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ZWO Atmospheric Dispersion Corrector

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#1 25585

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Posted 12 June 2019 - 02:16 PM

Where I live at 51N, the planets may not be high in my remaining lifetime. So another sixtysomething stargazer told me about an ADC that might help.  https://www.365astro...-Corrector.html

 

But how really effective are they for only visual? 

 

And generally can they make an achro more apo? 



#2 mistateo  Happy Birthday!

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Posted 12 June 2019 - 02:46 PM

If you are sensitive to the color fringing of atmospheric dispersion, it can help for visual.  It WILL NOT fix the chromatic aberration of a lens.


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#3 Redbetter

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Posted 12 June 2019 - 02:53 PM

I have one, haven't really used it much.  It only works on scopes that have a lot of inward focuser travel available (~2" IIRC), so it doesn't work at all for the scopes I would like to use it with.  It doesn't fix seeing, which is the primary obstacle here at 37 N.  Atmospheric dispersion would be a much greater problem though at 51 N, as that would greatly increase the amount of color spread.

 

The design is not optimized for visual.  It is too bad, because it could be built to insert into a 2" focuser and get rid of the worst of the massive inward travel.

 

It won't have any affect on the inherent color error of an achro, totally different thing.  It could be used to counter some wedge or other error in an objective that is producing a lateral color effect, but that is a different problem.


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#4 markb

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Posted 12 June 2019 - 02:55 PM

No on the achro to apo. The long-unavailable Chromacor could do that, but was semi custom and quite expensive. But I would pick up a compatible one in a flash of I happened on one.

Yes on the visual, if the focuser travel allows. I experimented with a single power version using a wedge window from Edmund long ago, and was happy with it visually.

I bought one of the ZWO units, very affordable, and look forward to using it this month. Unfortunately I left mine in NY while visiting my Arizona scope.

If you try one, go over the theory on how they work so you can set it up right. I have read several posts on how they don't work, when they are effective given a low angle planet and a correctly set ADC.

And Zoltan at 365-A is awesome to deal with.

Edited by markb, 12 June 2019 - 02:58 PM.

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#5 Ihtegla Sar

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Posted 12 June 2019 - 03:11 PM

Thanks for reminding me of this.  I had wanted to try one out but forgot about it, so I just ordered one from Agena.  I have an FC100DL, which has a lot of in travel if you don't use the long spacer and are not using a diagonal, which you would not need when the planets are low on the horizon.  I am at 45 N, and the planets are low for the next couple years, so this may be useful.  It got some good reviews on the Agena site and they link to a webpage with some detailed instructions on how to use it, so hopefully I will be able to make it work.


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#6 DrewFamily

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Posted 12 June 2019 - 03:35 PM

I love mine, especially during times like this when Jupiter and Saturn are low. It works fine for visual, and the in-focus is easily avoided with a Barlow. Go for it!


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#7 happylimpet

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Posted 12 June 2019 - 03:43 PM

I have a planetary imaging setup which includes a flip mirror for centering and and ADC before the barlow, Consequently I look through the ADC quite a lot with low planets and I can tell you for sure it helps hugely from the UK!!! Get one!

 

Youll want a barlow to extend the backfocus and also because ADCs should be used above f10 or you get astigmatism,


Edited by happylimpet, 12 June 2019 - 03:45 PM.

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#8 Gavster

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Posted 13 June 2019 - 09:39 AM

I tried the zwo for visual planetary and found it finicky to use and the views didn’t improve as much as I hoped. I then went for the more expensive Pierro Astro adc

http://pierro-astro.... astro adc.html

and still found it disappointing. For visual I much prefer without adc and so being based in the UK if I want to do good planetary observing I take a trip to Tenerife where Saturn and Jupiter will be getting to around 40 degrees which is high enough for me to do some nice observing.


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#9 SeattleScott

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Posted 13 June 2019 - 02:56 PM

At 47 degrees I found the ADC makes a detectable improvement. Haven’t ordered one myself. Debating whether the level of improvement warrants the additional effort and complexity, and there are usually higher priorities than something that will make a significant difference on a couple of targets that aren’t visible year round. But good chance I will get around to getting one someday.

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#10 happylimpet

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Posted 14 June 2019 - 03:03 AM

I dont think they're finicky in the least once you understand how they work, and there are good resources for that. Dont let any perceived complexity put you off.

 

An ADC will make a very real, noticeable improvement in your planetary images for the next few years, except for maybe Mars when thats north again, fairly soon I think.....



#11 Redbetter

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Posted 14 June 2019 - 05:30 AM

Had poor seeing tonight, but pulled out the ADC to try again with the 110ED.  It was disappointing.  I agree it is finicky, and more importantly, can be rather impractical.  I haven't tried Barlowing before the diagonal to some crazy high focal length & ratio for the contraption...I would have to drift time to work out the effective focal length. 

 

I played with it awhile getting it dialed in for Jupiter to get the sharpest image it would provide, adjusting, recentering, refocusing, repeat.  The end result was less detail than without the ADC (both before and after.)  But in poor seeing and with Jupiter at ~30 degrees, perhaps this is not a surprise.  I think I had somewhat better results with it last year on Mars before the dust storm hit, but don't recall the particulars.  I don't recall it degrading the image like this.   Red filters dealt with both the atmospheric dispersion and-to some degree-the dust, so I had no reason to use the ADC later and forgot about it.   Without it I could see that storm in Saturn's polar region with the 20", and that wasn't great seeing either.

 

The ZWO ADC won't work in the 20" f/5 and that blows away the scopes I have that it will work with, so it is of limited utility at my latitude.  I would like to catch some good seeing in the backyard to give it a fair shot with a couple of other scopes though.  I do expect it to have some benefit when the seeing is good.  Better seeing makes it easier to tune the thing, and should allow it to actually improve over the base level conditions.

 

And again, the ADC would likely be of considerably more use to me at latitudes in the high 40's and above.  It might be of use here as well if I was imaging (and could use that to better tune it) or had very good seeing to work with where the ADC would show noticeable improvement visually.


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#12 happylimpet

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Posted 14 June 2019 - 06:36 AM

Had poor seeing tonight, but pulled out the ADC to try again with the 110ED.  It was disappointing.  I agree it is finicky, and more importantly, can be rather impractical.  I haven't tried Barlowing before the diagonal to some crazy high focal length & ratio for the contraption...I would have to drift time to work out the effective focal length. 

 

I played with it awhile getting it dialed in for Jupiter to get the sharpest image it would provide, adjusting, recentering, refocusing, repeat.  The end result was less detail than without the ADC (both before and after.)  But in poor seeing and with Jupiter at ~30 degrees, perhaps this is not a surprise.  I think I had somewhat better results with it last year on Mars before the dust storm hit, but don't recall the particulars.  I don't recall it degrading the image like this.   Red filters dealt with both the atmospheric dispersion and-to some degree-the dust, so I had no reason to use the ADC later and forgot about it.   Without it I could see that storm in Saturn's polar region with the 20", and that wasn't great seeing either.

 

The ZWO ADC won't work in the 20" f/5 and that blows away the scopes I have that it will work with, so it is of limited utility at my latitude.  I would like to catch some good seeing in the backyard to give it a fair shot with a couple of other scopes though.  I do expect it to have some benefit when the seeing is good.  Better seeing makes it easier to tune the thing, and should allow it to actually improve over the base level conditions.

 

And again, the ADC would likely be of considerably more use to me at latitudes in the high 40's and above.  It might be of use here as well if I was imaging (and could use that to better tune it) or had very good seeing to work with where the ADC would show noticeable improvement visually.

It will work in an f5 scope, you just need to use a barlow, then the ADC, and then a correspondingly longer FL eyepiece.  I use it in my 12" f5 with a 2x barlow, ADC, then spacing to get effective 4x, then 32mm Plossl. Works out as a 8mm eyepiece. The ADC helps hugely. Perhaps its because as a planetary imager I have more experience using it, and we have tools in firecapture imaging software to help visualise the correction, and its been discussed a lot in the solar system imaging forum. However once you understand how it works its the easiest thing. I promise you it isn't intrinsically difficult!

 

Note that there were examples of some that were assembled 'backwards' so they had to be used rotated 180degrees from how described in order to help.....if you didnt realise this they would definitiely not improve things!



#13 Redbetter

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Posted 14 June 2019 - 02:55 PM

It will work in an f5 scope, you just need to use a barlow, then the ADC, and then a correspondingly longer FL eyepiece.  I use it in my 12" f5 with a 2x barlow, ADC, then spacing to get effective 4x, then 32mm Plossl. Works out as a 8mm eyepiece. The ADC helps hugely. Perhaps its because as a planetary imager I have more experience using it, and we have tools in firecapture imaging software to help visualise the correction, and its been discussed a lot in the solar system imaging forum. However once you understand how it works its the easiest thing. I promise you it isn't intrinsically difficult!

Oh, I can adjust it, and in the correct direction.  It is more of a question of the impact on the resultant visual image and is it worth the hassle where I am?  Visual fine tuning it requires some decent seeing, which is rare in the backyard.  The Catch 22 is that I am not inclined to fiddle around with it at my dark site which also has better seeing on average, particularly since that would be with the 20".  And when the seeing is particularly good, I don't want to waste it trying to do drift timings to work out Barlow factors, a new eyepiece table, etc. so that I can try to dial in a piece of gear that has shown marginal benefit for the latitude.  It runs into the same obstacle that the DSC's had--they didn't work acceptably right out of the box, and efforts to make them work didn't bear fruit early.  I soon decided they weren't worth the time invested since star hopping was accurate, and they...weren't.

 

I don't see how it would work in my 20" f/5 with low profile focuser, even with a  Barlow, but I haven't really experimented with using a Barlow to move the focal plane around.  I don't have a variety of Barlows and extensions to experiment with, just my old 2x TeleVue with 75mm Barlow focal length.  The scope's secondary size and focuser spacing are optimized for visual planetary while still providing good wide field illumination.  I already run the primary mirror up to the end of its adjustment so that I can use a 31 T5 Nagler along with my other eyepieces.  I don't see cutting a set of tubes to be used only when the ADC will be used...that wouldn't work for dark site viewing where the seeing is also better than in the backyard, but where I would want to do more than planetary.   

 

The ADC would be far more practical/versatile for visual if they had the 1.25" prisms set up to insert into a 2" focuser inside of nested cylinders, with the adjustment knobs on the upper edge near the focuser face, but well above the prisms below.  The focal plane wouldn't be pushed out well past the end of the focuser with that sort of arrangement.  



#14 nicoledoula

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Posted 14 June 2019 - 04:27 PM

"However once you understand how it works its the easiest thing. I promise you it isn't intrinsically difficult!"    So how exactly are they supposed to work? Not what they allegedly do, how they do it. 



#15 CPellier

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Posted 15 June 2019 - 03:03 AM

I don't see how it would work in my 20" f/5 with low profile focuser, even with a  Barlow, 

Hi, the point is not whether you can use a barlow with it, the point is that you must smile.gif. At F/5 and with the planets low in the sky, which means that they demand a strong correction, the optical setup will introduce a severe level of astigmatism, so much that it will just kill the image. ADC's are working well only with at least F/10 (and longer is better)

If properly used the ADC is not only correcting the color fringing : it also improves the resolution, because low in the sky, the intensity of the aberration of color dispersion is reaching several arcseconds, which is much more than the resolving power of any telescope even the smallest one.


Edited by CPellier, 15 June 2019 - 03:04 AM.

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#16 happylimpet

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Posted 15 June 2019 - 11:57 AM

"However once you understand how it works its the easiest thing. I promise you it isn't intrinsically difficult!"    So how exactly are they supposed to work? Not what they allegedly do, how they do it. 

You keep the midpoint of the two levers in line with east-west as seen through the eyepiece, and separate them until the disperion is gone. When you're used to your setup - and for example, Jupiter or Saturn will need the same correction night after night assuming you're looking near culmination, you can get the correction 90% right just through experience and then fine tune it.  

 

It's vastly easier than, for example, collimating. Perhaps on a par with lining up a finderscope. No-one reading this will have any trouble, though I do recommend some of the excellent advice given by people such as John Boudreau on the solar system imaging forum. I'll try to find a link.


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#17 Ihtegla Sar

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Posted 16 June 2019 - 08:09 PM

I got my ZWO ADC yesterday. I had no trouble reaching focus in my FC100DL, with and without a 2.5x (actually 2.1x) short GSO apo Barlow.

It was much easier to use than Collimation. Collimation makes me want to say words that you cannot say on this G Rated site. Not collimating the primary, that's not too hard. But collimating the secondary is worse that getting a root canal. After hours of fiddling with it to get it just right, the secondary always shifts when I go to tighten it down and then I have to start over -- I need a third hand. And when I'm done I'm never completely satisfied that its perfect.

The ADC gave me no such frustration. Its much more intuitive than all the weird angles and various reflections you have to sort out collimating the secondary.

I'm not sure how much improvement it made since seeing was not so good with the wind we were having last night but at least the wind kept the dew off my equipment.

But it definitely did make some improvement. And after a while I was able to see the red and blue tinge discussed in the linked instructions, something I hadn't noticed before reading about it, and the ADC can make it go away, or reverse polarity if you over correct.

Now I am debating whether I should get Takahashi's focus extender for use with the ADC or just keep using the GSO Barlow.

Edited by Ihtegla Sar, 17 June 2019 - 10:03 AM.

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