33 replies to this topic

[John Boudreau]

I haven't enjoyed any luck at all with seeing conditions for Saturn this entire apparition until recently. This is a low altitude Saturn taken on August 11 in good seeing considering it's relatively low altitude of 25°. It's presented here as a demonstration of what is possible with a properly adjusted atmospheric dispersion corrector. Top image is actually mislabeled as the ADC is in the imaging train but adjusted to the neutral position, essentially acting as if it was OFF and showing strong atmospheric dispersion. Middle image is the same as the top, but with the RGB channels realigned. Bottom image is Saturn taken with the ADC properly adjusted to cancel out the dispersion.

 

These images were taken minutes apart at the same focus point and with the exact same capture settings. Processing steps were exactly the same, right down to the color balance weights in Registax 6 and wavelet settings. Mild high pass tweaking in PhotoShop was also identical.

 

About the only thing that may skew the apparent differences here is that with alignment and stacking on AutoStakkert, I expect that the software had an easier time accurately aligning the ADC corrected video. However I tried several different alignment point settings with the uncorrected video and to the eye the results were consistent.

Attached Thumbnails

• 

Edited by John Boudreau, 23 August 2015 - 02:55 PM.

[RBChris]

John,

 

Thanks for the direct comparison! My experience here in Northern California has been very similar - the use of an ADC (in my case a Pierro Astro unit) makes high resolution planetary imaging possible even at low altitudes. Of course, the seeing still has to cooperate - I've had exactly one night of good seeing this apparition 

 

Randy

[Sunspot]

Dramatic! I tried an experiment with an image captured with the 224C. I split the channels into R, G, B and ran them through Winjupos derotate. I wasn't expecting much, I was surprised by the result. While not perfect, I think the improvement is marked. Processing on both images was identical and they are the same image. The first was off the camera, the second after Winjupos. Still I see an ADC in my near future, especially with Mars not being very high this year.

 

Paul M.

Attached Thumbnails

• 
• 

[qsipiMcKenna]

Hi John Boudreau.

What a dramatic improvement. The ADC has helped to make a really nice image of Saturn.

I don't do AP but was wondering if the ADC is worth having for purely visual?

[happylimpet]

Those ZWO ADCs are gonna sell like hot cakes.

[John Boudreau]

Thanks guys! I'm happy that these devices are getting more serious looks as time goes on, especially with those of us in the northern hemisphere who will be dealing with low altitude for Mars and Saturn for some years to come with Jupiter joining them all too soon.

 

As Randy mentioned, the seeing still has to cooperate. These ADCs can't make up for poor seeing, but if conditions are good, they are certainly the tool to use for best results. I actually own two different ones--- a rare Aries ADC and a Pierro-Astro, and have access to an Astro Systems Holland (ASH) ADC, which is the one uses in this comparison.

 

qsipiMcKenna--- yes, these ADCs are also a superb accessory for visual use. With Saturn so low now for many of us they may be the only way to clearly see the Cassini Division crossing in front of Saturn, and although it's declination is still reasonable for us in the north, when Jupiter is around improvements are had with smaller and finer features like white ovals and festoons. The wake disturbances from the GRS are also much improved. However I must add that improvements are more noticeable with increased aperture--- many will say they are best used on telescopes of at least 200mm aperture as smaller scopes may not fully resolve the features distorted by dispersion. ADCs should also be used with longer f-ratios, many times it's mentioned they should be used at f/12 or longer in order to minimize their minor aberrations but I've had good results with SCTs working at f/10. However it's best to meet or preferably exceed f/12 if possible--- use of a good Barlow gets most scopes up in that range.

[DesertRat]

Convincing demonstration John!  And a nice image too!

 

Not only does dispersion exhibit a rainbow effect in the 'raw' RGB, it also exists within the passband of each color filter.  The result is a blur, and in your great instrument that blur is quite a bit larger than the diffraction 'spot'.

 

I have seen really good seeing down to 25 deg elevation - not often however.  Glad you had the experience!

 

PS - just saw your latest post while I was typing - slowly.  For faster F/# at some point the ADC will induce a small amount of astigmatism.  But at F/10 the effect is probably much less than the smear of dispersion at this elevation.

 

Glenn

Edited by DesertRat, 23 August 2015 - 06:59 PM.

[Ethan Chappel]

   

 

The ZWO ADC can't come soon enough. 

[Kokatha man]

Excellent demonstration John & a very good Saturn outcome btw! The ASI120MC still shows its' capabilities in your scope!      

 

As your sequence of images displays vividly & Glenn also notes...& relevant for Paul (Sunspot) in his derotated individual channels after splitting - it isn't just the r-g-b alignment you are correcting but the blurred images themselves!

 

Probably will hold off - but maybe not - although we are pretty spoiled down here these coming years...

[John Boudreau]

Dramatic! I tried an experiment with an image captured with the 224C. I split the channels into R, G, B and ran them through Winjupos derotate. I wasn't expecting much, I was surprised by the result. While not perfect, I think the improvement is marked. Processing on both images was identical and they are the same image. The first was off the camera, the second after Winjupos. Still I see an ADC in my near future, especially with Mars not being very high this year.

 

Paul M.

 

Paul, while the WinJupos derotation effect looks interesting at first, it appears to have simply darkened the regions along the edges of the planet and ring system or at least strongly desaturated their color (I examined the images in Photoshop). That would probably vary with the LD compensation value chosen in WJ. Apparently the ring shadow is modeled much the same way as the edges are as it darkened the shadow in your image, but didn't clean up the Cassini Division in the same way. It appears to me that the luminance data that is dispersed remains the same.

[Sunspot]

 

Dramatic! I tried an experiment with an image captured with the 224C. I split the channels into R, G, B and ran them through Winjupos derotate. I wasn't expecting much, I was surprised by the result. While not perfect, I think the improvement is marked. Processing on both images was identical and they are the same image. The first was off the camera, the second after Winjupos. Still I see an ADC in my near future, especially with Mars not being very high this year.

 

Paul M.

 

Paul, while the WinJupos derotation effect looks interesting at first, it appears to have simply darkened the regions along the edges of the planet and ring system or at least strongly desaturated their color (I examined the images in Photoshop). That would probably vary with the LD compensation value chosen in WJ. Apparently the ring shadow is modeled much the same way as the edges are as it darkened the shadow in your image, but didn't clean up the Cassini Division in the same way. It appears to me that the luminance data that is dispersed remains the same.

 

It was a fun experiment, but I agree it definitely won't fix what the ADC will. Since you own both, which ADC would you go with, the ASH or the Pierre (knowing I'd want to use it in UV imaging as well)?

[John Boudreau]

Thanks for the kudos!

 

Glenn--- as you say, the dispersion creates a blur though the pass band of each filter with a mono camera. It's even more of a problem with a color camera as the Bayer RGB transmission curves have more overlap than the typical RGB filters used with a mono cam.

 

As happylimpet and AstroEthan have mentioned, I'd expect the upcoming ZWOptical ADC to be a strong contender amongst the crop of available ADCs. Emil has some superb images taken with one!

 

Darryl, while your location means an ADC isn't as critical for many of your results, knowing your dedication to squeezing the most out of your equipment means that you would indeed notice a gain in resolution even with targets at high altitude. Damian Peach has mentioned that he notices minor dispersion in the B up to about 70° with the C14. I've noted overall improvements with Jupiter over 60° with my 14.5" DK. Plus, even in places where the planets pass directly overhead, there are always hours of time when the planets are either on the rise or on the way down. So you'd have expanded periods of time for high resolution imaging.

 

Paul, my favorite ADC remains the Pierro-Astro. Not just for it's edge in UV imaging, but for the additional prism lever adjustment travel. The additional lever travel means that a camera can be left attached to the ADC for hours and only the levers need repositioning. The ASH ADC requires that the body of the device needs to be repositioned much more often than the Pierro-Astro as a planet moves along it's path of the sky over several hours. Both ADCs are supremely capable. The ASH has better lever 'feel' than a Pierro-Astro, but a very simple modification to the base of the levers give the P-A ADC excellent action. I'll post a picture of my mod within the next day.

[Sunspot]

John,

 

Thanks!! I was hoping to pick up a Pierro unit from Markus at Tucson in November, but the show was cancelled so I'll have to do it from the website. One last question (ya right!). What accessory(s) will I need to attach it to my filterwheel and camera?

[qsipiMcKenna]

Thanks for the positive info John.

I'm at 52° latitude so Saturn doesn't even get to 25° elevation from here. It really does appear as in your first image.

Just when I thought I had everything I needed for the Moon and Planets. Oh well, time to start saving again.

[John Boudreau]

John,

 

Thanks!! I was hoping to pick up a Pierro unit from Markus at Tucson in November, but the show was cancelled so I'll have to do it from the website. One last question (ya right!). What accessory(s) will I need to attach it to my filterwheel and camera?

 

Paul, here's the simple mod I did to the PA ADC levers. The PA ADC comes with metal dust caps and is sporting them in this photo. The prism levers and their plastic lever sleeve/handles are threaded and are designed to be slightly snugged to prevent the adjustment setting from slipping. Problem is, the ends of the thin lever handles contacting the knurled finish of the ADC's body causes a very rough action to the feel needed to fine tune the lever adjustment. Smoothing this action out requires a larger contact surface between the handles and the knurled body, so I found that #6 nylon washers worked perfectly. Any larger size washer wouldn't allow the levers to be 'zeroed out' as they would strike each other. I later added a tiny rubber O-ring to each lever, to act as a slight variable spring to change the drag as you tighten the handle. I don't recall the O-ring size #, but they are a fairly tight fit on the levers and seem to be about 1/8" ID. Silicon O-rings may be better in cold weather so I may upgrade to some during my next McMaster Carr order.

I picked up the nylon washers at Lowe's in the hardware section, and the O-rings in an auto parts store. 

 

As for adapters to work with your filter wheel, the PA ADC has male T-Threads on one end and female on the other. I use this adapter from OPT for the scope side of the ADC, and although one of OPT's photos of it show a 'safety' undercut, the later ones like mine do not have an undercut (I hate undercuts!).  It screws into the female end of the ADC leaving the other end's male T-threads to integrate with your filter wheel. Plenty of T-thread adapters exist to solve any mounting problems, but it may simply thread right into your FW.

 

BTW--- there is no dedicated front or back to the PA ADC--- it can be used either way. Same is true for the more recent ASH ADCs, as they now use T-threads at each end like the PA.

Attached Thumbnails

• 

[happylimpet]

Hi John thanks for posting all this info, very useful for those of us contemplating!  How many degrees can the 'mid point' of the whole setup be rotated...in the sense of allowing for the 'field rotation' of the dispersion, if you get my meaning.

 

ie if i incorporated it into my train, could i correct fully all the way from an object rising in the east to setting in the west? This would require approx 80degrees of rotation i think for me at +50N. (ie 2x (90-50)=80).

 

Of course this would very rarely happen....but it would be handy!

 

Cheers

 

Nick

[John Boudreau]

Hi John thanks for posting all this info, very useful for those of us contemplating!  How many degrees can the 'mid point' of the whole setup be rotated...in the sense of allowing for the 'field rotation' of the dispersion, if you get my meaning.

 

ie if i incorporated it into my train, could i correct fully all the way from an object rising in the east to setting in the west? This would require approx 80degrees of rotation i think for me at +50N. (ie 2x (90-50)=80).

 

Of course this would very rarely happen....but it would be handy!

 

Cheers

 

Nick

 

Good question Nick!

 

The overlap for the lever travel slots is 60°. I've never measured the total time available for imaging without resetting the body, but we're definitely talking periods measured in hours that allow referencing the levers to the horizon at zero correction. Once you get used to using it, you'll be able to reference the horizon without having to put the levers together to zero--- just imagine the line starting at the midpoint of the ADC's diameter and running exactly between the separated levers, and using that imaginary line as the horizon reference. Doing it that way you can gain even more hours before rotating the ADC's body in the focuser. It might be tough to picture that without an ADC in your hands, but once you get one I think you'll see what I mean. I just wish I knew how to explain it better.

 

It's mostly cloudy here tonight so my ROR observatory is closed, but I'll try to do a dry run at declinations for Jupiter and Saturn soon. It won't exactly match your situation as I'm at 42° 28', but it'll probably give you a better idea of what's possible. I'm curious about it myself now, and I've got some clear nights predicted for the rest of the week!

[John Boudreau]

 

Hi John thanks for posting all this info, very useful for those of us contemplating!  How many degrees can the 'mid point' of the whole setup be rotated...in the sense of allowing for the 'field rotation' of the dispersion, if you get my meaning.

 

ie if i incorporated it into my train, could i correct fully all the way from an object rising in the east to setting in the west? This would require approx 80degrees of rotation i think for me at +50N. (ie 2x (90-50)=80).

 

Of course this would very rarely happen....but it would be handy!

 

Cheers

 

Nick

 

Good question Nick!

 

The overlap for the lever travel slots is 60°. I've never measured the total time available for imaging without resetting the body, but we're definitely talking periods measured in hours that allow referencing the levers to the horizon at zero correction. Once you get used to using it, you'll be able to reference the horizon without having to put the levers together to zero--- just imagine the line starting at the midpoint of the ADC's diameter and running exactly between the separated levers, and using that imaginary line as the horizon reference. Doing it that way you can gain even more hours before rotating the ADC's body in the focuser. It might be tough to picture that without an ADC in your hands, but once you get one I think you'll see what I mean. I just wish I knew how to explain it better.

 

It's mostly cloudy here tonight so my ROR observatory is closed, but I'll try to do a dry run at declinations for Jupiter and Saturn soon. It won't exactly match your situation as I'm at 42° 28', but it'll probably give you a better idea of what's possible. I'm curious about it myself now, and I've got some clear nights predicted for the rest of the week!

 

 

Ok, I just did a dry run in my observatory simulating use of my Pierro-Astro ADC in an attempt to see how long it could be used without rotating it in the focuser. Normally to start, one would swing the levers so they are together (side by side), which puts the ADC in the 'zero' correction position. Then the ADC body would be rotated in the focuser so that these side by side levers would be parallel to the horizon. Doing this just as a target would be rising due east would allow me to continue to use the PA ADC past the meridian, perhaps a couple hours past depending up on the required correction setting.

 

But then I positioned the levers together (zero correction) in the middle range of the overlapping portion of their travel slots and with the scope pointed at the meridian, rotated the ADC's body in the focuser so that the levers would be parallel with the horizon. Once I did that, I found that the PA ADC could be used from horizon to horizon! Well almost--- my mount couldn't do it because of interference, but the ADC certainly still had plenty of correction travel to work. Only problem is that towards either horizon there isn't enough overlap to 'zero' the levers together as a horizon reference--- one would have to use the trick quoted from my previous post above from the mid paragraph, now underlined.

[Kokatha man]

Ok - just as an exercise in comprehension for me John...or in case one of those (not irregular) urges to buy something I never realised I couldn't do without until I saw/heard about them occurs..!

In this last post of your's:

"Scenario #1" - bring the levers together, then rotate the ADC so that those levers appear as if they are parallel to the ground. (assuming we are on a flat plain, or if we have placed our tripod/pier on the side of a steep incline we line the levers up with our - hopefully - flat horizon somewhere we can see it!)

Then we are set to go for a considerable time (first adjusting said levers to compensate for the dispersion of our target of course - & is checking every hour or so to possibly re-adjust the lever positions necessary..?)

"Scenario #2" - point your scope N or S along the meridian line with the levers side-by-side but this time such that they are together in that central portion of their slots that over-lap each other. (ie, the middle of the slot over-laps as per my annotation of your image below...)

Then rotate the ADC body such that these levers are parallel with the horizon...target your planet & adjust the levers so that there is no/minimal red/blue dispersion & away you go & should be good for a long session.

However, I am still asking you whether you need to check that this setting of the levers is correct for AD every couple of hours or so?!?

The under-lined bit I think I appreciate, but I'll wait for you to mark my comprehension here before asking anything more..!

[John Boudreau]

Ok - just as an exercise in comprehension for me John...or in case one of those (not irregular) urges to buy something I never realised I couldn't do without until I saw/heard about them occurs..!

In this last post of your's:

"Scenario #1" - bring the levers together, then rotate the ADC so that those levers appear as if they are parallel to the ground. (assuming we are on a flat plain, or if we have placed our tripod/pier on the side of a steep incline we line the levers up with our - hopefully - flat horizon somewhere we can see it!)

Then we are set to go for a considerable time (first adjusting said levers to compensate for the dispersion of our target of course - & is checking every hour or so to possibly re-adjust the lever positions necessary..?)

"Scenario #2" - point your scope N or S along the meridian line with the levers side-by-side but this time such that they are together in that central portion of their slots that over-lap each other. (ie, the middle of the slot over-laps as per my annotation of your image below...)

Then rotate the ADC body such that these levers are parallel with the horizon...target your planet & adjust the levers so that there is no/minimal red/blue dispersion & away you go & should be good for a long session.

However, I am still asking you whether you need to check that this setting of the levers is correct for AD every couple of hours or so?!?

The under-lined bit I think I appreciate, but I'll wait for you to mark my comprehension here before asking anything more..!

ADC.png

 

Hi Darryl,

 

First, just so everyone reading this is clear on the reason to first reference the levers to the horizon:
Atmospheric dispersion occurs along a vertical straight line from the target perpendicular to the horizon and extending directly towards the zenith. The ADC produces it's own dispersion , but of an opposite value to that of our atmosphere so that the two values can be canceled out. To do this, the ADC's line of correction must also be aligned vertically relative to the horizon. The levers are attached to the prisms 90° from the ADC's vertical dispersion line, so when they are set together to the 'zero' position (no correction), they must be aligned to the horizon.
I am referring to the true horizontal datum, not a local slope adding confusion. Recently for these tests I've picked up a small line/string level, which can be set on the levers to check their true horizontal alignment. In practice, we really don't need to be so precise, as 95% of a perfect alignment will appear the same to us as 100%, and a setting good to 90% is still far better than no correction at all. Eyeballing the setting is usually good enough. But adding such a level to our tool kit may be a good idea for some locations.

 

You’re understanding of the setup/alignment is correct Darryl, as is you're annotation of the mid slot position. As for your question on how often to check the correction settings--- ideally we would be constantly checking correction, but that's unrealistic. As an object is low in the sky but on the rise (or setting), it's altitude will be changing much more quickly than when it's near culmination. It's the rate of altitude change we have to watch as dispersion varies with altitude. Near culmination, I've gone an hour or more without adjusting correction. When low in the sky, I've adjusted as quickly as every 20 minutes. Again, not a perfect scenario, but far better than no correction at all.

 

You'll find it much easier to learn the ADC with a OSC camera as you can directly see what's going on in real time. Recently I've been using the temporarily increased saturation trick with my OSC to identify dispersion errors--- very sensitive and very quick! I even detected a very minor lever misalignment to the prisms with the ASH ADC that I hadn't noticed before--- I had to rotate it very slightly CCW to the horizon to get the dispersion seen on the ansae of Saturn's rings perfect left to right (Edit: That last sentence is very poorly worded. I could have easily adjusted the dispersion perfectly by moving the levers very slightly CCW, but I was trying to determine the amount of lever to prism misalignment with a bubble level at the time). So right now I'd say with your current use of the ASI224MC you have the perfect camera to learn with! You'll be a master by the time an equivalent mono camera becomes available.

Edited by John Boudreau, 27 August 2015 - 10:38 PM.

[volkerw]

I guess, the ADC is only useful for color-cams, or are there any advantages for monochrome-cams?

[John Boudreau]

I guess, the ADC is only useful for color-cams, or are there any advantages for monochrome-cams?

 

An ADC is useful for both color and monochrome. In fact most of my ADC experience is with monochrome cameras.

 

Dispersion still occurs within the bandpass of the filters used with mono cameras, and results in a vertical blur relative to the horizon. The effect of dispersion is a bit stronger in color cameras, because they typically have a greater overlap in their Bayer filter transmittance profiles than we have with RGB optical filters used with mono cams.

[Sunspot]

I would imagine that an ADC would be helpful when doing UV imaging, like on Venus or Jupiter.

[Kokatha man]

Thanks very much for your reply John!

Will think long & hard about this possible acquisition in the coming months...Sam's ZWO unit might be an option - I hope you're absorbing all this Sam to create a really good ADC!

Ok - I'm sure you have said this before (cannot seem to see it in this thread however) but once we set the levers as per above then I presume it's time to adjust each lever's specific position to correct the red & blue dispersion using the osc with increased saturation trick...could you point me to the thread where you've described this aspect please John? (or repeat it perhaps! )

You're right re good timing what with Saturn on the descent now & Uranus only 50 degrees maximum from where we are a bit North of home...

The very last part of your last post re slight adjustment might help me with understanding the final steps for correction...you say you needed to rotate the ADC body slightly ccw, wrt the horizon to obtain "proper" left to right correction...is this after (a) setting the levers together correctly & parallel with the horizon as understood now...then (b) adjusting each lever to correct for each of red & blue dispersion effects..?

Or am I completely "up a wattle tree" as we say here re this (b) operation..?!?

Thanks once again John!

[John Boudreau]

I would imagine that an ADC would be helpful when doing UV imaging, like on Venus or Jupiter.

 

The PA ADC works nicely with Venus in the UV:
http://alpo-j.asahik...5/v150530c1.jpg

 

Taken with Venus at 38° altitude, which is enough to noticeably effect UV recorded through the CWL 355nm  Astrodon UVenus filter.

 

I've never shot Jupiter in UV though. I guess I'll need a filter wheel with more slots.