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Relpet

Atmospheric dispersion corrector: Omegon v. ZWO

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Please post directly into SGL, 

 

Here is the whole text from your PDF.

 

I got a fairly lukewarm reception to a previous thread concerning the ZWO atmospheric dispersion corrector but threads in CN and postings on YouTube seem to indicate they are useful for planetary work at low altitudes, Now Omegon have introduced a similar product: https://www.astroshop.eu/field-flatteners-other-photo-accessories/omegon-adcatmospheric-dispersion-corrector/p,53049 Omegon will sell direct but as they share premises with Astroshop in Bavaria I guess Omegon products are bought in from the Far East to be branded by Astroshop.

Same price whether direct or through Astroshop. Omegon is not a big name in UK but their other products are well-reviewed The Zwo item, sold throughout the world, sells at a similar price and is listed here by TS, for example: https://www.teleskopexpress.de/shop/product_info.php/language/en/info/p8504_ZWO-ADC-AtmosphericDispersion-Corrector---T2-and-1-25--connection.html I like the red ring scale on the Omegon and it is slightly cheaper, otherwise cannot see much difference. If anyone has any views on any aspect of this topic I would be grateful for some feedback.

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12 hours ago, Ant said:

Please post directly into SGL, 

 

 

 

I

Thanks Ant.  My cut and past didn't work - hence the pdf.  Sadly, still no response.  Not a popular topic, I guess.

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I've tried to find out whether we'd benefit for visual only f/6, but the technology seems more geared to refractors and imaging than for dobs. Am I wrong that the benefit is chiefly (or even solely) color correction? Also, in image comparos of with and without, I couldn't discern any appreciable differences. Finally, it seemed to require in-focus and we're just too tight on that, but I'd be interested to understand it better. (We're also tree-lined and tend to aim higher, but maybe the benefits accrue even for commonly bad seeing?)

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19 hours ago, laowhoo said:

 

I've tried to find out whether we'd benefit for visual only f/6, but the technology seems more geared to refractors and imaging than for dobs. Am I wrong that the benefit is chiefly (or even solely) color correction? Also, in image comparos of with and without, I couldn't discern any appreciable differences. Finally, it seemed to require in-focus and we're just too tight on that, but I'd be interested to understand it better. (We're also tree-lined and tend to aim higher, but maybe the benefits accrue even for commonly bad seeing?)

The principle is to correct colour distortion caused by diffraction in the thicker atmosphere at low altitudes. By careful adjustment the diffraction is reversed.  There are lots of articles written on the subject.  This one by Damian Peach (http://www.damianpeach.com/images/articles/JBAA dispersion Peach.pdf) is an example.  Damian is one of the world's foremost imagers but other users confirm that there are tangible benefits for planetary observation.  Mars will be very close this year, but low in the sky.  For me it's proved impossible since I took up the pastime three years ago to get a decent look at Mars and I'm prepared to spend the money to get a better view than might otherwise be the case.  A number of members of the Cloudy Nights forum have given positive reviews of the ZWO ADC for planetary observing.  No one has been forthcoming on the Omegon.  Few people on SGL seem to have an opinion so check out Cloudy Nights, maybe, for more direct advice from satisfied users.  Both the designs my topic was about work on the same principle and are significantly cheaper than anything available previously - hence my interest.  I hope this helps.

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Fascinating. Airy actually developed an EP to do away w/ the need for a separate corrector

http://adsabs.harvard.edu/full/1967Obs....87...41F

Then there are these

http://paquettefamily.ca/astro/star_study/

http://skyinspector.co.uk/atm-dispersion-corrector--adc

http://www.ianmorison.com/combating-atmospheric-dispersion/

And Ian does add that our star diagonal is in the way. I had already read some CN feedback early in the day. I'll revisit those w/ a better understanding/appreciation, even if we can't use it (we only have about 1/4" add'l in travel for standard EPs).

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Yes, the principle has been known for many years, it's the affordability that is different.  The ASH and Pierro Astro ADCs can still be bought but the ASH for double the price of the Omegon and, in the latter case, for the same price as an 8" Dob.  It would be wonderful to be able to see one in use and make a judgment.  As prices drop perhaps they will become more commonplace.  In the meantime I'm setting aside my pocket money for a purchase in early summer.

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I’m visual only and did get a zwo adc when they first came out a year or so back. I tried it briefly on Jupiter. But it was quite fiddly to use and I found that binoviewing made a much better difference for me. I think combining an adc and a binoviewer would be tricky.

So I sold the zwo adc and just stick to my binoviewers now. I know another SGL member that also got a zwo adc and also found it fiddly and didn’t have much impact visually. We both use refractors.

I think there ADC may be more useful for imaging than visual.

For the mars opposition this summer I’ve decided to fly south to tenerife to catch it higher in the sky.

Edited by GavStar

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Dare I suggest a yellow filter would be a cheaper option? For visual at least...

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16 minutes ago, GavStar said:

I’m visual only and did get a zwo adc when they first came out a year or so back. I tried it briefly on Jupiter. But it was quite fiddly to use and I found that binoviewing made a much better difference for me. I think combining an adc and a binoviewer would be tricky.

So I sold the zwo adc and just stick to my binoviewers now. I know another SGL member that also got a zwo adc and also found it fiddly and didn’t have much impact visually. We both use refractors.

I think there ADC may be more useful for imaging than visual.

For the mars opposition this summer I’ve decided to fly south to tenerife to catch it higher in the sky.

That's a valuable comment when it comes from a user - even a disappointed user!  I do my observing mainly during the summer months at 43 degrees N but the planets this summer will all, so far as I can discover, be low in the sky.  Weather the last two summers in SW France has not been good for stargazing but if we do get a good summer this year atmospheric dispersion could be quite high.  I know from other reports about the need to adjust by minute amounts and would be prepared for that.  I have the kit for imaging but not much chance to use it so far.  Maybe this year.  Good luck in Tenerife.

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24 minutes ago, Ags said:

Dare I suggest a yellow filter would be a cheaper option? For visual at least...

Thanks Agnes.  I'll try every filter in my box once I get the opportunity but not convinced any of them will be that effective.

Off topic, my grandmother's name was Vanderhoek.  Her grandfather emigrated from Dordrecht in the 19th century so I've always had a soft spot for the Dutch.  Heel erg bedankt!

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Atmospheric dispersion is just refraction and so affect extreme reds and blues more. A yellow filter cuts off the extreme ends of the spectrum, leaving a sharper albeit lemony image. In theory anyway :happy11:

Edited by Ags
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Atmospheric dispersion is a real issue in spectroscopy!!! The entrance slit has to be orientated to collect all the spectral data....

 

Atmospheric_Dispersion_spots.png

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21 hours ago, Merlin66 said:

Atmospheric dispersion is a real issue in spectroscopy!!! The entrance slit has to be orientated to collect all the spectral data....

 

Atmospheric_Dispersion_spots.png

I'm delighted to leave that field entirely in your hands, Merlin!

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Actually I've been using a relatively inexpensive ADC (cost me about £130) for a few months now and I find it a game changer! It's true, one can use filters but unless it's an interference filter with a very narrow bandpass, one will only be eliminating about 2/3 of the dispersion (which is still pretty good for the £5 price tag I'll grant you!). I'm finding with the planets so low at the moment, having the ADC is the difference between a blurry mush and a half decent view. 

 

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39 minutes ago, timwetherell said:

Actually I've been using a relatively inexpensive ADC (cost me about £130) for a few months now and I find it a game changer! It's true, one can use filters but unless it's an interference filter with a very narrow bandpass, one will only be eliminating about 2/3 of the dispersion (which is still pretty good for the £5 price tag I'll grant you!). I'm finding with the planets so low at the moment, having the ADC is the difference between a blurry mush and a half decent view. 

 

Thanks, Tim.  That's exactly what I expected to learn if the ADC is used correctly.  The cost is about what one would pay for either the ZWO or Omegon.  May I ask which is yours, please?  By using coloured filters you inevitably block off an element of light whereas with the clear prisms in the ADC there may be some light loss but the full spectrum would presumably still be present, if slightly reduced.  I've been in England for a couple of months now and am impatient to get back to France where all me astro gear is kept.  The first clear night I'll try out various filters but I'm even more persuaded now that the ADC is top of the birthday present list.

Thanks again, Peter

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10 hours ago, Relpet said:

Thanks, Tim.  That's exactly what I expected to learn if the ADC is used correctly.  The cost is about what one would pay for either the ZWO or Omegon.  May I ask which is yours, please?  

 

Mine's a ZWO brand that I got from First Light Optics and I'm 100% happy with it and their service. The disadvantage of the cheaper designs (like ZWO) is that the image shifts a bit as you adjust the prisms but that's something I can live with given the very reasonable price. The total range of motion is only about 20 minutes of arc so it's not really an issue. And of course once adjusted for a given elevation, it stays very much the same from night to night. Now I just pop it in each morning observing jupiter - Other than rotating the whole thing to minimise the dispersion, no other messing about is usually required. I did this sketch this morning with my 7" refractor (Jupiter on the meridian and about 20° elevation) LHS with the ADC - beautiful view of the red spot  RHS without the ADC - can just about make out the equatorial bands. Of course it won't correct for the worse seeing associated with low elevation but I'd say it makes more difference than just about any other eyepiecey/filtery thing I've ever bought!

5a72de1223053_Jupiterwithandwithoutatmosphericdispersioncorrector.jpg.38fc4cc30df9f4958f6cd56c73efedbb.jpg

Edited by timwetherell
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7 hours ago, timwetherell said:

Mine's a ZWO brand that I got from First Light Optics and I'm 100% happy with it and their service.  I'd say it makes more difference than just about any other eyepiecey/filtery thing I've ever bought!

5a72de1223053_Jupiterwithandwithoutatmosphericdispersioncorrector.jpg.38fc4cc30df9f4958f6cd56c73efedbb.jpg

'Nuff said, Tim.  Brilliant sketches.  7" refractor, eh?  Oh. Blimey.  There's another giant step forward from my 3".

Thanks a lot.

Peter

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Blimey Tim. Is the difference that pronounced? I had no idea!

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13 hours ago, Moonshane said:

Blimey Tim. Is the difference that pronounced? I had no idea!

I'm finding it so most days. I guess the thing with low elevation is that it also comes with more pronounced seeing related problems too. On some days when the seeing is so bad one is looking through a shimmering river of air then it's seeing that contributes most to image degradation rather than dispersion so the effect of the ADC is less noticeable. But when the seeing is good, it's very, very much better with the ADC. Prior to owning one, I thought dispersion was just adding a coloured halo to the top and bottom of the planet. But thinking about it further, it's actually smearing the RGB right across the disc, so it's like looking through a photoshop motion blur filter. I'm definitely an enthusiastic convert to these things - especially for £130 which is not peanuts, but it's not crazy money in amateur astronomy terms either :)

 

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You' be better I reckon for this purpose (and if you have restricted budget and with planets low)  to buy a cheaper good quality plossl and an ADC rather than an ultra expensive planetary eyepiece. 

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On 17/02/2018 at 11:38, Moonshane said:

You' be better I reckon for this purpose (and if you have restricted budget and with planets low)  to buy a cheaper good quality plossl and an ADC rather than an ultra expensive planetary eyepiece. 

I never quite got how you define a planetary EP.  I do have some good, some very good and a couple of excellent EPs in 1.25 and 2 inch sizes ranging from 5mm to 38mm so I'm guessing I just need the ADC.  Still waiting for some indication that the skies might start clearing more than six hours a month before making a purchase.

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Absolutely! On an excellent night with jupiter or Mars near zenith I think a dedicated "planetary" eyepiece perhaps has a slight advantage over something like a skywatcher plossl. But with planets 15 degrees above the horizon dispersion and seeing effects will be 10,000x more significant than eyepiece differences. I think once upon a time before computer ray tracing, ED glass and multicoatings something like a monocentric would have been spectacularly better than say a Huygens eyepiece on planets. But today I don't think there's nearly so much difference especially since generic plossls are often really quite well made. 

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How much in-focus is required for visual use of these ADCs?

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Agreed guys. I was making the point that an ADC  appears more useful than a better eyepiece not that any eyepiece is a standout for planetary as such.

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