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Random refractor question

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#1 View2

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Posted 25 August 2019 - 09:19 PM

How important is it that a refractor be apochromatic if all you image is narrowband and big achromats are a lot cheaper with same 4 element FF/FR.

Edited by View2, 25 August 2019 - 09:35 PM.


#2 gezak22

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Posted 25 August 2019 - 09:45 PM

For narrowband imaging, an achro can be perfectly fine, but you still need to get a proper field flattener.



#3 AtmosFearIC

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Posted 26 August 2019 - 02:20 AM

You can do narrowband with a larger and cheaper achromat but you MUST refocus when changing filters. Colour correction isn't the only reason that an APO can be better for narrowband; spherical correction. A more expensive APO is more likely to have a better optical figure than a cheaper achro so the stars are likely to be smaller and tighter in an APO.

 

The short of it is that the colour correction isn't overly important with narrowband filters but a good optical figure still is and with cheaper achromats, that becomes the luck of the draw as they're built to a much cheaper price point.


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#4 Hesiod

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Posted 26 August 2019 - 05:28 AM

Remember also that mechanical features are just as critical.
If plan to replace the focuser anyway it's all right, but otherwise...
UNA lower end models can have very decent optics, but their mechanical features range from underwhelming to plainly awful.

EDIT: if want just more focal, reflectors would give this at much faster speed than "achros"

Edited by Hesiod, 26 August 2019 - 05:30 AM.



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