28 replies to this topic

[SeattleScott]

Side question: many here are commenting on how the long focal length of OP's scope will make it easier to use cheaper eyepieces. I would like to understand that better.

As I understand it, short focal ratios create spherical aberration (for refractors) and coma (for reflectors) at the edge of the image. Do expensive eypieces correct for that? Or is some other physical property of long focal ratios involved here?

Short F ratio refractors suffer from chromatic aberration, unless they are Apos. Expensive eyepieces will not help with coma or CA. If they did, they would over correct for these with other scopes that don’t suffer from coma or CA.

[Tony Flanders]

IMNSHO 90mm from Bortle 7 skies isn't going to do much for DSO observing.  My 4" Mak even has trouble resolving the Auriga clusters. I think a nice 6" F5 reflector is a minimum requirement for DSOs from Bortle 7.

Bigger is almost always better, and darker is definitely always better. But a 90-mm refractor in Bortle-7 skies can still do a mighty lot. Never as much as you would like, but that's always true, even when you're using a 30-inch scope under pristine skies.

[Jon Isaacs]

Bigger is almost always better, and darker is definitely always better. But a 90-mm refractor in Bortle-7 skies can still do a mighty lot. Never as much as you would like, but that's always true, even when you're using a 30-inch scope under pristine skies.

 

 

I do a fair amount of observing from I consider Bottle 8 skies with 80 mm refractors and see quite a few DSOs. I think Tony has done most if not all of the Messier's with a 70 mm from an urban site.

 

Jon

[Dave Mitsky]

There was an eyepiece design known as the Pretoria that corrected for mirror coma years ago but it was not commercially successful.  

https://astromart.co...toria-eyepieces

https://theskysearch...pic.php?t=17156