How to read filter graphs

[Dom1961]

A lot of people are recommending me filters and explaining why with the graphs....one problem...i have no idea what the lines on the graph means, HELP??

[ronin]

Usually they are transmission against wavelength.

Which filter ??

Something like the Baader UHC transmits 460 to 530 nm, then it drops to zero, remains at zero until 630nm then transmits until 700nm.

After that you are in the IR area and relevant (maybe) if imaging, but I tend to consider it less relecant for visual.

In effect the transmission is 2 sections of: 460 to 530, then 630 to 700, anything not in these being effectively zero.

Becomes really horriable if you think of combining 2 filters which are not very simple.

Again which filter are you looking at?

What do you want a filter for?

One aspect that is usually overlooked is that a filter will/can only remove wavelengths. whatever comes out is lees then went in.

[Cath]

A good place to start, might be to first understand what is meant by 'frequency'. Then once you have an understanding of that, graphs that show a frequency/wavelength response will suddenly make sense.

[Dom1961]

Explore scientific UHC

[John]

I should not worry about trying to interpret the graphs in detail. Most of them look more or less the same and many are not too accurately scaled anyway !

One UHC will have broadly the same light band pass width as another. There are slight variations brand to brand but if they vary too much then they are not UHC filters any more !

Similarly for O-III filters. The Baader O-III is a bit narrower than other O-III filters though so that might be one to pass over unless you have a 12" or larger scope.

Find out what others with a similar scope are using with some success and that are within your budget and go for the same.

Filters take away light (selectively), they don't make things brighter. It's important to understand that in considering the effect they will have. They are not really "magic bullets" apart from on a couple of objects.

[ronin]

From looking round it is transmitting 460nm to 530nm at 95% or a bit more gets through.

Then nothing until 640nm to 660nm where again about 90% or more gets through.

460nm is blue and green: The filter is specifically passing through the Hydrogen beta lines and the OIII lines.

640 to 660 is the red and again it is specificattly passing the Hydrogen alpha and Sulpher II lines

[Dom1961]

That is still confusing me....

[Cath]

Yes the numbers will not mean anything to Dom if they don't understand the basics.

One question I would have for you Dom, is not meant to as anything but to first enquire about your visual sense of the world - do you have full colour vision of the world around you do you know ?

[LukeSkywatcher]

I have quite a few filters and have no clue as to what the graphs mean. I put my faith in the members of SGL and tell them what i want and they set me in the right direction. I usually pick the middle of the road (price wise) filters (LP,UHC,OIII). The only filters i didnt skimp on were the ND3.0, Solar Continuum and Polarizing filter for my Hershel wedge.

[reiner]

I have measured a lot of filter spectra and written a short tutorial on how to read filter graphs and how to decide on the right filter.

It is here

http://www.reinervogel.net/Filter/Filter_e.html

Maybe this will help.

Reiner

[Dinosour23]

You may inquire about the nature of rainbows through google, then you will understand what light is made of and how filters may remove some "colors" so you get a better contrast in your eye.

[Cath]

Yes but you're assuming the OP knows what 'colour' means/is .. something we don't yet know the case too.

Not everyone does.

[jetstream]

I would buy either the Lumicon or Astronomik UHC. I purchased a different brand and it doesn't work well, a poor example I think. My money could have (should have) gone towards one of the mentioned two.

[SpaceWalker]

FWIW, there is a short tutorial on filters on this site,   Primer - Understanding and Choosing Filters for Visual Use.

[SpaceWalker]

Similarly for O-III filters. The Baader O-III is a bit narrower than other O-III filters though so that might be one to pass over unless you have a 12" or larger scope.

John,  I respectfully disagree with your last statement. I have the Celestron OIII which is actually the Baader rebrand. It  works good on my 8" Dob. The Baader /Celestron OIII is slightly narrower than similar competitors but quite acceptable as a budget solution.  IMO, as long as  one maintains optimal magnification range (e.g. 5-9x/inch of aperture recommended by David Knisely) everything should work fine independently on aperture size. Just saying.

[Dom1961]

FWIW, there is a short tutorial on filters on this site,   Primer - Understanding and Choosing Filters for Visual Use.

That is great thanks!!

[bingevader]

If this is a question about understanding how to read the graphs, then they're not that difficult actually, interpreting them is another matter!

Basically, if you are plotting light transmission against wavelength and both values increase along their individual axis, then a peak in the graph indicates that light is being transmitted.

The height of the peak indicates the amount of light (often a percentage) being transmitted.

The width of the peak indicates the spread of wavelengths of light being transmitted.

The troughs in the graph indicate where light of a particular wavelength is being blocked.

As John has said, a UHC filter will only remove unwanted light from other objects and so make your desired target more obvious, it will still be faint and fuzzy if that's what it was before.

In fact, it's likely to be a bit dimmer too, as most of the UHC filters have the side effect of not transmitting the full 100% of the desired wavelength.

They are useful however, and I've enjoyed using my ES UHC, if still only on a limited number of occasions.

It does have the other side effect of giving some objects a slightly green tinge.

I didn't like this much on M42 when observing the nebula as a whole, but the filter did help to tease out detail that I couldn't see otherwise.

[John]

John,  I respectfully disagree with your last statement. I have the Celestron OIII which is actually the Baader rebrand. It  works good on my 8" Dob. The Baader /Celestron OIII is slightly narrower than similar competitors but quite acceptable as a budget solution.  IMO, as long as  one maintains optimal magnification range (e.g. 5-9x/inch of aperture recommended by David Knisely) everything should work fine independently on aperture size. Just saying.

I guess it depends on your observing preferences. I've owned both the Baader O-III and the Celestron branded version (you are right - they are the same) and found that the narrow band width pass reduced the brightness of background stars too much for my observing preferences - I like to see some stars to set the DSO in context. I used these filters in 4" to 10" scopes but found the views delivered by the Astronomik O-III and latterly the Lumicon O-III preferable to me.

So not a disagreement - just different viewing preferences

[alro]

I have skys the limit ht uhc, for me it is fantastic. Although at £40 it alllegedly holds it own and fairs well to the top leading brand names in this field, read into that what you want.....

worth a punt imo.

al

[SpaceWalker]

I guess it depends on your observing preferences. I've owned both the Baader O-III and the Celestron branded version (you are right - they are the same) and found that the narrow band width pass reduced the brightness of background stars too much for my observing preferences - I like to see some stars to set the DSO in context. I used these filters in 4" to 10" scopes but found the views delivered by the Astronomik O-III and latterly the Lumicon O-III preferable to me.

So not a disagreement - just different viewing preferences

Thanks for the clarification, now I see what you mean. Indeed, just different preferences. When I observe nebulae I don't care about the contest too much, that's why I was surprised.