Atmospheric dispersion

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  • #573706
    James Dawson
    Participant

    Damian Peach’s article on Atmospheric Dispersion and Atmospheric Dispersion Correctors has just gone up as a tutorial on the website (https://britastro.org/node/9058) and I wonder how many people use an Atmospheric Dispersion Corrector and if they have any comments about them, or tips on their use?

    James

    #577978
    Chris Dole
    Participant

    I’ve been using a ZWO ADC for several months now. Simple to use, especially with a Mak, SCT or refractor. With the planets being lower down from Great Britain in the coming years, getting one could be a worthwhile investment. The ZWO version is about £130 I believe, not cheap but they were a lot more expensive in the past.

    The bigger the aperture the more benefit you’ll see, and I see a noticeable improvement even in my 7″ Maksutov when imaging below about 30° altitude.

    Chris.

    #577979
    Robin Leadbeater
    Participant

    This look pretty good value as a couple of decent wedge prism eg from Edmund Optics alone would be close to this figure. This arrangement though would generate astigmatism placed in a converging beam wouldn’t it?  Shouldn’t there ideally be some form of collimator ?

    Robin

    #577981
    Robin Leadbeater
    Participant

    Thinking about it though, I guess in planetary imaging where focal ratios are invariably very high it is not so much of a problem

    Robin

    #577982
    Chris Dole
    Participant

    Although I’m far from an expert in these matters, I believe your correct about the converging beam introducing astigmatism. To achieve the correct image scale, these ADCs are usually used behind a Barlow. Would this limit the astigmatism? And does the long resulting focal ratios achieved (>f20) limit the effect? I know that in practice I see improvement in image quality.

    The attached image was taken at 17° altitude with a 7″ telescope in July. I can’t replicate this without the ADC without nasty colour fringing.

    Chris.

    #577990
    Damian Peach
    Participant

    Chris – that image of Saturn is a fine example to the benefit of using ADC’s. Had this been taken without the result would be no where near as good with severe colour fringing and smearing. With all of the major planets skirting through the southern part of the ecliptic over the next few years it is an ideal time for getting an ADC and learning how to use it.

    #578000
    Martin Lewis
    Participant

    As Damian says, now is a good time to get an ADC if you don’t already have one. About the price of a good eyepiece and a sound investment especially if you want to image or view Mars and Saturn from the UK in the next few years.

    Es Reid and I did a study of the levels of astigmatism generated by ADCs in use and we found that for a given scope the degree of astigmatism generated by a correctly adjusted ADC is primarily related to the altitude of the object. You can read more about this study at; http://www.skyinspector.co.uk/adcs-part2

    I also have a more general page on ADCs those those interested in learning more about these useful devices at; http://www.skyinspector.co.uk/atm-dispersion-corrector–adc

    Cheers, Martin

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