What are some must have filters for dob?

[mbaddah]

Just wondering if there are any "must-have" filters someone with a dobsonian should have, many thanks.

[erick]

In my short experience?

Short answer is none.

Long answer is that most people end up with a pile of filters, used somewhere between sometimes and never!

My 2 cents worth:-

I guess you have a "Moon filter" already, which is some form of neutral density filter. Try this also on Jupiter and Venus when they are particularly bright.

A better option for this application is to buy two polarising filters and fit both as a set of crossed polars - just rotate the bottom one relative to the upper one until you have the brightness that is comfortable for you. A couple of 1.25" polarising filters are cheap (I got mine from www.aoe.com.au (http://www.aoe.com.au)).

I've never had great success with coloured filters, but I'm still learning. Here is a good site describing their application:-

http://sciastro.net/portia/advice/filters.htm

I do put a yellow filter on when I'm showing the sun through a white light solar filter (Thousand Oaks, over the end of the scope) - so people see the sun "nice and yellow", the way they drew it in kinder! Remember, the only place for a solar filter is at the field end of the scope, not the eyepiece end!

That web reference also describes other specialist filters that attempt to remove light pollution or enhance nebula emissions relative to the sky or other light sources.

I particularly like the DGM Optics NPB filter:-

http://www.iceinspace.com.au/forum/showthread.php?t=22373
http://www.iceinspace.com.au/forum/showthread.php?t=23307

Lots of good threads on filters in the forum, just search, eg:-

http://www.iceinspace.com.au/forum/showthread.php?t=21056


Size:-

Your problem is deciding whether you need a filter in 2" since or 1.25" size?? If you can afford the 2" size, buy it. Then it can go on the end of a 2" eyepiece, or on the end of the 2"--> 1.25" adapter. Then you can change 1.25" eyepieces and not have to worry about swapping the filter to the new eyepiece. BE CAREFUL - some 1.25" eyepieces (eg. GSO plossls) are long and will hit the filter when you drop them into the adapter. Either tighten them up not fully in (and you'll probably forget) or put some form of ring on the eyepiece to prevent it going fully in (Your local Clark Rubber store has a stock of rubber O-rings. I have some which fit fairly neatly over the 1.25" barrel and prevent the eyepiece going all the way in. Not a precise solution, but it works.) However, if you are going to drop in a 2x or 3x 1.25" barlow, it will definitely be too long and will hit the filter.


Tip: If everything looks wrong, just check carefully that you haven't left a filter on somewhere and forgotten about it. Gee the sky is dark tonight! Woops, Moon filter still in place! :lol:


Have fun!
Eric :)

[PhilW]

I too find the Omega Optical/DGM NPB filter to be the one essential for deep-sky observing (link below).

One handy accessory is a filter slide. My homemade one is shown in this photo. This removes the possibility you'll drop your precious filter in the dark. This slide also has an O-III filter fitted, but the NPB is the one I use 80% of the time.

http://www.omegafiltersebuyer.com/servlet/the-NPB-Filters/Categories

Phil

[Starkler]

Im with Erick on this.


Coloured filters - Tried them once, didnt do much for me. Taking up space in my eyepiece case now.

Variable polariser - Cuts the brightness, but by throwing away light you throw away ability to see the finest of details. I traded mine for a ronchi ep.

Green moon filter - ergh!!!

UHC filter - useful for teasing extra detail from nebula. Amount of use that it gets its relating how much I can bothered screwing filters on and off as i change eyepiece, ie not much. If i got hold of a nice filter slide I might invest in a 2" OIII filter.

[janoskiss]

I agree with short answer: none.

Narrowband nebula filters are nice especially on larger Dobs. On big monster Dobs they are amazing. DGM NBP is a good value well performing general purpose nebula filter.

Lot depends on what aperture you have in mind.

[rmcpb]

Possibly the only one that could be considered "essential" is the variable polariser for the moon. Otherwise, my coloured filters spend most of their time just taking up space in my case and the OIII and UHC filters are for later on in your observing career.

Make haste slowly.

[OneOfOne]

I would have to say none are essential, although it is surprising the difference a UHC makes when observing nebulae in the 'burbs. The contrast between the sky and target is boosted quite a lot. I tried putting the UHC on the scope at Snake Valley, and I believe it made the image worse. I guess if the light pollution in the sky is a limiting factor, it will help. If the sky is basically pretty dark then the filter will just reduce the amount of light you see...the contrast is as good as it is going to get.

I have a polarizer for the Moon when it gets bright, but when it is near full I have no real interest in looking at it anyway!

If you "had" to buy some filters, I would get an UHC and an OIII. I very rarely use my colour filters.

[GeoffW1]

:rofl::doh:Never thought of that!:lol: I'm going to try it this very weekend with some colleagues at the farm:thumbsup:

Cheers

[wavelandscott]

Basically I agree with the above statement...none are essential or completely necessary.

However,

A Moon filter is a nice thing to have and I use mine regularly...

If I was only going to have one filter it would be the DGM Narrow Band Pass (sold by Omega optics in the US can be bought via internet) filter is also a good all purpose nebula filter...or Astronomics UHC (I've owned both but prefer the DGM)

To complete the visual package an OIII would round things out...I have the DGM OIII...I haven't really compared others...

Good Luck!

[ngcles]

Hi MBADDAH & All,

I'd have to respectfully disagree with a few opinions offered above, while agreeing with some others. In making this post I assume you own a telescope of at least 8" aperture. I won't comment on brand though.

For mine, the coloured filters are a waste of time. I prefer the natural view of planets and I don't think there are big gains to be made in contrast with the use of coloured filters. They are of no help I can see on deep-sky stuff. I own a 23A light red that gets used for 2 mins, once every 5th pancake day on Mars. I also think a SWAN-band filter can occasionally be useful on comets -- particularly on the ion tails.

Deep Sky observing is very different.

There are three basic types of visual-use filters --

(1) "Deep-sky" type filters that are supposed to make a general improvement on all types of objects by blocking light-pollution type wavelengths.

(2) "Narrow-band" filters (like the UHC filter) that are designed to pass both the doubly ionised oxygen lines (ie O III) _AND ALSO_ the nearby H-Beta Line.

(3) "Line-filters" that are designed to _either_ pass the O III _OR_ H-Beta.

The Deep-sky filters are of very limited assistance. They really only work on nebulae and despite the claims, I think, actually make any other types of objects (eg galaxies, clusters) dimmer.

The Narrow-band or UHC type filter is an extremely useful one. It will make a considerable improvement on virtually any emission or planetary nebula, whether you are in the suburbs or under a dark sky. It is like bolting on an extra 2" of aperture _for those types of objects_. There are very few nebulae that won't look better with a UHC in 8" and above 'scopes. The exceptions are those few nebulae that shine at odd-wavelengths for some reason and/or don't have strong O III or H-Beta emission.

Line filters can be pretty useful too. The O III is best for planetary nebulae that frequently shine strongly in the dual O III wavelengths. The H-Beta filter is very specialised and only works well on a few select objects. The California Nebula (NGC 1499) and the Horsehead Nebulae complex (IC 434) are two prominent examples. Because these filters have such a narrow band-pass, I wouldn't use either of these line filters on 'scopes less than 25cm. I probably wouldn't use the H-Beta on less than 40cm actually.

In summary,

Deep-sky filter -- waste of time IMHO.

UHC -- Excellent choice for 8" and above 'scopes for a big general improvement on planetary and emission nebulae. Even works well on Horsehead and comets (ion tail).

O III -- For 25cm and above on Planetary Nebulae in particular will provide a further small contrast enhancement over the UHC.

H-Beta -- For 40cm an above on very faint nebulae that have significant H-Beta emission. In a 46cm 'scope, the improvement on the horsehead has to be seen to be believed.

Over time I've acquired

1.25" -- UHC and O III

2" -- UHC and H-Beta.

Best wishes with your choice!


Best,

Les D
Contributing Editor
AS&T

[mbaddah]

Thanks guys for all your replies, very informative.

It seems the two most popular filters to grab are the UHC and OIII. I currently own a 6" dob but will be purchasing a 10" really soon. Will listen to all your advice and stick to those two for now, perhaps slowly build up over time.

Cheers :D

[§AB]

I own a 10" Dob and I have recently bought Astronomik's UHC, OIII and H-Beta filters. I've tried the UHC and OIII on M42 and Eta Carinae and the contrast gain is MASSIVE, especially with the OIII. A good first impression.

Now if the moon would just kindly bugger off.

I can back you up on that....

[goober]

Great post, Les.

I have a narrowband filter that is only marginally useful (so far) in my 10cm refractor. I accidently left it in a couple of weeks ago when I swung over to Mars, and the detail I could see was amazing - no idea why!

Someone did a very comprehensive test of various filters against objects with a 10" newt., and weighted the results. It's interesting (but long)....

http://pages.sbcglobal.net/raycash/filters.htm

[ausastronomer]

I agree 100% with everything Les said above. I use a couple of different coloured filters every 3rd pancake day on the planets as opposed to Les who uses them every 5th pancake day.

Save your money on the variable polariser. I observe the moon in the 18" without a filter. Whilst it appears bright, you will not damage your eye. It takes a couple of minutes to re adjust, that is all. It is only "reflected" light off a surface with a similar colour and texture to asphalt. It only appears so bright because everything else is so dark. Look at the moon in daylight through your telescope and it isn't very bright. In fact 1/2 the detail is washed out. Use a moon filter, lose detail, your choice. I own a 2" variable polariser for the sole purpose of stopping my wife's moaning :)

The DGM NPB is a very narrowband UHC filter and is almost a "hybrid" UHC/OIII filter. It offers a noticeable improvement on more objects and a wider range of objects than any other filter I have used. Whilst on some selected objects the Astronomiks or Lumicon UHC's and OIII's will offer a more noticeable improvement, the DGM NPB does "something" to a lot more targets. It does surprisingly well on the Horsehead Nebula, Californian Nebula and Crab Nebula and a few other targets that normally a UHC or OIII doesn't help much with, notwithstanding the UHC's do help a little on the horse.

I would be buying a 2" DGM NPB filter if your 1.25"/2" adaptor is threaded. Otherwise a 1.25" version.

Cheers,
John B

[ngcles]

Hi All,

The post above by Goober contains a link to a page maintained by Ray Cash. The info was originally generated by Dave Kinsley. They are both US based amateurs. The page contains information (subjective evaluation -- it has to be said) about various improvements seen using different pass-band filters on a great variety of objects. Both Ray and Dave are expert observers and I would happily rely on their opinions.

This was the link posted by Goober (many thanks):

http://pages.sbcglobal.net/raycash/filters.htm

I believe it is a _must read_ for anyone considering a visual filter purchase for a mid-sized telescope.

BTW Ray Cash and another expert observer Steve Gottlieb have a terrific site containing a plethora of info on various deep sky observing projects here:

http://www.astronomy-mall.com/Adventures.In.Deep.Space/



Best,

Les D

[ngcles]

Hi All,

In addition, John B wrote above:

"I own a 2" variable polariser for the sole purpose of stopping my wife's moaning" :)

The only thing that would stop my wife's moaning is if the my telescope somehow rolled off a cliff ... :D

Best,

Les D
Contributing Editor
AS&T

[chunkylad]

Don't give 'em any ideas.............:doh:



BTW, I agree with the comments about the DGM NBP filter. It's the most used filter in my case.

Cheers

[jjjnettie]

I think that a UHC filter would be a good choice to start with.
One of my favourite objects is The Veil supernova remnant in Cygnus. Using a UHC filter makes all the finer whispy filaments stand out more. Beautiful.
I hardly ever use my colour filters.