Quote:
Originally Posted by glenc
but it was better with the 21mm.
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4mm exit pupil thats why.
I have found that a 4mm exit pupil, or thereabouts, provides the best contrast balance for observing the Horsehead. Thus
ANY 1/2 decent 20mm eyepiece is a good choice in an F5 scope. Conversely if your scope is an F8 to F10 SCT or long tube reflector you will need a 30mm to 40mm eyepiece. In addition, any filters you are likely to use to try observing the horsehead deliver optimium performance with a 3mm to 5mm exit pupil.
Cheers,
John B