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Chromatic Abberation filter - what size?

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#1 CeeKay

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Posted 20 February 2019 - 02:14 AM

Hello,

 

     I hope this is the correct forum to post this question.  I purchased an Explore Scientific AR 102 refractor and after a couple of weeks, was able to finally take it out on Sunday night.  To say I was floored with what I saw is an understatement - but I noted some CA when I was looking at the Moon, and saw it in the photos that I took of it during the session. 

 

     I knew that CA was going to be an issue before I had purchased it, so my question is - should I by a 1/25" CA filter for attaching on to my eyepieces (all I have are that size to fit my 30 year old C-8) or should I by a 2" to put on the diagonal and leave in place permanently? thinking1.gif

 

Thanks in advance to all. smile.gif

 

Chris


Edited by CeeKay, 20 February 2019 - 02:14 AM.


#2 Jim Waters

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Posted 20 February 2019 - 02:23 AM

Leaving it on the 2" diagonal may be better.  You will not have to move it from eyepiece to eyepiece.  Sorry you have so much CA.

 

What's the cost difference between the two sizes?


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#3 photomagica

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Posted 20 February 2019 - 02:25 AM

I have mine on a 2" diagonal. That's really convenient and still easy to remove when not wanted.

Bill



#4 Jond105

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Posted 20 February 2019 - 02:44 AM

Diagonal.

#5 sg6

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Posted 20 February 2019 - 04:18 AM

Using a 2" diagonal will be better, so that implies a 2" filter.

Not sure you will want it there all the time however owing to how a filter performs.

 

A CA filter is usually just removing a selection of wavelengths so what gets to the eye is less in brightness and also a change in color. In effect is it worth it but the idea of "remove" is not realistic since that means complete blocking of a fair selection of wavelengths.

 

I would also say a CA filter cannot "remove" CA, if you have a purple effect then to remove that means removing all red and all blue. A filter can realistically at best reduce CA, consider that a CA filter reduces CA, but does not remove or cure it.

 

Seeing CA on what is generally the edges of the moon makes me think "So what?". One side has a bit of red the other a bit of blue, hardly going to initiate the end of astronomy as we know it. I have a 102, Bresser 102/600, not sure which 102 you have, seems to be several of them between ES and Bresser I think there are 4 or 5 at assorted focal ratio's.


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#6 AndyBooth

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Posted 20 February 2019 - 06:44 AM

I have a skywatcher 102/600 achromat.

when I view an object that gives objectional CA to me, I use a 2” Baader fringe killer on the diagonal.

it gives a slight yellow cast, but removes the CA.

for fainter stuff, including doubles, i remove it to give a better colour rendition.


Edited by AndyBooth, 20 February 2019 - 06:47 AM.


#7 SeattleScott

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Posted 20 February 2019 - 08:42 AM

I thought the AR102 was fine for medium power DSO so I would just use it for Moon and planets, rather than having it dim and discolor my DSO views. Now maybe 2” is still better because then you can play around with magnification a bit without having to swap out filter. But not a big deal if just using it for planetary.

It can depend on which filter you get. For example fringe killer is mild and does not dim or discolor much, so you miss decide to leave it on. The more aggressive filters would probably be better just for planetary in your scope, unless you really like yellow stars.

Scott

Edited by SeattleScott, 20 February 2019 - 08:43 AM.


#8 Sketcher

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Posted 20 February 2019 - 02:06 PM

As the owner of a larger, fast achromat (among other scopes), I prefer having a 2" CA reduction filter screwed into the diagonal -- whenever I want to reduce the CA.

 

Why?  I find the filter most useful for lunar and planetary observing.  For those purposes, I tend to swap in and out eyepieces (or use a zoom) until I find the most suitable magnification for what I'm looking at under the atmospheric conditions I have to deal with at the time.  With the filter in the diagonal, I can swap in and out eyepieces without having to also swap the filter from eyepiece to eyepiece -- which could quickly become rather unpleasant on cold winter nights.

 

I would not leave the filter in the diagonal on a permanent basis.  For seeing the true (or as close to true as possible) colors of double stars, etc. the filter needs to be removed.

 

It basically boils down to a personal preference/budget issue.  A 1.25" filter would also work, but it looks like most of us prefer the added convenience offered by a 2" filter -- in the diagonal.

 

Hopefully you've already determined the specific CA reduction filter that you prefer to use with your telescope.  Different telescopes can react differently to different filters -- even when ignoring personal preference issues.


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#9 alex_d

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Posted 20 February 2019 - 10:08 PM

Hello,

 

     I hope this is the correct forum to post this question.  I purchased an Explore Scientific AR 102 refractor and after a couple of weeks, was able to finally take it out on Sunday night.  To say I was floored with what I saw is an understatement - but I noted some CA when I was looking at the Moon, and saw it in the photos that I took of it during the session. 

 

     I knew that CA was going to be an issue before I had purchased it, so my question is - should I by a 1/25" CA filter for attaching on to my eyepieces (all I have are that size to fit my 30 year old C-8) or should I by a 2" to put on the diagonal and leave in place permanently? thinking1.gif

 

Thanks in advance to all. smile.gif

 

Chris

I was under the impression the AR102 all but removed CA due to it being a "doublet" seems weird your noticing it at all, have you asked anyone at ES about it? My first scope was a Meade infinity 102 and CA was fairly obvious through that as well on the moon, Jupiter etc, i figured id use the scope for a couple of years making sure astronomy was my thing I then purchased a used SW120ED doublet, CA gone with that scope, immense difference I could never go back, I love this thing.



#10 BFaucett

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Posted 20 February 2019 - 10:30 PM

I was under the impression the AR102 all but removed CA due to it being a "doublet" seems weird your noticing it at all, have you asked anyone at ES about it? My first scope was a Meade infinity 102 and CA was fairly obvious through that as well on the moon, Jupiter etc, i figured id use the scope for a couple of years making sure astronomy was my thing I then purchased a used SW120ED doublet, CA gone with that scope, immense difference I could never go back, I love this thing.

 

Umm... Well, the ES AR102 is an achromat.  "Doublet" does not automatically mean ED glass.  

 

EXPLORE SCIENTIFIC AR102 AIR-SPACED DOUBLET REFRACTOR - DAR102065-01

The Explore Scientific AR Air-Spaced Doublet Series telescopes are classic air-spaced, crown-and-flint, doublet achromatic f/6.5 refractors with a superior-class optical figure.

 

https://explorescien...ublet-refractor

 

Bob F.


Edited by BFaucett, 20 February 2019 - 10:30 PM.

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#11 nicoledoula

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Posted 20 February 2019 - 10:30 PM

You might have a bad AR 102, or were using way too much magnification. I get objectionable CA on planets and bright stars but hardly any from the Moon.    But regardless, that scope is designed for wide fields. You could throw money at a problem that can't be fixed (turn the purple yellow?), or maybe a better solution would be to buy a 6" F/8 dob for planets. (around $300) Some will say an 8" or 10" of course. But I want cheapes, lightest weight, fastest cooling and easiest to collimate with a long focal length that's easy on EP's for planets. I already have a wide field. Want to kill planets but only have $300?  A 6" F/8 Dob can't be beat. Someone should make a collapsible version. For PLANETS.  If you someday decide you like Dobs and want to go bigger the next step up IMO is an 8" F/7, not an F/6. And those cost 2X more than an 8" F/6.   For $300 a 6" F/8 Dob can't be beat on planets. Nothing even comes close. 



#12 nicoledoula

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Posted 20 February 2019 - 10:33 PM

 "all but" is subjective. If comparing to a singlets level of CA, a doublet does remove most of the CA. 


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#13 BFaucett

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Posted 20 February 2019 - 10:37 PM

 "all but" is subjective. If comparing to a singlets level of CA, a doublet does remove most of the CA. 

 

I think alex_d thought the AR102 was an ED doublet.  See my post above (#10). 

 

Bob F.


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#14 alex_d

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Posted 20 February 2019 - 10:54 PM

I think alex_d thought the AR102 was an ED doublet.  See my post above (#10). 

 

Bob F.

No i was just going off what the website says about the performance of the AR102

 

"The AR102 102mm Achromatic Refractor Telescope from Explore Scientific features a doublet achromatic lens system which virtually eliminates chromatic and spherical aberrations. This translates to a distortion-free viewing experience with true color rendition across the entire field of view."

https://www.bhphotov...air_spaced.html

 

In other words, any CA should be negligible at best.



#15 Jond105

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Posted 20 February 2019 - 11:13 PM

No i was just going off what the website says about the performance of the AR102

"The AR102 102mm Achromatic Refractor Telescope from Explore Scientific features a doublet achromatic lens system which virtually eliminates chromatic and spherical aberrations. This translates to a distortion-free viewing experience with true color rendition across the entire field of view."
https://www.bhphotov...air_spaced.html

In other words, any CA should be negligible at best.


I think ES tried pulling a fast one on that description. It's an achro, let's call it, midspeed?, at 6.5. It probably keeps the CA down in its nice star cluster views, but high mag on planets and the moon will show the CA. Depending on the observer it could be bothersome or looked passed.
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#16 alex_d

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Posted 20 February 2019 - 11:20 PM

I think ES tried pulling a fast one on that description. It's an achro, let's call it, midspeed?, at 6.5. It probably keeps the CA down in its nice star cluster views, but high mag on planets and the moon will show the CA. Depending on the observer it could be bothersome or looked passed.

Still looks like a nice scope, I almost purchased one myself last year, if it hadn't been for Terry giving me a great price on his mint SW, id own one now.


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#17 Jond105

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Posted 20 February 2019 - 11:21 PM

Still looks like a nice scope, I almost purchased one myself last year, if it hadn't been for Terry giving me a great price on his mint SW, id own one now.


I actually did purchase one in December. Never even opened the box when it arrived. 5 minutes after I ordered it a used Ed100 came up for sale with a steal of a deal that I couldn't pass on.
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#18 SeattleScott

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Posted 20 February 2019 - 11:27 PM

Good old American advertising for you. Any bright object showed CA in mine. It was fine for low-mid power DSO but CA really interfered on planets and bright stars, and was noticeable on the Moon (although the Moon was still worth a look). Also DSO would lose some crispness compared to a 4” Apo at magnification over 100x. So virtually eliminate is kind of a joke. Virtually eliminated CA when viewing the double cluster at 30x, sure, I can go along with that.

Scott
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#19 alex_d

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Posted 20 February 2019 - 11:30 PM

I actually did purchase one in December. Never even opened the box when it arrived. 5 minutes after I ordered it a used Ed100 came up for sale with a steal of a deal that I couldn't pass on.

Nice!! Something special about that ED glass, couldn't ever go back.


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#20 alex_d

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Posted 20 February 2019 - 11:32 PM

Good old American advertising for you. Any bright object showed CA in mine. It was fine for low-mid power DSO but CA really interfered on planets and bright stars, and was noticeable on the Moon (although the Moon was still worth a look). Also DSO would lose some crispness compared to a 4” Apo at magnification over 100x. So virtually eliminate is kind of a joke. Virtually eliminated CA when viewing the double cluster at 30x, sure, I can go along with that.

Scott 

Ya, sounds like the infinity i had but probably a bit better however.. For a starter scope to learn/make mistakes on and decide if Astronomy is your thing, i think there is value to be had.



#21 Jon Isaacs

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Posted 21 February 2019 - 06:02 AM

I think ES tried pulling a fast one on that description. It's an achro, let's call it, midspeed?, at 6.5. It probably keeps the CA down in its nice star cluster views, but high mag on planets and the moon will show the CA. Depending on the observer it could be bothersome or looked passed.

A few years ago, the AR-102 was on sale for $300.  Despite the fact that I already owned a very well corrected 4 inch apo, temptation got the better of me and I purchased an AR-102.  

 

The chromatic aberration was quite apparent at higher magnifications, double stars and planets were the most affected.  In terms of various rules of thumb for chromatic aberration, the AR-102 has a chromatic ratio of 1.6, similar to an ST-80.  I liked the scope except the very large diameter dew shield, I could never get over the thing when I was using the scope.  

 

But, it is nice mechanically and optically within the limits of a 4 inch F/6.5 achromat and provided reasonable views of the planets and doubles, again within the limits of a 4 inch F/6.5 achromat.  I think it makes a nice companion to a larger reflector or SCT that can be used when the best possible views of the planets are desired.. 

 

As far as 1.25 inch versus 2 inch CA filter, I have never used a 2 inch, mine have been 1.25 inch filters that I just swapped out.  A 2 inch would avoid that but for me, there were only a few eyepieces that I have used it with so it's worked for me.  If one has a 1.25 inch Barlow like the 2X GSO, the lens can be removed and the filter threaded on in place of the lens and the swapping can be avoided.

 

Jon


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#22 AxelB

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Posted 21 February 2019 - 08:01 AM

If you have a 2" diagonal, I suggest you put a 2" Baader semi-apo filter in front of it as soon as you plan to use magnification above 80x. Here I’m extrapolating from the experience with my AR127. Actually, unless you need as much light as possible for a particular dim and large target, you can leave it there all the time. That filter will not only reduce CA significantly, it will also reduce light pollution and skyglow (it’s like two filters in one: minus violet and a Moon&Skyglow). That filter will be great for Jupiter and also for genersl observing and it doesn’t impact color perception too much. Some CA will still be visible at high power on super bright targets but it will be well tamed compared to unfiltered view.

Because I have another better scope for planetary observations, I rarely push my AR127 above 80x but a filter still helps at medium power (+ the light polution and contrast enhancement properties are still appreciable).

If you want total reduction of CA, you can use the super agressive Baader Contrast Booster but it will dim the image a lot and turn everything greenish. That filter is super nice for Mars observing. I rarely use it unless I’m observing Mars. For this planet, I’ll often use the Contrast Booster even if using my C8 sct (no ca reduction needed).

Edited by AxelB, 21 February 2019 - 08:10 AM.


#23 REC

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Posted 21 February 2019 - 10:24 AM

Hello,

 

     I hope this is the correct forum to post this question.  I purchased an Explore Scientific AR 102 refractor and after a couple of weeks, was able to finally take it out on Sunday night.  To say I was floored with what I saw is an understatement - but I noted some CA when I was looking at the Moon, and saw it in the photos that I took of it during the session. 

 

     I knew that CA was going to be an issue before I had purchased it, so my question is - should I by a 1/25" CA filter for attaching on to my eyepieces (all I have are that size to fit my 30 year old C-8) or should I by a 2" to put on the diagonal and leave in place permanently? thinking1.gif

 

Thanks in advance to all. smile.gif

 

Chris

Get a Baader Semi-Apo filter and leave it in the diagonal. CA is not really a big deal on DSO's in my C102.


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#24 nicoledoula

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Posted 21 February 2019 - 02:44 PM

Key words: "Because I have another better scope for planetary observations, I rarely push my AR127 above 80x"  A 6" F/8 Dob is a much better scope for planetary and costs $300. You already have a nice wide field (that you can also look at the Moon with) Buying things that make your scope live up to it's potential makes sense IMO. If you buy anything for your AR 102,  maybe it should be the widest field best corrected 2" EP's that you can afford. (And not waste money changing purple to yellow)  $300 6" Dob beats the planetary detail that can be seen in a $3000 4" APO.  AR 102, best in 4" class widefield.  6" F/8 Dob best in ($300) class planetary.  I might eventually switch to an ST 120 for wide fields, but the 6" Dob has to stay as the cheap planet killer on my to buy list.  https://www.amazon.c...2/dp/B0018IWQC8   Halfway to the price of something that could actually see way more planetary detail. A filter or another scope that could see deeper DSO's as well. Edit: If the color is objectionable send it back for a refund (save up a little more $) and try the A.T. 102 ED or SW Pro 100 ED, or even the new F/11 ED from Altair astro.  If you like refractors


Edited by nicoledoula, 21 February 2019 - 02:52 PM.


#25 Sketcher

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Posted 21 February 2019 - 03:43 PM

Key words: "Because I have another better scope for planetary observations, I rarely push my AR127 above 80x"  A 6" F/8 Dob is a much better scope for planetary and costs $300. You already have a nice wide field (that you can also look at the Moon with) Buying things that make your scope live up to it's potential makes sense IMO. If you buy anything for your AR 102,  maybe it should be the widest field best corrected 2" EP's that you can afford. (And not waste money changing purple to yellow)  $300 6" Dob beats the planetary detail that can be seen in a $3000 4" APO.  AR 102, best in 4" class widefield.  6" F/8 Dob best in ($300) class planetary.  I might eventually switch to an ST 120 for wide fields, but the 6" Dob has to stay as the cheap planet killer on my to buy list.  https://www.amazon.c...2/dp/B0018IWQC8   Halfway to the price of something that could actually see way more planetary detail. A filter or another scope that could see deeper DSO's as well. Edit: If the color is objectionable send it back for a refund (save up a little more $) and try the A.T. 102 ED or SW Pro 100 ED, or even the new F/11 ED from Altair astro.  If you like refractors

Well, it looks like we've drifted a bit from the original topic.

 

I've pushed an AR152 beyond 300x.  To me, it doesn't matter if another telescope can do one thing or another better.  I'm more interested in getting as much as I can out of whatever telescope I happen to be using.

 

With that philosophy in mind, I use my AR152 as a general purpose, lunar - planetary - deepsky - etc. telescope -- just as I use my Astro-Physics apochromat, my larger Newtonians, my smaller achromats and even my small, lowly (but still capable!) singlet refractor.

 

It doesn't matter which telescope does better than the other.  It doesn't matter to me when I have other telescopes.  It shouldn't matter to those who have only one telescope.  No matter what telescope a person uses, someone, somewhere has access to a more capable telescope.

 

So, unless a person wants to forever be buying larger and better telescopes; we all might as well learn to be happy with whatever we're using.

 

P.S.  adding a CA-reducing filter to most of the current crop of achromats will improve their performance when it comes to high magnification lunar and planetary work.  There's nothing wrong with trying to gain more performance out of a telescope.


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