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Need advice about Atmospheric Dispersion Corrector

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#1 AldebaranWhiskey

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Posted 11 July 2018 - 02:19 AM

I have been considering purchasing a ADC for planetary imaging, but I have read that they are more effective on "slower" scopes with higher f ratios. I have an f/4.9 203mm (8 inch) newtonian. I am aware that it is tricky using an ADC on an EQ mount, which is something I am willing to deal with. I just want to know if anyone has experience with using ADC on faster scopes and if it yields any benefits, especially for imaging.



#2 CygnusBob

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Posted 11 July 2018 - 10:43 AM

If you are doing planetary imaging and want to capture fine detail, you will probably need a fairly high f ratio.  ADCs make use of wedge prisms, that will introduce astigmatism into the optical system.  The higher the f ratio the lower the amount of astigmatism that is created.  What camera are you using and what is the pixel size?



#3 AldebaranWhiskey

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Posted 11 July 2018 - 03:48 PM

If you are doing planetary imaging and want to capture fine detail, you will probably need a fairly high f ratio.  ADCs make use of wedge prisms, that will introduce astigmatism into the optical system.  The higher the f ratio the lower the amount of astigmatism that is created.  What camera are you using and what is the pixel size?

I am using a zwo asi290mc, pixel size is 2.9um. With the "5x pixel size" rule of thumb I figure I should be using around f/14.5, and with a 3x barlow I'm at around f/14.7. Does the introduction of an ADC into the imaging train alter the f/ratio?



#4 Dan Crowson

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Posted 11 July 2018 - 04:17 PM

This would probably get more replies in the Solar System Imaging Forum. Let me know if you'd like me to move it there.


Dan



#5 AldebaranWhiskey

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Posted 11 July 2018 - 04:39 PM

Sure, if you think that would help. Thanks Dan

This would probably get more replies in the Solar System Imaging Forum. Let me know if you'd like me to move it there.


Dan



#6 CygnusBob

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Posted 12 July 2018 - 12:05 PM

If this was a monochrome camera with 2.9 micron pixels you would achieve critical sampling at around an f ratio of 12 (at 500 nm).  However, since it is a color camera with a Bayer pattern of pixels with different color filters I would estimate an f ratio of 18 would be required for critical sampling.  However, it is not necessary to hit this exactly.  So a 3X Barlow giving you f/ = 14.7 sounds about right.  This arrangement will not create a lot of astigmatism as long as the Barlow is placed before the ADC.  I would go with that. 

 

The ADC will not change the f ratio of the system.

 

Good luck.



#7 RedLionNJ

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Posted 12 July 2018 - 12:33 PM

If this was a monochrome camera with 2.9 micron pixels you would achieve critical sampling at around an f ratio of 12 (at 500 nm).  However, since it is a color camera with a Bayer pattern of pixels with different color filters I would estimate an f ratio of 18 would be required for critical sampling.  However, it is not necessary to hit this exactly.  So a 3X Barlow giving you f/ = 14.7 sounds about right.  This arrangement will not create a lot of astigmatism as long as the Barlow is placed before the ADC.  I would go with that. 

 

The ADC will not change the f ratio of the system.

 

Good luck.

Incorrect.

 

The additional spacing the ADC introduces between a barlow and the sensor WILL increase the f-ratio.  You might want to consider a lower-powered Barlow (or a PowerMate) to compensate for this.



#8 charotarguy

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Posted 12 July 2018 - 12:46 PM

Red,

 

Does it matter where the barlow is placed in the i-train? Before or after the adc? 



#9 PiotrM

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Posted 12 July 2018 - 12:50 PM

Does it matter where the barlow is placed in the i-train? Before or after the adc?

ADC works better at slower f/ratios, like f/10 or more.
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#10 AldebaranWhiskey

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Posted 12 July 2018 - 04:26 PM

Incorrect.

 

The additional spacing the ADC introduces between a barlow and the sensor WILL increase the f-ratio.  You might want to consider a lower-powered Barlow (or a PowerMate) to compensate for this.

Okay that makes sense, but let's try the math.

 

The specs on the ADC i'm considering says the body is 30mm long. (not sure if that is the actual space that will be added to the train or if it includes the adapter nose, but let's assume it's the spacing as an example.) My native focal length is 1000mm and mirror diameter is 203mm. so 1000/203 = f/4.9

 

if I add in the 30mm spacing of the ADC I get 1030/203 = f/5.1 Not much of change adding 30mm

 

So if I use a 3x barlow my f/ratio will be 3*5.1= f/15.3

and using a 2x barlow my f/ratio will be 2*5.1= f/10.2

 

Is this correct or am I missing something here?



#11 CygnusBob

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Posted 12 July 2018 - 04:37 PM

Yes, there is extra space when the ADC is in place (I am not talking about the 30 mms of housing, but the optical path), but it does not change the f ratio.   I checked this using an optical design program of an f10 SCT with and without an ADC.  The f number did not change.

 

The extra space is similar to what happens when a filter or window is added to an optical system.  The extra space is given by T*(n-1)/n  where T is the filter thickness and n is the index of refraction.  In the case of the ADC, the thickness is thickness of the two wedge prisms against each other.  As an example a 4 degree wedge prism, the prisms might have a thickness of ~ 8 mm so the extra length would be ~ 3 mms.

 

The ADC must be placed after the Barlow lens.  If you do it before, there will be a significant amount of astigmatism.


Edited by CygnusBob, 12 July 2018 - 05:58 PM.


#12 Kokatha man

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Posted 12 July 2018 - 07:59 PM

Bob - I respect the fact that you are trying to help A/W here but I also notice you are a newcomer here. (this does not of course necessarily mean you aren't an old hand at planetary imaging, but I cannot recall seeing your work at anytime...)

 

Folks here work with these imaging trains 24/7 & any additions to the imaging train's length after the barlow (whatever barlow) must add additional length & alter the barlow's amplification & thus f/l as per here: http://www.televue.c...=52&Tab=_photo 

or for their powermates here: http://www.televue.c...id=53&Tab=_app 

 

It is rare to be able to fit an ADC into the train without coupling in additional componentry although I have shown a few examples to minimise this situation...note also that with many/most SCT's where an after-market focuser is essential for planetary imaging, that the back-focus will need altering to throw the image back to the barlow lens - & altering the primary-to-secondary spacing to achieve this will affect the f-ratio also.

 

Happily the OP only has to make sure he can achieve focus with his newt. smile.gif

 

Most other barlows follow a similar gradient to the Televues in the above link ime.

 

You're correct of course in placing the barlow after the ADC, especially with a scope such as the OP's with a short native f/l. waytogo.gif

 

These situations become quite critical at times when considering the smaller pixel-sizing in the sensors of a lot of the current crop of cameras...the ASI290 being one such... 


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#13 CygnusBob

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Posted 12 July 2018 - 08:59 PM

Kokatha Man.  

 

Well yes, changing the distance from the Barlow to the camera sensor and then focusing by varying the objective to Barlow distance will change the system f number whether an ADC is present or not.  However after the objective to Barlow distance is established, adding an ADC afterward will not change the f number.  Of coarse the design of the ADC may force the user to change the Barlow to imager distance.  Ideally you would pick a Barlow to sensor distance that would result in the magnification factor required and then figure out how to come up with the required spacers.   If that turns out too be hard to do, select a different Barlow lens with a different focal length which may result in a Barlow to sensor distance that is easier to configure.



#14 Kokatha man

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Posted 13 July 2018 - 12:53 AM

...I'm really not quite sure what you are trying to say there Bob - the simple fact is that whatever distance is added to the barlow to sensor spacing affects both the barlow's amplification/power/multiplication factor & therefore the f/ratio the scope is operating at - fullstop..!

 

This is the nub of the matter which everyone involved in planetary imaging is aware of...trying to arrive at the oft-quoted "rule of thumb" of 5x the pixel-sizing of whatever camera (f15 approx for the ASI290MM, although this is quite flexible, which I personally demonstrated some time ago here) is the challenge when the range of barlows providing varying amplification is somewhat limited...although as I said previously I have frequently listed several options for this...utilising only the lens element of many proprietary barlows being the most simple.

 

I get the impression that you are trying to address this matter from a limited theoretical awareness without any real practical experience Bob - apologies if I am wrong but there are physical practicalities that simply cannot be ignored! wink.gif


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#15 ToxMan

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Posted 13 July 2018 - 08:43 AM

Aldebaran Whiskey,

 

You are on track...don't get bogged down in little details.

 

Use a 3x barlow...it wouldn't hurt to have another barlow, ie 4x, in case really good seeing presents itself. Flexibility helps.

 

Setting up the ADC for Newtonian on an equatorial mount can be found here: http://skyinspector....-corrector--adc

 

Personally, I hate these devices...But, I got one in my image train. 

 

BTW, is there a story behind your screen name?



#16 Tavi F.

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Posted 13 July 2018 - 09:00 AM

- The main thing to know about the necessity of using an ADC corrector is that this device forces the observations (visual or photographic) to be off-axis. So, the corrector preserves the amount of inherent aberrations for the particular objective or system (rather it is refractive, one mirror newtonian or two mirrors cassegrain) and brings it in to use, in the useful field of view.
- The amount of correction needed increase as the object gets lower in the sky (which is evident).
- In turns of the focal length of a given objective and the altitude for the object in the sky, the angular correction needed increases directly proportional with the focal length of the objective, which implicitly means that the off-axis height (or angle) will respect the same directly proportional relation to the focal length. This is why even a proper design Dall-Kirkham will outperform a similar size newtonian with a paraboloidal main mirror of the same f-ratio as the DK's primary. Because the DK performs better off-axis (by intrinsical design) than a newtonian, for the same given angle.        
- The classic commercial solution for an ADC corrector is with two wedge discs, which can be rotated in relation one to the other, increasing this way the correction from 0 to a maximum. And also the hole assembly could be rotated in the focuser that the shifting line of correction to be vertical.
- The angle of one wedge disc has no theoretical effect on the correction, if both prisms are the same. Just the maximum correction will be higher or lower. The higher values might be needed for larger (than "normal") two mirrors systems (large cassegrains with focal lengths longer than 7..8 meters). One practical aspect is the sensibility of the adjustment in order to obtained the desire correction.

 

- Finally, there is another possibility of varying the linear correction needed in the focal plane, not by increasing the power of the prismatic group (as a unit), but by varying the distance between the prism and the focal plane... using just one wedge disc (prism) with axial movement in it! This solution is the most optic efficiently, having the only drawback that it does not start from zero, but it doesn't need to... if you have a dedicated planetary telescope for use. This is what I am using as an ADC corrector, with a two degrees single prismatic disc.



#17 CygnusBob

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Posted 13 July 2018 - 10:29 AM

Kokatha Man.

 

I do have some practical experience with ADCs.  I build a DIY ADC that makes use of a longitudinal design.  I do not like most of the ADCs that are sold using the rotating wedge prism design.  The reason I do not like them is because in the presence of turbulence, they are hard to adjust.  With the longitudinal design, all you need to know is the elevation angle of the planet you are observing.  You then directly set that angle using a scale on the ADC.  It is not necessary to even look at the imagery or insert an eyepiece to get a correct adjustment.  It would nice if some vendor would start selling this type of ADC.

 

I described this ADC in an article in Astronomy Technology Today titled "An Easy to Use Atmospheric Dispersion Corrector"  Volume 8 Issue 4 July-August 2014.


Edited by CygnusBob, 13 July 2018 - 10:33 AM.


#18 PiotrM

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Posted 13 July 2018 - 11:13 AM

Well, the correction level needed also depends on the atmospheric conditions. There were subsequent nights that at pretty much same planet elevation I used different ADC settings. I doubt you can pre-set ADC and get a perfect correction without actual adjustment.

Also if there is turbulence preventing ADC setup then also imaging is pointless - that would have to be something really big and noticeable.

#19 AldebaranWhiskey

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Posted 13 July 2018 - 04:47 PM

 Thanks for the discussion, people! I appreciate the help. I'll keep your advice in mind. I'm going to go ahead and start experimenting with the ADC, and see what use I can get out of it for imaging.  If I don't like it I'm sure it will come in handy for visual observation or with a future different scope.

 

(  ToxMan - It's just a fictional whiskey from the Star Trek universe. The name is from a funny scene from Star Trek the next generation, where Data and Picard share a bottle of booze with Scottie. I think the episode is called "Relics."  http://memory-alpha....debaran_whiskey  )

 

  


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#20 Kokatha man

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Posted 13 July 2018 - 08:42 PM

Bob - first up apologies for some (possible) ambits re my comments about your "theoretical Vs practical" awareness/involvement with ADC's - you obviously have some very practical background in them! waytogo.gif

 

But the issue of spacing in the image train is still very acute wrt keeping image scales under control where the ADC & any interconnectors proceed the barlow, which is almost every time.

 

Indeed after it was pointed out to me that increasing the distance between ADC & camera sensor assisted in their setting/useage (at least with the common wedge variety) the distance from barlow to sensor became an even greater issue...but of course not insurmountable. smile.gif

 

I appreciate your comments re turbulence & ADC use but as Piotr suggests it is probably not worth imaging in those situations anyway - I often advocate here about the futility of imaging in poorer seeing although I can appreciate some folks might find that precludes them from imaging at all...so it's a point.

 

Good input also Tavi: in your last paragraph are you speaking of the prime focal plane (ie, that of the objective/lens/mirror) or referring to that (ie, the sensor) after any barlow..?

 

All in all some interesting points - A/W, "experimenting with the ADC" is indeed your way forward & I certainly endorse their use at low eleveations.



#21 CygnusBob

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Posted 14 July 2018 - 11:24 AM

Yes, If the turbulence is too great one might just as well call it a night.

 

However, there is another issue with the rotary design that makes the adjustment a bit more involved.  The 2 prisms must both rotate.  One clockwise and the other one counterclockwise.  The idea is the plane of symmetry between the two prisms should be in the vertical direction (optically).  In other words if one prism is rotated by 23 degrees clockwise the other prism should be turned 23 degrees counterclockwise.  This is straight forward for a refractor or an SCT, but for a Newtonian it is not, because the "vertical" direction is the direction that appears vertical if you were looking into an eyepiece.   Ideally this rotation should be accomplished with a gearing mechanism, so that the equal and opposite rotations occur automatically.  There is a rather expensive ADC that has does have a gearing mechanism, however the cheaper models do not have this feature.


Edited by CygnusBob, 14 July 2018 - 12:30 PM.


#22 John Boudreau

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Posted 14 July 2018 - 12:04 PM

Yes, If the turbulence is too great one might just as well call it a night.

 

However, there is another issue with the rotary design that makes the adjustment a bit more involved.  The 2 prisms must both rotate.  One clockwise and the other one counterclockwise.  The idea is the plane of symmetry between the two prisms should be in the vertical direction (optically).  In other words if one prism is rotated by 23 degrees clockwise the other prism should be turned 23 degrees counterclockwise.  This is straight forward for a refractor or an SCT, but for a Newtonian it is not, because the "vertical" direction is the direction that appears vertical if you were looking into an eyepiece.   Ideally this rotation should be accomplished with a gearing mechanism, so that the equal and opposite rotations occur automatically.  There is an rather expensive ADC that has does have a gearing mechanism, however the cheaper models do not have this feature.

Aries Optical has made at least 2 runs of a geared Risley Prism-type ADC--- one from the 2nd run was my first ADC, and I still own it. The latest Pierro-Astro (MkIII) introduced earlier this year is also geared:

https://www.teleskop...connection.html

 

For years I had assumed only a vertical direction to atmospheric dispersion. Except for rare instances, I had been using only monochrome cameras, and was making the ADC adjustment visually with an eyepiece parfocal to the camera. Many of the basic tutorials here on CN were posted by me, and as more people started using ADCs FireCapture camera capture software introduced an interesting  and quite effective ADC adjustment tool (only works with color cams). Several people noted that their best adjustment via the FireCapture tool did not result from equal spacing of the ADC prism levers (Kokatha man was one of the first to mention this to me). Then during a particularly strong jetstream here in the northeast USA, it happened to me during use of my color cam--- proper dispersion correction was noticeably offset from the vertical. Turns out this can happen if there's enough air in lateral motion as seen by the observer. Since then, I've seen as much as about a 15° offset! Now, when I mention ADC adjustment I tend to mention that dispersion usually occurs along a vertical line from the horizon. Also, I agree with a point made by Piotr here--- I have seen different ADC lever adjustments with the target at the same altitude on different nights where the humidity was noticeably different. Perhaps this isn't as common in typical conditions in Nevada. 

 

The Aries ADC required periodic rotation within the focuser as the target moves across the sky with an equatorial mount. The new Pierro Astro MkIII's whole geared prism section can be rotated within the unit's main body for a number of hours to allow resetting the unit horizon/dispersion reference without loosening the focuser to reset the unit. It's the neatest overall design I've seen so far.


Edited by John Boudreau, 14 July 2018 - 12:10 PM.

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#23 wargrafix

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Posted 14 July 2018 - 12:11 PM

Here is a question. When should you not use an adc?

#24 CygnusBob

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Posted 14 July 2018 - 12:27 PM

wargrafix

 

If a planet is observed at a high elevation angles, like above 60 degrees I would not use an ADC.  It would be too much bother for too little gain.

 

John Boudreau

 

I was not aware that such atmospheric conditions existed that would produce that large a rotational shift.  Interesting!



#25 wargrafix

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Posted 14 July 2018 - 12:36 PM

At neutral position, does the adc affect images?


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