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Atmospheric Dispersion Corrector (ADC) test images

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#1 John Boudreau

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Posted 10 February 2015 - 03:18 AM

While I've owned an Aries ADC for a few years now, I've recently had the chance to test out another model from Astro Systems Holland (ASH) for a friend who can't run it through it's paces until the snow disappears around here--- if ever. :grin: Fortunately I have my scope permanently mounted in a roll off roof observatory, and I'm only limited by all too few clear, relatively steady nights during this New England Winter.

 

Most of the currently available ADCs like this ASH unit and the Pierro Astro ADC use counter-rotating Risley prisms to offset the effects of atmospheric dispersion, as does my somewhat limited production Aries ADC. I now consider an ADC an essential tool for both visual and imaging use--- In visual use under good seeing, I can see an improvement with Jupiter even when it's at 60° altitude. Dispersion varies with altitude--- lower targets have the most dispersion, while the problem is reduced with a gain in altitude. During the recent Jupiter triple transit, tweaking my Aries ADC setting as the shadows of Io and Callisto merged allowed a noticeably sharper view of the short-lived partial eclipse by Callisto's shadow on the N hemisphere of Io. Jupiter was 63° in altitude at that time.

 

During my typical RGB imaging sequences an ADC certainly helps as there is still noticeable dispersion within the bandpass range of each filter, particularly the B filter as dispersion is strongest towards the shorter wavelengths. I feel that results from a one shot color camera benefit even more in part because it's much easier to focus on the planet itself once the ADC is adjusted correctly.

 

The attached images were part of a test of the latest version of the ASH ADC, a recently updated design that can now interface with common T-thread systems like the Baader T2 adapter series, as does the Pierro Astro ADC:

 

https://www.astromar...and?language=en

 

http://www.pierro-as...pherique_detail

 

For these test images I used an ASI120MC color camera with a Baader UV/IR cut filter on my 14.5" D-K scope at it's native f18, about 6650mm FL. The planet was 44° in altitude during the video captures, each of 2 minutes duration. Seeing was not great, but probably about 5 of 10 on the Pickering Scale on average. Video capture settings and processing was identical for the left and middle images. There was no WinJupos derotation used as the videos were short enough to tame much of the rotational smearing. The images are presented at a scale of 75% of the original. Please click on the thumbnail below.

 

On the left is Jupiter with the ADC set to neutral--- no correction. It's actually mislabeled 'W/O ADC' as the ADC is indeed in place, but set to zero correction which essentially acts as if there was no ADC in line with the camera. You can easily see the atmospheric dispersion causing a blue fringe at the top of Jupiter; and a red fringe at the bottom. Features are also slightly smeared top to bottom throughout Jupiter's disk. In the middle image, The ADC was properly set to correct for dispersion, and you can see the improvement in fine-scale features along with the disappearance of the color fringing. The image to the right is a copy of the left image with one more processing step--- the RGB channels were realigned at the sub-pixel level in MaxIm DL 5 to remove the color fringing, but you can see that the fine scale features are still not as distinct as in the ADC corrected middle image.

Attached Thumbnails

  • jupiter_ADC.jpg

Edited by John Boudreau, 10 February 2015 - 03:25 AM.

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#2 PiotrM

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Posted 10 February 2015 - 03:24 AM

If you would try L filter with a mono camera then the difference would be even bigger, up to a point where L without ADC would always be blurry. Saturn will be a nice showcase target too :)

#3 Kokatha man

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Posted 10 February 2015 - 05:40 AM

A good representation John! :) Even with the limited resolution of Pat's "field laptop" the differences between each image are quite tangible...an ADC has been something I have steadfastly put on the back-shelf of "items I could do with..." but will probably bite the bullet on later this year...

I can see how easy it would be to use/adjust with an OSC, but how much of an imposition are they for mono cams - seemingly one needs to constantly re-appraise the amount of dispersion/diffraction over an imaging session as elevation changes..?

#4 PiotrM

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Posted 10 February 2015 - 12:41 PM

I can see how easy it would be to use/adjust with an OSC, but how much of an imposition are they for mono cams - seemingly one needs to constantly re-appraise the amount of dispersion/diffraction over an imaging session as elevation changes..?


Depends how fast it's rising. In case for Saturn at my place it's rising quite gently so in general no re-alignment would be needed during average imaging session. Jupiter rises bit faster, but I don't change the ADC if the imaging run isn't very long (noticeably more than an hour). I set the ADC before imaging using an eyepiece and an eye... but I was able to do the same settings by looking at L-filtered image of Saturn rings at good seeing.

#5 HowardK

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Posted 10 February 2015 - 01:49 PM

 

I can see how easy it would be to use/adjust with an OSC, but how much of an imposition are they for mono cams - seemingly one needs to constantly re-appraise the amount of dispersion/diffraction over an imaging session as elevation changes..?


Depends how fast it's rising. In case for Saturn at my place it's rising quite gently so in general no re-alignment would be needed during average imaging session. Jupiter rises bit faster, but I don't change the ADC if the imaging run isn't very long (noticeably more than an hour). I set the ADC before imaging using an eyepiece and an eye... but I was able to do the same settings by looking at L-filtered image of Saturn rings at good seeing.

 

Very interesting.

can you tell me the process for setting the ADC 

if you use an eyepiece i guess focus must not be touched

 

also...how come you used a Baader ir/uv filter on your ASI 120MC ..... This camera has a ir cut window built into its body?



#6 Mike Phillips

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Posted 10 February 2015 - 08:36 PM

Wonderfully represented John.  In some strange twist of fate, I was thinking of asking YOU this question as you're the top dog for Mercury, and here's your post!  So, do you image Mercury with the ADC?  Does it help?  Would you pick one ADC vendor over the other and why?

 

Thanks,

 

Mike


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#7 PiotrM

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Posted 11 February 2015 - 02:06 AM

can you tell me the process for setting the ADC 
if you use an eyepiece i guess focus must not be touched

I focus the image and then I start setting the ADC (starting from "0" position) pulling the levers until I can't see colored edges. Sometimes I over-correct to see the reversed edge-colorisation (to notice where is the upper border of good correction. The focus stays in good shape. It's important to look straight through the eyepiece (not from an angle) and to look for colored edges when the planet image is stable, not blurred - that may give false positive effect.

also...how come you used a Baader ir/uv filter on your ASI 120MC ..... This camera has a ir cut window built into its body?

I have mono, John has color. Maybe he removed the cover glass ;)
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#8 John Boudreau

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Posted 11 February 2015 - 05:45 AM

If you would try L filter with a mono camera then the difference would be even bigger, up to a point where L without ADC would always be blurry. Saturn will be a nice showcase target too :)

 

Piotr, I find that successful L imaging relies on very good seeing with the planet fairly high in the sky even with an ADC. In normal seeing at least for me, there are moments when even with the ADC set correctly the L image disperses noticeably and this effects the quality of the frame stack. Such errors are much more noticeable when trying to tame dispersion over a 300nm wide band as with the typical 400 to 700nm range we L image in, as opposed to momentary ADC errors that occur within the typical 100nm or so bandpass for each color filter. For this reason I abandoned L channel imaging and put more time into the separate R,G, and B channels. But sure, for those lucky enough to have great seeing I agree that adding a corrected L channel is certainly an option.



#9 John Boudreau

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Posted 11 February 2015 - 05:48 AM

A good representation John! :) Even with the limited resolution of Pat's "field laptop" the differences between each image are quite tangible...an ADC has been something I have steadfastly put on the back-shelf of "items I could do with..." but will probably bite the bullet on later this year...

I can see how easy it would be to use/adjust with an OSC, but how much of an imposition are they for mono cams - seemingly one needs to constantly re-appraise the amount of dispersion/diffraction over an imaging session as elevation changes..?

 

Thanks Darryl! I feel that the true differences are probably a bit larger than the images show. The corrected image suffered from some interaction with a passing thin cloud that may have hurt it, reducing the number of quality frames to stack. Two minutes later I took another shot at the same settings and the image is even sharper. But it could have also hit an area of better seeing in the wake of the cloud. In the end, I decided to go with the result from video with the thin cloud as it was taken closer to the uncorrected result.

 

ADCs are certainly something that require tweaking. Since dispersion occurs along a line perpendicular to the horizon, the ADC has to be rotated to match that. With a Risley prism ADC, when you adjust the unit the planet will move up/down in the FOV far enough to move the planet off of the typical camera's sensor. For this reason I prefer adjusting the ADC visually using an eyepiece set to be parfocal with the camera, especially with a monochrome camera--- it is possible to adjust the correction well enough from the live view with a color camera as long as the seeing is reasonable. The amount of correction varies with the light path length after an ADC, so if your using an eyepiece that has a noticeably different focal point than the camera, you are not setting the ADC correction accurately. Of course fine tuning the focus once the camera is back in place is necessary, but since the refocus requirement is fairly close to that of the parfocal eyepiece, the ADC correction setting will still be OK.

 

Despite the required horizontal orientation and prism setting tweaking, in practice I seldom tweak these adjustment more than every half hour. Being just a bit out of adjustment is still much better than no ADC at all. I know Damian Peach has also mentioned only worrying about ADC settings every 30 to 60 minutes depending on the path through the sky of the target planet. Maybe in years to come there will be motorized ADCs that can be set up to self adjust, but for now they are manually adjusted, but one gets used to it and it's not really a big hassle with experience. It eventually becomes second nature.



#10 John Boudreau

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Posted 11 February 2015 - 05:52 AM

 

Very interesting.

 

can you tell me the process for setting the ADC 

if you use an eyepiece i guess focus must not be touched

 

also...how come you used a Baader ir/uv filter on your ASI 120MC ..... This camera has a ir cut window built into its body?

 

 

Hi Howard,

 

Please see my answer above to Darryl, which includes my ADC adjustment and focusing suggestions.

 

As for the Baader UV/IR cut used with my ASI120MC: When I got the camera, the sensor only had one noticeable dust mote--- which happened to be so close to the sensor's center that with a tight ROI, the dust was always somewhere on the disk of Saturn! So I had to remove the stock filter window to clean the sensor. Since then I've been chasing a dust problem with it that requires frequent cleaning so I no longer bother to reinstall the stock filter window. Since I prefer removing the nosepiece for sensor cleaning, I find it's simply easier to leave the Baader filter on the camera's nosepiece.



#11 John Boudreau

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Posted 11 February 2015 - 06:33 AM

Wonderfully represented John.  In some strange twist of fate, I was thinking of asking YOU this question as you're the top dog for Mercury, and here's your post!  So, do you image Mercury with the ADC?  Does it help?  Would you pick one ADC vendor over the other and why?

 

Thanks,

 

Mike

 

Thanks Mike!

 

Oddly enough, my best Mercury from last year was indeed shot with my Aries ADC--- as was my best near-IR Uranus image. They were both imaged with deep red/near-IR longpass filters though. I do think an ADC helps in those cases, but it's probably minimal. I actually plan to buy one of those new Astronomik 842BP filters to try for Mercury, as it's a ~ 200nm bandpass filter ranging from ~640nm to the advertized 842nm. Odd that the filter is named after the cut-off wavelength instead of the CWL, but they already make a 742 longpass, so that's probably the reason. Since these longer wavelengths are less effected by atmospheric dispersion, I think that with a 200nm wide bandpass, the 842BP filer may control dispersion well enough for Mercury without use of the ADC.

 

As for ADC brand suggestions, I have only used 2 types: The currently discontinued Aries ADC which uses a rotating knob to control geared counter rotating prisms, and the ASH ADC which uses levers to move the prisms. Both designs have their own merits, but for imaging use I feel the lever adjusted prisms make the most sense. The Pierro-Astro also uses levers, and these levers have longer travel than those of the ASH. One can plan ahead, and position the Pierro-Astro ADC so that the body of the ADC may not have to be repositioned in the focuser during a long imaging session--- the extra adjustment range for prism adjustment can allow simple repositioning of the levers as the planet arcs across the sky. This is also true for the ASH ADC, but to a much lesser extent as the lever adjustment range is shorter than with the Pierro-Astro unit. I have held a Pierro-Astro in my hand at NEAF last year and it's very nicely made (as is the ASH). But the extra lever travel would seal the deal for me, and in addition the Pierro-Astro has UV shifted AR coatings which should be advantageous for Venus UV imaging, along with 1/10 wave prisms. The ASH has 1/4 wave prisms, but I really can't notice a difference with the ASH unit's performance vs. my Aries ADC, which uses 1/10 wave prisms.
Then there's this interesting ADC design: http://www.gutekunst-optiksysteme.com/
Looks like an interesting concept, but one would be paying a ton of money for advantages that may be more theoretical than real world ones. You'd get a bigger advantage by putting that extra cash towards a bigger telescope!


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#12 Kokatha man

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Posted 11 February 2015 - 08:35 AM

...thanks for that answer John! :) Interesting Piotr's comments re adjusting onscreen using the L channel image...I'm presuming (if I haven't misinterpreted his comments) that he's saying that the luminance channel's onscreen appearance will show some sort of dispersion/image blurring along Saturn's ring edges - chime in again Piotr if I've misinterpreted..! ;)

Jupiter wouldn't offer the same opportunity though I suspect...I'm drawing up some plans to machine a completely new imaging train assembly so I might have to think hard about incorporating a visual element to the options...

This is all very interesting, but tbh with my plethora of upcoming expenses (headed by a new laptop! :() & the fact that we'll like image Saturn only for around an hour or so either side of culmination when it's riding really high means it might stay on the back-burner a bit longer... ;)

#13 John Boudreau

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Posted 11 February 2015 - 02:02 PM

Darryl, I agree with Poitr that Saturn's rings present a nice target for tuning an ADC even while viewing live L channel video with a monochrome camera although I still prefer adjustment with a parfocal eyepiece, and I see that Poitr prefers the eyepiece method too. With Saturn already quite low up here, the Cassini division is tough to detect along the fore and aft zones of the rings even in fair to good seeing. But it clearly appears in those rings zones when the ADC is properly adjusted, and in good seeing it gets very sharp! I've got Saturn results from the last couple of years that detect Encke's at the ring ansae down to 32° altitude and seeing was probably only P6 with moments of P7 at the very best. That would be impossible without an ADC in those conditions.

 

While I haven't adjusted an ADC in live view with Jupiter and my mono camera, I'm sure it's possible. Visually, I find the festoons to noticeably increase in contrast and any visible barges to really sharpen up as the ADC is tuned. The fine details in the wake of the GRS really helps in fine tuning too.  I'm sure that these features would also be of help in watching the L channel onscreen view. Might be worth a try.

 

In post #7 Poitr touches upon a trick I tend to use--- When tuning the ADC visually, it's easy to watch the planets move up/down along the dispersion line while tuning the prisms. With the prisms set to 0 correction, note the amount of dispersion visible. Next adjust to correct, but then also overshoot by the same amount--- the opposing red and blue fringing of the planet's disk will swap ends. Going back and forth a few times on either side of the correction point will give you a good feel for what the ADC is doing, and it may make it easier to find the point of best correction. In less than ideal seeing, this can help when judging by the visibility of fine details is less than certain.

 

Since you are thinking of fabricating a new imaging train, here's a few important points with an ADC:
As I mentioned previously, an ADC's correction varies with distance to the camera or eyepiece. In some cases I use the ADC on targets at such a low altitude that I run out of correction--- even at the maximum setting it's not enough. Adding a 30mm or longer spacer will give you more correction for those lower altitude ranges, so you may want to have the flexibility of adding an additional spacer if the need arises. Also, since most of us use Barlows to increase our EFL for planetary imaging or even visual work, it's best to use the Barlow in front of the ADC (the scope side). ADCs are intended for the longer light cone of f/12 and slower systems, and most are at their very best working with an f/20 or slower light cone. The Risley prisms do induce aberrations, but with these slower f-ratios these aberrations are lessened to the point of being insignificant.



#14 HowardK

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Posted 11 February 2015 - 02:43 PM

Darryl, I agree with Poitr that Saturn's rings present a nice target for tuning an ADC even while viewing live L channel video with a monochrome camera although I still prefer adjustment with a parfocal eyepiece, and I see that Poitr prefers the eyepiece method too. With Saturn already quite low up here, the Cassini division is tough to detect along the fore and aft zones of the rings even in fair to good seeing. But it clearly appears in those rings zones when the ADC is properly adjusted, and in good seeing it gets very sharp! I've got Saturn results from the last couple of years that detect Encke's at the ring ansae down to 32° altitude and seeing was probably only P6 with moments of P7 at the very best. That would be impossible without an ADC in those conditions.

 

While I haven't adjusted an ADC in live view with Jupiter and my mono camera, I'm sure it's possible. Visually, I find the festoons to noticeably increase in contrast and any visible barges to really sharpen up as the ADC is tuned. The fine details in the wake of the GRS really helps in fine tuning too.  I'm sure that these features would also be of help in watching the L channel onscreen view. Might be worth a try.

 

In post #7 Poitr touches upon a trick I tend to use--- When tuning the ADC visually, it's easy to watch the planets move up/down along the dispersion line while tuning the prisms. With the prisms set to 0 correction, note the amount of dispersion visible. Next adjust to correct, but then also overshoot by the same amount--- the opposing red and blue fringing of the planet's disk will swap ends. Going back and forth a few times on either side of the correction point will give you a good feel for what the ADC is doing, and it may make it easier to find the point of best correction. In less than ideal seeing, this can help when judging by the visibility of fine details is less than certain.

 

Since you are thinking of fabricating a new imaging train, here's a few important points with an ADC:
As I mentioned previously, an ADC's correction varies with distance to the camera or eyepiece. In some cases I use the ADC on targets at such a low altitude that I run out of correction--- even at the maximum setting it's not enough. Adding a 30mm or longer spacer will give you more correction for those lower altitude ranges, so you may want to have the flexibility of adding an additional spacer if the need arises. Also, since most of us use Barlows to increase our EFL for planetary imaging or even visual work, it's best to use the Barlow in front of the ADC (the scope side). ADCs are intended for the longer light cone of f/12 and slower systems, and most are at their very best working with an f/20 or slower light cone. The Risley prisms do induce aberrations, but with these slower f-ratios these aberrations are lessened to the point of being insignificant.

John

 

how do you adjust the ADC in the Barlow or focuser if not using a Barlow such that it is level with the horizon?



#15 Kokatha man

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Posted 11 February 2015 - 07:36 PM

Hmmm, plenty of food for thought there John! :thinking:

 

The fact that I might need a longer imaging train after the barlow is a bit worrying...I currently use a TeleVue 2X right up against the sensor for approximately 1.2X minimum amplification on my variable system so the likelihood that I'll be reverting to one of the more "standard-sized" pixel cameras is probably a good thing...I tend to "go against the grain" a bit for my own reasons regarding f/l but it is good to know this important aspect. :)

 

Tbh I'm not drawn to the swapping in of ep's but it might be possible to incorporate this facility into the new fitting...it's very much still at the sketching stage & one option is to change the current mode of releasing a band-clamp screw to slide/increase the extension distance, to one that uses either a helical adjuster or something more sophisticated (& expensive!) Looks like it could be right back to the start with the sketches..! :)

 

<"While I haven't adjusted an ADC in live view with Jupiter and my mono camera, I'm sure it's possible. Visually, I find the festoons to noticeably increase in contrast and any visible barges to really sharpen up as the ADC is tuned. The fine details in the wake of the GRS really helps in fine tuning too.  I'm sure that these features would also be of help in watching the L channel onscreen view. Might be worth a try.">

 

...but as focus is probably the prime component of our imaging sessions (leaving aside collimation & cooling) then it is gratifying to hear the above - meaning it would suggest that this might alleviate ep adjustment: but definitely one of those areas where philosophising won't get me too far...a "suck it & see" hands-on experiential approach would seem to be essential ultimately..! :lol:



#16 John Boudreau

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Posted 12 February 2015 - 10:19 PM

John

 

how do you adjust the ADC in the Barlow or focuser if not using a Barlow such that it is level with the horizon?

 
Hi Howard,
 
It's not the Barlow or focuser that needs to be used as a reference with the horizon. It's the ADC itself. It corrects only along one axis in it's light path, and one simply has to install the ADC so that it's correction axis aligns with the direction of dispersion in our atmosphere. And that dispersion correction is always perpendicular to the horizon.
 
Obviously as you look around at the sky, the horizon lies along a line that's left to right. For these lever-type ADCs like the ASH and Pierro-Astro units, when the levers are positioned together the ADC has zero correction. The zero correction position also establishes the plane which needs to be parallel with the horizon.
 
Attached is a photo of the ASH ADC installed in the focuser of my 14.5" Dall-Kirkham scope. In this photo the levers are adjusted slightly apart so are not at zero correction. However the point between them is still the 'zero' position and from that point, drawing an imaginary line (yellow dashed line shown) through the ADC and then in line with the very large thumbscrew represents the line that needs to be level/parallel to the horizon--- simply rotate the ADC in the focuser until this levels with the horizon. The correction axis of the ADC is perpendicular to this dashed yellow line.
 
Now I'll get into some complications. The above description works for viewing straight-through refractors or Cassegrains that give an inverted image, and in those cases the ADC's lever zero point should be pointing to the left. Add a diagonal and things change. A diagonal corrects the image up/down, along with mirror reversing it. In correcting the image up/down it reverses the correction value--- the ADC then needs to be inserted into the diagonal with the levers to the right to counteract that correction value reversal. But how you angle the diagonal itself also causes the horizon to rotate in the field--- so if you MUST use a diagonal you need to be aware of this and make necessary adjustments to the ADC's position. It's simply best not to use a diagonal for critical imaging work anyway, so these complications become another reason to simply use the ADC straight-through. Because of their diagonal secondary mirrors Newtonian owners are probably already well aware of how the horizon appears to be slanted in those scopes depending upon focuser location, and need to keep in mind that the dashed yellow line in the attached image needs to be lined up to how the horizon appears within the focusers of their particular setups. There may be other systems that have their unique issues that one would need to experiment with ADC positioning--- Gregorian reflectors with their erect image come to mind, but I think I've covered the vast majority of situations.
 
Here's the manual for the Pierro-Astro ADC. Reading it may help with further understanding of ADC concepts:
http://nimax-img.de/...User_Manual.pdf
 
It may look much more complicated than it really is. Once you see how it works in person, the learning curve should be pretty quick.

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  • ADC_DK_133.jpg


#17 HowardK

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Posted 13 February 2015 - 01:39 AM

John

 

thanks for the photo and description...very helpful.

i do not use a diagonal.

 

am i right in saying that once the ADC is set level with the horizon this does not have to be adjusted again?..... Or does this perpendicular level change as the scope follows the object in the sky?......and does this vary with an alt/az or GEM mount?

 

howard



#18 John Boudreau

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Posted 13 February 2015 - 08:38 AM

John

 

thanks for the photo and description...very helpful.

i do not use a diagonal.

 

am i right in saying that once the ADC is set level with the horizon this does not have to be adjusted again?..... Or does this perpendicular level change as the scope follows the object in the sky?......and does this vary with an alt/az or GEM mount?

 

howard

Hi Howard,

 

With an alt-az mount ADC horizontal alignment can be set once and then you only have to worry about the lever setting for changing dispersion of the target as it's altitude changes. That certainly doesn't have to be done continuously, especially when the planet is within +/- an hour or two of the meridian, as the altitude stays roughly the same during that time span.  Another thing to consider is that dispersion error is much more noticeable with larger scopes--- for instance an 8" scope would require less ADC adjustment than a 12" because the resolving power is noticeably better in the 12" and will resolve a dispersion error more easily. In fact while I'm mentioning scope size here It's important to note for some people that ADCs are rarely worthwhile for scopes smaller than 8" as much of the time dispersion error isn't resolved well enough in those smaller scopes. Scopes smaller than 8" are rarely used for planetary/lunar imaging anyway.

 

With an equatorially mounted scope the ADC horizontal alignment does need to be adjusted now and then, as of course do the ADC levers to adjust the amount of dispersion correction itself. But the majority of my adjustments are in the 30 minute or longer interval range--- only very rarely do I adjust as quickly as 20 minute intervals (and even then I admit that may be overkill). Only with experience with an ADC over time will you settle on adjustment intervals that work well for you. Tolerance in horizontal alignment is pretty loose which is why manufacturers don't put a precision reference line or a bubble level on the ADC--- eyeballing it is good enough. In most cases a good ADC dispersion setting should work well enough for maybe a 5° change in altitude before the error is noticeable even in excellent seeing.



#19 HowardK

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Posted 13 February 2015 - 12:48 PM

 

John

 

thanks for the photo and description...very helpful.

i do not use a diagonal.

 

am i right in saying that once the ADC is set level with the horizon this does not have to be adjusted again?..... Or does this perpendicular level change as the scope follows the object in the sky?......and does this vary with an alt/az or GEM mount?

 

howard

Hi Howard,

 

With an alt-az mount ADC horizontal alignment can be set once and then you only have to worry about the lever setting for changing dispersion of the target as it's altitude changes. That certainly doesn't have to be done continuously, especially when the planet is within +/- an hour or two of the meridian, as the altitude stays roughly the same during that time span.  Another thing to consider is that dispersion error is much more noticeable with larger scopes--- for instance an 8" scope would require less ADC adjustment than a 12" because the resolving power is noticeably better in the 12" and will resolve a dispersion error more easily. In fact while I'm mentioning scope size here It's important to note for some people that ADCs are rarely worthwhile for scopes smaller than 8" as much of the time dispersion error isn't resolved well enough in those smaller scopes. Scopes smaller than 8" are rarely used for planetary/lunar imaging anyway.

 

With an equatorially mounted scope the ADC horizontal alignment does need to be adjusted now and then, as of course do the ADC levers to adjust the amount of dispersion correction itself. But the majority of my adjustments are in the 30 minute or longer interval range--- only very rarely do I adjust as quickly as 20 minute intervals (and even then I admit that may be overkill). Only with experience with an ADC over time will you settle on adjustment intervals that work well for you. Tolerance in horizontal alignment is pretty loose which is why manufacturers don't put a precision reference line or a bubble level on the ADC--- eyeballing it is good enough. In most cases a good ADC dispersion setting should work well enough for maybe a 5° change in altitude before the error is noticeable even in excellent seeing.

 

Thanks John for taking the time to explain this.

i mostly use an Edge 8" at f/22 with an ASI 120Mcolor camera...this on an altaz mount.

sometimes i use a 12.5" planewave at f/16..this GEM mounted

 

i have pretty poor seeing here....P3-4 at best really..maybe 5 as object rises above 50 degrees

i am going to order the pierro astro ADC ....there is a mk 2 version ready now and thanks to you i feel that there would be a benefit to using it with both scopes....it seems very easy to use.

 

i plan on using firecapture to over expose and over saturate the planet's image revealing the out of balance dispersion around the object's edge...this should make it easy to correct with the levers....i plan on noting where the levers are positioned at various altitudes so that in future runs i will know where the levers are to be set depending on the object's altitude in the sky.

 

i will post here with my thoughts once i have got used to the ADC....

sure can't hurt...right?

 

howard



#20 John Boudreau

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Posted 14 February 2015 - 11:21 AM

 

Thanks John for taking the time to explain this.

 

i mostly use an Edge 8" at f/22 with an ASI 120Mcolor camera...this on an altaz mount.

sometimes i use a 12.5" planewave at f/16..this GEM mounted

 

i have pretty poor seeing here....P3-4 at best really..maybe 5 as object rises above 50 degrees

i am going to order the pierro astro ADC ....there is a mk 2 version ready now and thanks to you i feel that there would be a benefit to using it with both scopes....it seems very easy to use.

 

i plan on using firecapture to over expose and over saturate the planet's image revealing the out of balance dispersion around the object's edge...this should make it easy to correct with the levers....i plan on noting where the levers are positioned at various altitudes so that in future runs i will know where the levers are to be set depending on the object's altitude in the sky.

 

i will post here with my thoughts once i have got used to the ADC....

sure can't hurt...right?

 

howard

 

 

Glad to help out Howard!

 

The alt/az mounted 8" Edge HD should help with the learning curve of an ADC. You'll see a bigger difference with it on the 12.5" Planewave though. A few years ago the Planewave CDK optics designer, Dave Rowe, was thinking about designing a large aperture ADC for Planewave. I don't think anything's become of that yet so he may have shelved the idea, at least for now. It probably was going to be very expensive!

 

The Pierro-Astro ADC interfaces nicely with Baader T2 adapters and T2 adapter from other sources, so you should have no problem interfacing the ADC with either scope.

 

While taking measurements of your dispersion at different altitudes is a good way to help set up the ADC quickly, the dispersion level will vary somewhat with weather conditions, especially humidity. So only use those figures as a guide, as you still may find yourself tweaking the settings a bit differently at times. I've noticed a difference in setting between fair vs. great transparency. Also keep in mind that varying projection distance from the ADC to the camera or eyepiece changes the degree of correction.

 

I've never tried the over exposure/over saturation trick to adjust an ADC. One of the possible issues I see with that is that Jupiter and Mars can actually really have a bit of blue along the day/night terminator when it is visible on the limb opposite the Sun up to a few weeks or so on either side of opposition. In those cases that technique may still work as long as you keep that in mind, and use the sunward limb as your main datum point.

 

Best of luck with the new toy and looking forward to seeing your results! :)



#21 HowardK

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Posted 14 February 2015 - 08:21 PM

 

 

Thanks John for taking the time to explain this.

 

i mostly use an Edge 8" at f/22 with an ASI 120Mcolor camera...this on an altaz mount.

sometimes i use a 12.5" planewave at f/16..this GEM mounted

 

i have pretty poor seeing here....P3-4 at best really..maybe 5 as object rises above 50 degrees

i am going to order the pierro astro ADC ....there is a mk 2 version ready now and thanks to you i feel that there would be a benefit to using it with both scopes....it seems very easy to use.

 

i plan on using firecapture to over expose and over saturate the planet's image revealing the out of balance dispersion around the object's edge...this should make it easy to correct with the levers....i plan on noting where the levers are positioned at various altitudes so that in future runs i will know where the levers are to be set depending on the object's altitude in the sky.

 

i will post here with my thoughts once i have got used to the ADC....

sure can't hurt...right?

 

howard

 

 

Glad to help out Howard!

 

The alt/az mounted 8" Edge HD should help with the learning curve of an ADC. You'll see a bigger difference with it on the 12.5" Planewave though. A few years ago the Planewave CDK optics designer, Dave Rowe, was thinking about designing a large aperture ADC for Planewave. I don't think anything's become of that yet so he may have shelved the idea, at least for now. It probably was going to be very expensive!

 

The Pierro-Astro ADC interfaces nicely with Baader T2 adapters and T2 adapter from other sources, so you should have no problem interfacing the ADC with either scope.

 

While taking measurements of your dispersion at different altitudes is a good way to help set up the ADC quickly, the dispersion level will vary somewhat with weather conditions, especially humidity. So only use those figures as a guide, as you still may find yourself tweaking the settings a bit differently at times. I've noticed a difference in setting between fair vs. great transparency. Also keep in mind that varying projection distance from the ADC to the camera or eyepiece changes the degree of correction.

 

I've never tried the over exposure/over saturation trick to adjust an ADC. One of the possible issues I see with that is that Jupiter and Mars can actually really have a bit of blue along the day/night terminator when it is visible on the limb opposite the Sun up to a few weeks or so on either side of opposition. In those cases that technique may still work as long as you keep that in mind, and use the sunward limb as your main datum point.

 

Best of luck with the new toy and looking forward to seeing your results! :)

 

Thanks, John

 

your images on your website are excellent...shows well the improvement from photographic film way back when to video webcams.


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#22 John Boudreau

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Posted 05 April 2015 - 08:29 AM

An update to this thread:

 

In post #11, I wrote "The ASH has 1/4 wave prisms, but I really can't notice a difference with the ASH unit's performance vs. my Aries ADC, which uses 1/10 wave prisms."

 

It's recently come to my attention that the newer Astro Systems Holland ADC with T-thread adapter interface like the one I tested now uses 1/10 wave wedge prisms. It was the older one piece model that had used 1/4 wave prisms.

 

 



#23 KpS

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Posted 06 June 2015 - 12:29 PM

After buying ADC, I faced the problem of setting the correct angle between the prisms for a given altitude. Relatively simple solution occurred to me when focusing with Bahtinov mask. The first image shows the light from Spica after passing through the mask oriented horizontally and the L filter. Well are displayed the second order diffraction spectra. The blue color is higher above the horizon than the red one. The second picture shows correction made by ADC. It seems that the blue end is slightly overcorrected here. Dispersion of air has a slightly different wavelength dependence than fused silica.
For a given optical train is now sufficient to perform a half-dozen measurements, and later just interpolate ADC angles for different altitudes.

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#24 John Boudreau

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Posted 07 June 2015 - 01:00 AM

After buying ADC, I faced the problem of setting the correct angle between the prisms for a given altitude. Relatively simple solution occurred to me when focusing with Bahtinov mask. The first image shows the light from Spica after passing through the mask oriented horizontally and the L filter. Well are displayed the second order diffraction spectra. The blue color is higher above the horizon than the red one. The second picture shows correction made by ADC. It seems that the blue end is slightly overcorrected here. Dispersion of air has a slightly different wavelength dependence than fused silica.
For a given optical train is now sufficient to perform a half-dozen measurements, and later just interpolate ADC angles for different altitudes.

 

Interesting--- thanks for posting this. I've tried the W47 filter method where because of the IR leak in a W47 filter two images of a given star are formed that can be seen by a camera, and adjusting the ADC to the best merge of these images also allow an accurate point of ADC adjustment.

 

If you're just creating a library of points for the basic adjustment point I think that's fine, but beware that the exact point will change to some degree with humidity and temperature of the air so checking with the Bahtinov mask just before you image would still be the most accurate. I've watched the ADC correction level change on the color live video while watching a thin, translucent cloud move over Jupiter as the cloud acted as a weak lens.

I've recently picked up a Pierro-Astro ADC myself. I see a very slight difference in your corrected image from L to R so there may be a slight misalignment with the horizon but in practice such minor errors are not an issue (you may well be closer than I usually get here).

 

As you indicate there apparently isn't an exact match in dispersion between air and the fused silica prisms of the Pierro-Astro ADC, and while I know there also isn't an exact match with BK7 prisms either I don't know which material is closer. Again with dispersion variances in the air itself, an exact match may not be possible. But we can certainly get pretty darned close! :grin:



#25 KpS

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Posted 07 June 2015 - 11:33 AM

Thank you for your comment, John. W47 procedure is certainly more accurate than any canned table but my filter wheel is already occupied. For myself I think that even ninety percent correction is much better than none.
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