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ADC v no ADC on Venus UV imaging

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About this observation
Observer
Martin Lewis
Time of observation
15/04/2020 - 18:44
Object
Venus
Observing location
St Albans
Equipment
444mm Dobsonian
ASI290MM
Baader Venus filter (type 2)
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Illustration of the benefit of using an atmospheric dispersion corrector (ADC) on UV imaging of Venus. Identical processing for both. Even at 40° altitude it helps improve surface detail.

Even though the Baader Venus filter filter bandpass is only 60nm wide, the change in atmospheric refraction with wavelength is dispersion is high at these shorter wavelengths. As a result you get some vertical smearing as the atmosphere acts like a giant prism.

Note on the R image the sharp right hand side (vertical edge) but the blurred bottom edge (hztl). Also note the asymmetric horns that result too from not correcting for the dispersion.
Martin

Comments

Peter the Fraudfinder's picture

Martin

That is fascinating.  I have an ADC but have been deliberately avoiding using it on Venus with the Baader UV filter as I thought that any additional glass reduces the amount of UV that will get through.

If I get an othe rchance soon I will try with the ADC.

Peter

Fossil Light's picture

Hi Peter,
Some makes will attenuate the UV more than others. Compare with and without the ADC at the same camera settings to see how much it impacts the brightness (but keep the same Barlow to camera spacing otherwise the image scale will be thrown out affecting the image brightness).

If the image is 30% dimmer just record for 30% longer and the signal to noise in the stacked image will be identical to before!
Cheers
Martin

Peter the Fraudfinder's picture

Thanks, Martin

That is very helpful.   I use a Pierre Astro ADC I bought some years ago.   I have been avoiding Barlows on Venus for the same reason - I still get a nice image scale at the moment with the 14inch at f/10 and it means I can run the camera at 200-250 frames per second even with the Baader U Venus fiiter - getting the pick of the seeing with it.   I have certainly been picking up more cloud detail than with a Barlow or PowerMate in the past.

Peter

Fossil Light's picture

Is your Pierro  Astro ADC the UV transmissive version? Early on there were two versions sold but now they just do the version which extends into the UV with fused silica prisms and UV transmissive coatings.    

Martin           

Peter the Fraudfinder's picture

Martin

That is a question I can't answer unless you know a means of checking by examining it.  I bought it before my current computer and email records (which go back to 2013) and at the time it was one of only three ADCs available in Europe. There was a rival at around £550 and another at a mind numbing price (£999) but the Pierro Astro version I bought was (from memory) a more 'reasonable' £450.   UV wasn't on my radar at the time so I didn't notice - I wanted it for Jupiter, Saturn and Mars with my then 12inch SCT. I hadn't considered clouds on Venus as a viable imaging option.  I bought the 12inch in late 2008 so the ADC could have been bought any time between 2009 and 2012.

Peter

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