often commented that you need larger aperture to benefit from using them but I don’t agree. I’ve used OIII filters with 72mm refractors successfully, the key is to be properly dark adapted, get to a decent skits and also to keep the exit pupil quite large (4mm or more) which basically means not using high power. In your scope which is f5, something like a 25mm Plossl would give a 5mm exit pupil which would work well.

Aperture does help with objects like globule clusters as the additional resolution helps resolve the individual stars, but on the larger ones such as M13 you should still resolve stars around the edges.

[catburglar]

I personally don’t find much benefit from a UHC filter, but my skies may not suffer so much light pollution-my NELM is around 4.5. 

However I do find an OIII filter works well- even on smaller scopes. As Vlaiv says- it’s a bit more specialist than a UHC, but I find it more rewarding, so I no longer own a UHC filter at all.

I’ve tried a few different brands over the years- currently a Castelli from 365 astronomy which has a slightly higher bandwidth than some others and hence better for smaller scopes with less light grasp…

Planetary nebulae usually have quite high surface brightness (and there’s quite a few within the range of a 130mm aperture) so I find I can use an eyepiece  with an exit pupil of around 1-2mm on the ring, dumbbell and cat’s eye etc in a 90mm Mak.

And….just to tempt you a little more- by way of example. One of my Astro “Wow” moments was seeing the whole of the veil nebula in an 80mm refractor with this filter and a 30mm wide angle eyepiece from my back garden- so it’s not just for PN’s. I’ve also seen it in an Edmund Scientific Astroscan (approx 105mm f4ish) with a 16mm ultra wide angle eyepiece to- so for the dim stuff I think a 4-5mm exit pupil is better.

I know this goes against the trend somewhat, but based on my personal experience I would recommend an OIII as the first step into filtered views and get away to dark skies for the galaxies, globs and the like rather than use a UHC filter.

 

Edited September 7, 2021 by catburglar
Typos in 4th paragraph

[Giles_B]

Thank-you very much for these responses and apologies for my delay in replying - I guess picking up my scope and getting out of town is the best, and maybe I'm just getting over influenced by catburglar's recommendation, but I think I may start to seek out some emission nebulae with a view to getting an OIII if I can get a reasonable view (i.e. not obstructed by trees and houses from my garden) - there's nothing like the convenience of observing the sky from my backyard

[Giles_B]

16 hours ago, Stephen Waldee said:

The ONLY time I have ever heard a really elite, highly experienced deep sky observer recommend a simple colored filter for anything other than planets, is the suggestion to use a Wratten 12 (deep yellow) to see some faint details on certain galaxies. I can recall trying that a few times -- with NO significantly positive result.

So the only potentially useful filters for *extended objects* (i. e., nebulae or galaxies) will be in the following categories, according to most experts and filter marketers:

1. The LPR or light pollution rejection filter: it's a filter that cuts out the wavelengths of light generated by OLDER streetlights (high and low pressure sodium bulbs.) Since modern streetlights are often LEDs or halogen, the color balance is not yellowish, but almost bluish--and the LPR filters no longer work very effectively for VISUAL observing.  They CAN still help increase exposure times for imaging.  Now, this is the "mainstream" opinion.  I differ a bit, because I like to use the LPR or broadband type for faint objects at HIGH magnifications.  They may be ineffective at 30-50x, but try *200x* on some very faint objects, particularly planetary nebulae--even, sometimes, galaxies when you're trying to detect almost IMPOSSIBLY vague spiral arm structure.  In other ways of expressing this, use "small exit pupils" which, when you consult simple formulas for figuring this out from the eyepieces you are using in your scope, means in effect higher power.  I could expand this to about 15 pages of text at this point--neither you nor I would have the patience!

2. The other type of filter -- MUCH more effective than the above -- for "extended objects" EXCLUDING galaxies and star clusters, is the "interference line filter".  Depending on what wavelengths of light they block or admit, they are in some general types: "UHC" (ultra high contrast, which is really just a marketing term) which means in effect they are for gaseous nebulae, for the visible greenish wavelength of light known as H-beta (from atomic hydrogen.) Another type is the "O-III" filter: again, they are for nebulae but specifically ones that radiate predominantly the light waves of atomic oxygen: planetary nebulae in particular, though quite a few gaseous nebulae (like Orion's M-42) also feature the oxygen lines.  These filters work better at LARGER exit pupils than SMALL ones: i. e., at low to moderate rather than high magnification.

There are some others, including the Hydrogen-beta filter (pretty much for a handful of objects like the Horsehead and California nebulae); and the Hydrogen-alpha PHOTOGRAPHIC filter (not for visual observers.)

You can get sometimes, depending on many factors, SOME slight enhancement of contrast on star clusters, globular clusters, and galaxies with the LPR type; but it's pretty much hit-and-miss.  However, the LPR filter DOES often work well on *reflection nebulae* like the glow suffusing the star cluster M-45, the Pleiades.

I should stop here, because I've written a very large and comprehensive web paper on the entire subject of filters for deep sky observing, with internal sections that have hyperlinked observing reports of hundreds of objects with which I've used filters (mostly in scopes from 3 to 11 inches' aperture.) Let me merely state that *filters are NOT only for large scopes* (though some people think so if they have not done the correct experimentation with the right amount of magnification for the filter-type.)

There is also a general introduction, as well as some 'niche' material for more advanced users.  

http://celestialregina.x10.mx/horsehead/NebulaFilters.htm

 

I started developing this web report in 1997 and am still adding to it, refining it, and (yes, I have to admit) occasionally CORRECTING it!

In years previous to that I worked here in California USA with Orion Telescopes as the Product Development Manager and was responsible for testing and putting together the catalogue articles and user sheets for the "UltraBlock" filter; I later worked with my friend Dr. Jack Marling of Lumicon on a software program -- a "telescope/filter simulator" -- for calculating best performance ranges for his line of filters, with specific objects, scopes, and magnifications.  I have used these kinds of filters since around 1983...

Good luck--I could not IMAGINE observing without a full complement of them!  Below is my "filter case", with the specific models described on this webpage:

http://celestialregina.x10.mx/horsehead/filter/filter14-SRW-eypc-filter-case.htm

 

Steve Waldee

http://celestialregina.x10.mx

or http://reginacelestial.byethost3.com

 

 

Cripes, that's a long post, but a really informative one! Thanks Steve, I'll try experimenting with different magnification and filter combinations,which is something I've not considered before...

[Stu]

Great post @Stephen Waldee, informative and amusing, what’s not to like? 😀👍

I totally agree with your comments about the merits of dark adaptation and the time it can take. Certainly in the limited times I’ve been to decently dark sites, an hour of dark adaptation is a good measure of how long it can take. I recall on a couple of consecutive nights observing the North America Nebula. The first night I looked at my phone on full brightness and was not able to see it even forty five mins later. The following night with similar conditions (transparency) I was much more thorough in getting dark adapted and saw it easily.

I think one of the problems many of us face in regular observing from gardens and local sites is being able to get properly adapted because of glare from streetlights or security lights. I know that in my new location, although the skies in my garden are pretty good, there are some very bright security lights on a community building nearby which make life difficult. I’m thinking about some form of shielding to allow me to maintain dark adaptation better.

I confess I do use a phone with SkySafari  when observing, set to red and dimmed down as low as possible. Most of the time this makes little difference due to the general surrounding brightness but I’m sure under the darkest of skies it will still affect adaptation.

I’ve done a couple of outreach events with local schools recently and whilst it is understandable, I find the Health and Safety aspects quite frustrating. Rather than teaching the children to safely move around in the dark with a torch, bright lights are left on which precludes observing anything other than the brightest objects.

Finally, I again agree with the point about dark adaptation being important when using filters such as the OIII. I think there is perhaps a temptation to think that you pop a filter in and everything becomes easy to see. They obviously make things darker, but improve the contrast, so if anything dark adaptation is MORE important when using filters.

Thanks for the post, I shall try to find time to follow up the links you’ve posted.

[Pete Presland]

Fascinating thread, thank you to all that have contributed 👏 @Stephen Waldee you will find SGL a very friendly and open to nearly all opinions, so nothing to worry about there.

Very interesting articles you have shared there regarding dark sky adaption and filters, really nice to hear it is from personal experience. 

The idea of preserving the "night vision" of your eye piece eye (that looks odd 🙂 ), is something i have also done on many occasions. The difference even after a short time in my light polluted garden is remarkable. One of the big mistakes also, is observers do not spend enough time on one object.  As @Stu said "pop a filter in and everything becomes easy to see. They obviously make things darker, but improve the contrast, so if anything dark adaptation is MORE important when using filters" is spot on!

As @Stu said one of the major problems we face, certainly in the UK is finding somewhere local with anything like dark skies. I went to Norfolk last year, not far from Kelling where Asto camps are held. The skies were amazing compared to what i have locally. 

Thanks once again for the in-depth links, P.S are the "Astrogoggles" available in the UK? 🙂 

[Stu]

Don’t worry @Stephen Waldee, we are a fairly laid back and friendly bunch over here and don’t put up with overly heated discussion or putting down of views. Passionately held views are fine, so long as differing opinions are respected or responded to pleasantly and I’m sure that is your style too. If your intent is good, we will come and pull you out of any scrum you get stuck under 😉

You have a wealth of knowledge  which exceeds that of many or most of us here, so I’m sure we can learn a lot from you.

The challenge most of us face is being able to relate to what it is like observing under a properly dark sky. They are few and far between in the U.K. so true dark adaptation is something which few of us experience very often. Doesn’t mean we don’t like reading about it though!

Stu

[Louis D]

13 hours ago, Stephen Waldee said:

This same guy chided my wife at another star party.  Regina knows ALL ABOUT dark adaptation; she's an occasional observer.  She had brought her iPad and was showing some kids around 6 or 7 years old her star chart--the iPad screen was set to "all red".  This same guy -- well meaning chap, but misguided -- came over and in a booming loud voice said. "TOO BRIGHT! You will ruin their dark sensitivity!!" The instant he said this, a car turned the corner in the parking lot were we had the star party and pulled up RIGHT in front of all of us...his brights on!  There are SOME situations where you just CANNOT get dark adapted; so why try?  (Regina was so peeved that she said she would rather NEVER see this guy again.  Club President!  Some diplomat...)

Pretty much the reason I refuse to join the local university based astro club.  The leadership is very officious and takes all the joy out of observing.  The suburban clubs are much more laid back.

[Louis D]

13 hours ago, Stephen Waldee said:

Below: My Better Half, wearing her Astrogoggles (one of the products I introduced when I

was the Product Development Manager of an American telescope sales company.)

I remember those in the Orion catalog (maybe not with your wife wearing them, though).  So we have you to thank for them?

[Dr Strange]

Welcome to SGL!

Time for me to put in my (inflation adjusted) £0.07...

A couple of things...

First and foremost the more you observe, even from your garden, the more details you will see. Observing is like any other activity. The more you do it the better you will get at it and your eyes are no different.  When I started in the hobby I could barely make out details on things like Andromeda and Hercules Globular. As I kept plugging away more and more details showed up. Now, even from my near city center light polluted skies, I can see details I couldn't see when I started. Including on Orion. Before the nebulosity was very faint and I was lucky to catch the A-C stars. Now it is very distinct and I can easily see the A-D stars and many times the E star as well as a detailed amount of nebulosity on good nights. 

Second, make sure you have a comfortable chair. This isn't for your rear end it is for your eyes. The more comfortable you are the more you will see.

Third, there is no way around it. Petrol to take you to a dark(er) sky is the best investment you can make. The darker the sky the more you can see. Even a modest change in light pollution makes a big difference. For example going from my Bortle 8/9 home garden skies to a Bortle 6/7 site on the outskirts of the city make a very noticeable difference. While somewhat bulky and awkward to carry your scope and accessories can ride with you on public transportation if need be so this is one option. The other is put it in the boot and off you go. Look for a local club you can join. Many times they will have scheduled dark sky nights where the club gathers at a certain location. It can be a nice outing.

If there is a She Who Must Be Obeyed in the picture I have found that if I pack a simple but pleasing dinner basket, a blanket to sit on, some pleasing music softly playing on the phone, and a bottle of wine it changes from "Oh Lord! You are going out to do that stupid thing you like to do in the dark! AGAIN!" to a "date night under the stars" where she shares the experience with me. 

Lastly, in answer to your question, there are some objects where a filter will help. That would be a OIII filter used on nebula. I have tried both UHC and OIII and OIII works better. This was also found by a gentleman on Cloudy Nights website who did a test of various filters. On galaxies and clusters (open and globular) a light pollution or OIII/UHC filter will actually dim the object down because certain bands of light will not get through. So if you are going to invest in a filter OIII would be it. Don't bother with colour filters are a waste of time and money. Even on the planets. And I don't like a moon filter. It tends to degrade the image in my opinion. I like to view the moon in all her light. Of course she can be a harsh mistress (boldly pinched from Robert Heinlein's The Moon is a Harsh Mistress book title)  and if the light bothers you then one is warranted but I would rather be moon blind for a few seconds in one eye and see her in all her glory than diminish her. 

Also consider double star observing and carbon star observing. This can easily be done from your garden and it is quite fun. Or pick up a solar white light filter for the front of your telescope and take a (safe) look at the sun. A nice atlas of doubles is the Cambridge Double Star Atlas. FLO has it. https://www.firstlightoptics.com/books/the-cambridge-double-star-atlas-2nd-edition.html