ADC- essential for planetary?

[mikeDnight]

4 hours ago, andrew s said:

I think it does hold up quite well . I will try to find the published research papers I read some years ago that supported this view. Do you know of any pubished evidence against it or have you perhaps studied it?

On comparing refractors and reflectors I think you need to look at why the best planetary images are from those terrible SCTs. Yes you need a larger reflector to give the same resolution as a refractor that's physics and well understood.  It is interesting aside to note P Lowell tended to stop down the 24" refractor for his observations.

On resolution, while the resolution limit is a convention it applies to reflectors and refractors alike. Physics limits what can be seen even if you don't  belive it. Small telescope have a more limited resolution than larger ones (assuming any central obstruction is accounted for correctly). There is no magic in optics.

I never claimed telescopes produced a pixelated image just used that as an example of where we tend to prefer small and sharp as to large and more blured. Just as you find a natural preferred magnification not too high and not too low.

So I have bitten back😜

Regards Andrew

PS I have tried to track down the papers I read years ago but it was in the age  of getting paper copies via the library! However, the Fried parameter is alive and well and the following might be worth a look.

https://www.telescope-optics.net/induced.htm

https://www.handprint.com/ASTRO/seeing1.html https://www.handprint.com/ASTRO/seeing2.html https://www.handprint.com/ASTRO/seeing3.html

and references their in.

Simple view here https://skyandtelescope.org/astronomy-equipment/beating-the-seeing/#:~:text=So will every other telescope,after bringing a telescope outdoors.

Most modern references concentrate on adaptive optics but the Fried parameter (https://en.wikipedia.org/wiki/Fried_parameter )  still persists.

Those terrible SCT's do indeed produce some of the best planetary images, but that's probably due to their popularity and compactness, and not because they produce a high definition visual image. And they were originally designed as cameras, not visual scopes at all. Then you have to consider the vast improvement in imaging technology and computer software, which has aided the SCT no end to create something close to recognizable. Try using a film camera and the pictures are not so good. I'm convinced I can hear the poor photons scream as they realize after their long journey,  that their demise will be as photon splatter as they bellyflop on that awful Schmidt plate. 😈

Lowell stopped down the 24" because of the chromatic aberration it produces. The telescope was built to fit the observatory dome and not the other way around, so the Lowell refractor is far too short in focal length to be properly colour corrected.

I promise I'll read the links you so kindly attached Andrew, but I doubt anything will convince me that atmospheric cells are the reason why some larger scopes fare badly at times. ☺

 

Edited July 23, 2020 by mikeDnight

[fate187]

No I had no barlows or PM back then. 

I just measured the height of the quick changer: 10mm. Add to that 5mm from the dovetail/bayonette part with T2 thread you get 15mm. However, in my bino the dovetail is similar to the one in Baaders Mark V/Maxbright II, which may reduce the optical path length. The Maxbright also comes with an T2 adapter to directly thread it on a male T2 adapter (like on their T2 diagonals).

 

[fate187]

I had another pleasant last night. I used my TEC140 with the ADC/mirror diagonal/1.7GPC/bino and 7, 9 and 12.5mm eyepieces. Conditions were not sogood during the beginning of the session at around 10-10:30 p.m. Jupiter was around 5-7° altitude. The problem was not AD but the atmosphere. It was like looking through a waterfall ;). Still, detail was visible: the GRS and clouds around it, the bands north and south of the equator, etc. It greatly improved later on towards the meridian. Jupiter's moons were tight defined spheres discs, with different size and magnitude. 

I did another test: I switched to view without ADC and what is easily noticeable is, that the moons are not defined spheres discs anymore, but washed out/elongated due to the AD. Also details on Jupiter were less defined and less sharp. Less sharpness was also very prevalent on Saturn without ADC.  

That being said, an ADC will help you in good nights to make (some) extra detail/features visible, but will not help when the jet stream or local seeing is just not there :).

Edited July 24, 2020 by fate187

[Rusted]

Probably irrelevant to this discussion but I now routinely use a 2.6x GPC in my 6" H-alpha f/10 1500mm refractor. I fit the GPC without hesitation before each solar session since discovering that its resulting images were no worse than without it. I am using an ASI174 at high frame rates at 800x600 in a PST modified 150/10 H-a in short, selective bursts while constantly monitoring the image on a 27" quality screen for lucky moments of best seeing in the shade of a dome.

I have never gained anything in image quality from stopping down any of my refractors over the years. Nor by reducing the magnification to a lower level than average. I have experimented with stopping down my old 6" f/8 Celestron CR150 "Kaleidoscope," Vixen 90, iStar 180/12 R35 and iStar 150/10 H-a. None of them showed the exercise of stopping down to be worthwhile. Though I did once set fire to an experimental cardboard ring in the dewshield of my old 6". 

There are a number of solar imagers now achieving truly remarkable results from SCTs up to 30cm using Airies full aperture ERFs in the right seeing conditions. Usually early morning and late afternoon. A habit I have adopted myself. Since it regularly provides stunning clarity and steadiness of greatly enlarged images. Meanwhile, the incredible scale and detail of the SCT solar images are more suggestive of orbiting imaging platforms than amateur astronomers working from the ground.  Solar Chat has shared examples of these incredible images.

According to reports on CN it was established some years ago that an optimised, long focus Newtonian of between 30-35cm aperture would trash anything else in the park on planetary image quality in good seeing. It required careful optimisation to ensure thermal issues were addressed, a high quality mirror and a suitably small high quality secondary, were well collimated in a thermally neutral [cardboard] tube with "cooling fans" to scrub the front surface free of convection currents and speed the glass to ambient temperatures. No doubt thinner, or meniscus mirror blanks, suitably supported, would provide even better results. No doubt the argument could easily be made that all the other telescopes present of much larger aperture. Or even large and costly APOs. Were all much too "warm" to perform to their theoretical limits. A bit like bringing a large and jewel encrusted knife to a gunfight?

Stop me if I'm rambling. 

[markse68]

57 minutes ago, fate187 said:

I did another test: I switched to view without ADC and what is easily noticeable is, that the moons are not defined spheres anymore, but washed out/elongated due to the AD. Also details on Jupiter were less defined and less sharp. Less sharpness was also very prevalent on Saturn without ADC.  

You probably need better quality optics or you lack experience in your observing or you should have been using a smaller scope.

[chiltonstar]

10 minutes ago, markse68 said:

You probably need better quality optics or you lack experience in your observing or you should have been using a smaller scope.

I agree with Michael (fate187); looking at Ganymede at the moment without the ADC  (180 Mak, 10mm and 6mm Baader orthos) I can't see the dark patterns I would expect to see under better conditions as the disk is blurred vertically by the AD. With the ADC, Ganymede becomes a disk once again with detail. Maybe I'm more aware of AD than some because I have very good colour vision apparently (according to the workplace optician I went to when using high power lasers in the lab) but ppoor steroscopicity. Use of a smaller aperture refractor will of course reduce the amount of AD in line with theory, but of course will also reduce the amount of available resolution - no way around the laws of physics here I'm afraid. I see using an ADC as maximising the amount of resolution potentially available with more aperture.

Chris

[BGazing]

1 hour ago, fate187 said:

No I had no barlows or PM back then. 

I just measured the height of the quick changer: 10mm. Add to that 5mm from the dovetail/bayonette part with T2 thread you get 15mm. However, in my bino the dovetail is similar to the one in Baaders Mark V/Maxbright II, which may reduce the optical path length. The Maxbright also comes with an T2 adapter to directly thread it on a male T2 adapter (like on their T2 diagonals).

 

Well your astigmatism was a factor of the f/l. You have to push it past f/10 to do away with astigmatism...

But yes, Gutekunst is a proper ADC for that scope.

 

[fate187]

You could be right there BG. The Gutekunst does not have this issue. Strehl including astigmatism and tilt 0.998.... 

BTW: Some were arguing the two levers of the ZWO unit. There is an ADC from Pierro Astro (Version 3) which has only one knob for tuning the ADC, so somewhat less "complicated", of course its more than the ZWO unit: https://www.teleskop-express.de/shop/product_info.php/language/en/info/p5992_Pierro-Astro-ADC-MK3-Atmospheric-Dispersion-Corrector-with-T2-connection.html . 

I would like to try out the ZWO and Pierro Astro device again, after I gained some knowledge in this field in the recent months.

best regards

[CraigT82]

41 minutes ago, fate187 said:

I would like to try out the ZWO and Pierro Astro device again, after I gained some knowledge in this field in the recent months

For what it's worth, Damian Peach uses the ZWO ADC in his personal scope for imaging (250 Mewlon), he says he has tried all the commercially available ones but sees no benefit over the humble ZWO (for his own requirements)

Edited July 24, 2020 by CraigT82

[BGazing]

4 hours ago, fate187 said:

You could be right there BG. The Gutekunst does not have this issue. Strehl including astigmatism and tilt 0.998.... 

BTW: Some were arguing the two levers of the ZWO unit. There is an ADC from Pierro Astro (Version 3) which has only one knob for tuning the ADC, so somewhat less "complicated", of course its more than the ZWO unit: https://www.teleskop-express.de/shop/product_info.php/language/en/info/p5992_Pierro-Astro-ADC-MK3-Atmospheric-Dispersion-Corrector-with-T2-connection.html . 

I would like to try out the ZWO and Pierro Astro device again, after I gained some knowledge in this field in the recent months.

best regards

Yes, MK3 PA looks very elegant except that it has been unavailable for quite some time.

I like the one knob adjustment. MkIV with a bubble level would be perfect.

[fate187]

Just imho bubble level is not necessary. I think some degree mismatch is ok and may not be visible visually. Maybe when doing AP? And the bubble level needs careful alignment on the device during production

[BGazing]

Tried some more things last night - seeing was again meh, clouds came and went and distant flickering of the thunderstorms did not look like I will be observing much.

C8 was not used, again only Tak with binos, with and without ADC.

At around 100x in binos you can see AD. At around 150 it is not so pronounced (dimming, i guess, but it is still there). At 100x the moons have a bit of AD elongation. ADC clears that up and they become points. You certainly can use Tak without ADC and it is not overwhelming, but it is there and it blurs the picture. Also, the seeing  somehow appears better with ADC (this was also observed by the others). Jury is out there if seeing seems better subjectively with or without binos...there's certainly less eyestrain when viewing through the running water with both eyes.

More measuring stuff...when using binos without ADC I finally put the GPC in front of the diagonal to simulate 130x...and, believe it or not, it provides TOO MUCH back focus, I racked focuser out all the way with my extension and could not come to focus. It quite possibly would focus with the normal Tak extension (I use 47mm clicklock instead), or using normal diagonal with ep holder and 1.25 nose on Binoviewer, but at the expense of more magnification as per calculations above (approx 3.8x altogether). But if you are in dire need of backfocus, there's your solution, just put it in front of the diagonal and you are good to go.

Edited July 25, 2020 by BGazing

[fate187]

Nice report BG :).

I also observed Jupiter and Saturn last night with the TEC140 and ADC/bino. I used the 1.7 GPC with which I come to focus nicely. However for magnification I also tried the 2.6 GPC. This let to some extrafocal blueish tint... The focused image was nice though.

Jupiter put up quite a show. Io was moving in front of the Jovian disc followed by the moons shadow. Remarkably, the disc of Io was very well defined against the planet during the first 20min after the beginning of the transit. Did I make out surface details on the moon? Don't know. But the disc definitly seemed to have some coloration compared to is typical disc-like appearance when not in transit. Maybe the lighter background of Jupiter provides some less drastic constrast between the surface of the moon and the Jovian disc. Because the moons appear as homogenous discs (although with well defined magnitude differences and a little bit of different coloration).

Unfortunately, I wasn't able to observe the appearing GRS, because I was too tired from the night before and conditions were becoming worse. Nontheless, a beautiful observation.

[knobby]

On 23/07/2020 at 19:52, andrew s said:

I think it does hold up quite well . I will try to find the published research papers I read some years ago that supported this view. Do you know of any pubished evidence against it or have you perhaps studied it?

On comparing refractors and reflectors I think you need to look at why the best planetary images are from those terrible SCTs. Yes you need a larger reflector to give the same resolution as a refractor that's physics and well understood.  It is interesting aside to note P Lowell tended to stop down the 24" refractor for his observations.

On resolution, while the resolution limit is a convention it applies to reflectors and refractors alike. Physics limits what can be seen even if you don't  belive it. Small telescope have a more limited resolution than larger ones (assuming any central obstruction is accounted for correctly). There is no magic in optics.

I never claimed telescopes produced a pixelated image just used that as an example of where we tend to prefer small and sharp as to large and more blured. Just as you find a natural preferred magnification not too high and not too low.

So I have bitten back😜

Regards Andrew

PS I have tried to track down the papers I read years ago but it was in the age  of getting paper copies via the library! However, the Fried parameter is alive and well and the following might be worth a look.

https://www.telescope-optics.net/induced.htm

https://www.handprint.com/ASTRO/seeing1.html https://www.handprint.com/ASTRO/seeing2.html https://www.handprint.com/ASTRO/seeing3.html

and references their in.

Simple view here https://skyandtelescope.org/astronomy-equipment/beating-the-seeing/#:~:text=So will every other telescope,after bringing a telescope outdoors.

Most modern references concentrate on adaptive optics but the Fried parameter (https://en.wikipedia.org/wiki/Fried_parameter )  still persists.

Really interesting read 👍🏼

[fate187]

On 23/07/2020 at 20:52, andrew s said:

...

https://www.telescope-optics.net/induced.htm

https://www.handprint.com/ASTRO/seeing1.html https://www.handprint.com/ASTRO/seeing2.html https://www.handprint.com/ASTRO/seeing3.html

and references their in.

Simple view here https://skyandtelescope.org/astronomy-equipment/beating-the-seeing/#:~:text=So will every other telescope,after bringing a telescope outdoors.

Most modern references concentrate on adaptive optics but the Fried parameter (https://en.wikipedia.org/wiki/Fried_parameter )  still persists.

Nice references!

Interestingly in the S&T link there is reference to AD and seeing. See the lowest grafic where AD and influence of poor and good seeing are plotted against star image size. This showes, that even in good seeing objects below 60° will have an added effect due to AD. Mr. Gutekunst who is the designes and manufactures the ADC, also suggested to use the ADC even for objects above 50-60°.
Of course the graph also tells, that in bad seeing situations ADC's won't help and seeing dominates the image or viewing quality.

On the thought about smaller scopes giving better views than larger in bad seeing the S&T link clearly says, that this is not the case.
I had a thought experiment: Consider a 100mm scope, an air mass with height of 1000m, eddy size of 300x300x300mm³ and altitude straight above at 90°. Could you calculate the statistical chance for non-overlapping eddies on a circle with 100mm diameter projected on the scope. The same for a larger scope, smaller/larger eddys and for different altitudes with its increased airmass. 🤔

[John]

So it's use an ADC or don't bother then ?

As per the title of this thread - it appears that they are essential for good (serious ?) planetary observing even when the planets are significantly higher in the sky than they are now.

 

 

 

[Paz]

It's been very interesting following this thread - I'm looking forward to trying a few new approaches but I've been clouded out ever since I mentioned the clouds!

 

[fate187]

27 minutes ago, John said:

So it's use an ADC or don't bother then ?

As per the title of this thread - it appears that they are essential for good (serious ?) planetary observing even when the planets are significantly higher in the sky than they are now.

 

I would say, that the benefit decreases with increasing altitude of the planets :D. Larger apperture users may notice this easier because they can magnify more and retain brightness of the object and thus the AD effect.

For the low heights the planets are currently, I don't want to observe without one. For me it is a similar gain in viewing quality as the switch from mono to bino. Which I also don't want to miss during then planetary season. And just as mono vs. bino discussion, you have to try it for yourself

Edited July 25, 2020 by fate187

[John]

Having invested several £thousand in planetary scopes a few years back I guess it would seem churlish not to try a device for £100 or so at some point

 

[markse68]

Essential is a strong word but I was trying to draw attention to what I was witnessing I guess  It’s early days yet for me and my adc- maybe i’ll get bored of the extra hassle though it’s not really a hassle- I got bored of my motorised focuser so who knows. If i lost it or forgot to take it with me I’d of course not refuse to look at the planets 😂 I’d miss it though 

[BGazing]

Essential? Well, having a scope and an EP is essential.

But really, in a small frac you might give it a pass...and in a bigger scope you will clearly see AD. It's one of those things when you remove it and you go 'a-ha!'. Sort of like when you go to your ophthalmologist and you think they got your prescription right and then they do the little astigmatism correction and it just gets better...

The cutoff altitude may vary by aperture. Jove at current altitude takes a particular AD hit, just the nature of the target.

[John]

5 hours ago, John said:

Having invested several £thousand in planetary scopes a few years back I guess it would seem churlish not to try a device for £100 or so at some point

 

On a practical note, how much inwards focuser travel do the ZWO and Altair ADC's eat up ?

I've seen a figure of 57mm for the ZWO but that sounds a lot to me

Thanks

[markse68]

8 minutes ago, John said:

On a practical note, how much inwards focuser travel do the ZWO and Altair ADC's eat up ?

I've seen a figure of 57mm for the ZWO but that sounds a lot to me

Thanks

it probably is about that- the optical part is ~30mm then add the displaced ep holder. Adding a barlow complicates things- shortening it somewhat i think

[John]

A barlow isn't mandatory is it ?

 

[markse68]

20 minutes ago, John said:

A barlow isn't mandatory is it ?

 

well if the scope is faster than f10 then it’s a good idea as the prisms can introduce other aberrations apparently. otherwise not but it could help with in- focus travel too? I’m using a 2x with my f8. gives my orthos a bit more er too which is nice. The ultra-ultra-expensive Gutekunst model (they do 2 versions)  has a built in barlow

Edited July 25, 2020 by markse68