Refractor - Achromat (Chromatic Abbreviation) > SEMI APO

[daverave83]

Hey all,

i am looking at different filters to see which results reduce CA, hopefully by the end of my investigation i can provide people who have Refractor Scopes better imaging views particulary on Deep Sky Objects.

So what is Chromatic Abbreviation?

This is where the spectrums of light don't focus at the same length, for example the focus is correct for Yellow and Green, but the out of focus is Blue/Red which is why you get Purple Halos.

I have searched all over the internet for guides and way around to remove the CA, unfortunately there is no way to remove 100% of this,  unless you focus on each spectrum of colour which can be very time consuming and expensive.

Whats Filters will be the investigation?

Wratten 8 =

Baader Longpass 495nm =

Baader Finge Killer =

SkyGlow & Moon Filter =

Once the weather clears i will try out each filter and post the final result.

[emadmoussa]

Thanks for that.

 CA will not have much of an effect on DSOs though. It will be a problem if you're looking at bright objects such as the moon and planets...bright stars will produce negligible amount of CA depending on how corrected your achro refractor is.

[michael.h.f.wilkinson]

It's chromatic aberration. CA is caused by the fact that light of different wavelengths is refracted differently by glass (an effect known as dispersion). This is how a prism works. In an achromatic lens, two types of glass with different dispersion values are used to cancel out the CA for two wavelengths.

[daverave83]

Apparently the Wratten 8# and Baader Long pass 495nm filter reduce CA on stars upto 80-90%.

for Example M42

Notice how the blue stars can't focus properly, this creates an Halo affect

Another example is IC434 - Flame and Horse

CA is alot more distinctive.

[michael.h.f.wilkinson]

The mean issue is that reducing CA in an achromat boils down to reducing the range of wavelengths to those for which correction is best. That usually means clipping off the red and blue. So if a scope has been corrected for e.g. one orange-red wavelength, and one blue-green, selecting two passbands close to these corrected wavelengths can help (or one passband spanning the two spectral lines). Alternatively, just pass a single narrow line of an arbitrary wavelength. Using an H-alpha filter eliminates CA, at the expense of seeing only one red line, and having to refocus when moving to another wavelength.

[daverave83]

That kind of makes sense.

I currently have an HA Filter, so this means i would have to refocus on the blue spectrum? the question is how would i do this?

So lets say i got M42 in my live view on my DSLR and got the focus to a pinpoint star, i would assume that it is looking at the Green/Yellow wavelengths how would i refocus on the Blue Wavelengths? could a OIII filter do this?

[ronin]

The way to focus on one colour is to remove the others.

When you said:

This is where the spectrums of light don't focus at the same length, for example the focus is correct for Yellow and Green, but the out of focus is Blue/Red which is why you get Purple Halos.

You also have the option to focus on the Blue/Red and so get Yellow halos.

Adding filters simply removes part of the spectra, that is the reason it is called a filter: It filters out wavelengths.

There is a history of people trying to add filters to an achro to try get an apo, probably over 20 years of it without any success. If you do not want CA with an achro then as you cannot alter the angle of refraction of a wavelength that leaves removing the offending wavelengths from the system. If you put a good narrowish red filter in you get a good image with no CA, just it is completely red, same for yellow, green and blue.

[daverave83]

Agreed that you can't remove the wavelength completely as this due to the glass and the way it bends colours, but looking to ways to minimise it with a low cost.

Looking at the prices of APO's and EQ5 you are looking at spending 1k+, however if you are thinking of dipping their feet into AP at a low cost i want to look at filters that can reduce the amount of CA to semi APO standard.

[Macavity]

I think part of the problem is that e.g. the Baader "Fringe Killer" only removes about 50% of the acromatic "purple haze". Ever worth checking out filter transmission curves! Maybe indeed you might look for a more drastic "minus blue" filter - Essentially, I suspect, "light yellow" or yellow "long pass"? F5 achromats are great fun, but for apertures greater than 3", the false colour increase a fair bit? But, for (theoretically) the same amount of colour, you could go to a 5" F8. The problem with FAST acromats is not merely limited to chromatic aberration though... There is a fair amount of spherical aberration too. But an EvoStar 120mm retails for £240 i.e. less than the price of an 80mm APO. On a restricted budget, or with an existing scope, always worth a go. Might work for you. 

You could do a (forum) search re. "Chromacorr". But I sense no FREE lunches? 

(But worth checking out John et. al.'s experiments / thoughts on this device...)

[daverave83]

Thanks Mac
 
Baader Fringer Killer does indeed only remove 50% of the CA, and the price of them is about 50 quid.
 
However reading various forums Baader Longpass 495nm filter can almost turn an Achromat into an AP, the only difference is the stars are either orange/yellow instead of the normal contrast.
 
http://www.cloudynights.com/ubbthreads/showflat.php/Number/5047210
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6113911/page/0/view/collapsed/sb/5/o/all/fpart/1
 
My experiment is to see if at low cost you can get near enough the same images you get from an APO, eventually i will go for a Skywatcher Evostar Apochromatic ED80 Pro on a EQ5 Pro Mount, but that is a long term save.

This the detail i have found 

Application:
Suppresses the secondary spectrum (blue) at achromatic refractors. Image definition is increased.

[Macavity]

Well, great minds (here and elsewhere!) clearly.

But I had been pondering simple ways to reduce the "halo" around stars, on my "Electro Finder".

- Essentially a standard finder (60mm f/3 achromat!) with a high-sensitivity B&W video camera.

Sadly, I just got rid of a set of (Wratten-type) filters, but the Baader looks interesting...

(Greener filters are usually band-pass, and would clobber the redder stars etc. etc.)

[Space Dragon]

I've tried the Baader Semi-Apo, Fringe-Killer and 495 Longpass with my 152mm F5.9 and have kept the 495.

It removes all of the CA on bright objects and also tightens up focus....stars are pinpoint.

There appears to be no light loss either, no darkening of the image and it now lives in the 2" diagonal.

[daverave83]

I've tried the Baader Semi-Apo, Fringe-Killer and 495 Longpass with my 152mm F5.9 and have kept the 495.

It removes all of the CA on bright objects and also tightens up focus....stars are pinpoint.

There appears to be no light loss either, no darkening of the image and it now lives in the 2" diagonal.

Excelent 

[lensman57]

Thanks Mac

Baader Fringer Killer does indeed only remove 50% of the CA, and the price of them is about 50 quid.

However reading various forums Baader Longpass 495nm filter can almost turn an Achromat into an AP, the only difference is the stars are either orange/yellow instead of the normal contrast.

http://www.cloudynights.com/ubbthreads/showflat.php/Number/5047210

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6113911/page/0/view/collapsed/sb/5/o/all/fpart/1

My experiment is to see if at low cost you can get near enough the same images you get from an APO, eventually i will go for a Skywatcher Evostar Apochromatic ED80 Pro on a EQ5 Pro Mount, but that is a long term save.

This the detail i have found 

Application:

Suppresses the secondary spectrum (blue) at achromatic refractors. Image definition is increased.

Hi,

I am really sorry to to disagree with the notion that a £50.00 filter will turn a badly designed Achro into a Takahashi. Kodak no8 mid yellow and no 12 , minus blue , have been in use in terrestrial photofraphy for decades to reduce the Lum value of the blue or cloudy sky and increase contrast with Panchromatic film emulsions , no 12 has the more severe effect as it totally balckens the sky and the shadows that are mostly illuminated by blue portion the light as they bend easier than the red. As these filters remove a great portion of the blue light IMHO they have no place in AP whatsoever and are of limited use in observing, if an achro is what you are observing with and either because of the design or fast F ratio it sufferes from excessive CA then perhaps a filter that attenuates the blue the least maybe used for casual observation purposes. Personally I'd prefer a little CA to false star colours and lack of a full spectrum of light.

Regards,

A.G

[daverave83]

Hey there.

I didn't say that a filter would turn an Achro into Tak, however it is proven that a Longpass 495nm does reduce CA upto 80-90%, these filters were specially made for Achro due to it's limits hence the reason why they also brought out the semi apo also, so people dont have to spend vasts amount of money on a tak and still get good performance.

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6294607/Main/6294607

check the 3rd image, for an achro that is impressive, so understand how you can disagree and that is your opinion however proof is in the pudding as they say and good quality images can be done but not as superior as a tak.

[lensman57]

Hey there.

I didn't say that a filter would turn an Achro into Tak, however it is proven that a Longpass 495nm does reduce CA upto 80-90%, these filters were specially made for Achro due to it's limits hence the reason why they also brought out the semi apo also, so people dont have to spend vasts amount of money on a tak and still get good performance.

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6294607/Main/6294607

check the 3rd image, for an achro that is impressive, so understand how you can disagree and that is your opinion however proof is in the pudding as they say and good quality images can be done but not as superior as a tak.

You are correct, I think for visual they should be fine as long as the yellowing effect is not too much, stiil I'd say that a well designed ahcro F10~F15 does not have that much CA so a filter may not be necessary, some of these have as little CA as a fast imaging ED or even APO. Please keep us informed as to the results.

Regards,

A.G