Excellent article on refractors for visual obs.

[cajen2]

https://www.dropbox.com/s/bgfoalq0umf11ut/Understanding%2C Using%2C and Choosing Refractors.pdf?dl=0

It's American and some manufacturers/retailers are ignored but explains a lot about CA, spherical aberrations and different sorts of glass.

Edited November 4, 2023 by cajen2

[Louis D]

Yep, a pretty good and well organized summary of discussions on the various refractor forums.

He does completely ignore high quality, slow achromats like those that were made by D&G Optical.  Admittedly, they're only available on the secondary market now.

[John]

A useful piece - thanks for posting it 🙂

Good to see spherical aberration covered. IMHO it can make a more substantial impact than CA on optical performance.

Edited November 4, 2023 by John

[glafnazur]

The paragraph below interested me. I used to do this and then read that quite a few people leave the lens cap off so I started doing that, maybe I'll go back to cap on.

When bringing from a cold environment to a warm environment, put lens caps on for the scope and eyepieces, and put them in their case if possible. Close the case to trap in the cold air, then bring the cases inside and let them slowly warm up over 12-24 hours. This is to prevent frost from forming on a lens surface (even on the inside of the tube

[cajen2]

I like "... don't get caught up in refractor elitism. Some people treat their gear as a status symbol..." 😆😉

[Louis D]

56 minutes ago, glafnazur said:

The paragraph below interested me. I used to do this and then read that quite a few people leave the lens cap off so I started doing that, maybe I'll go back to cap on.

When bringing from a cold environment to a warm environment, put lens caps on for the scope and eyepieces, and put them in their case if possible. Close the case to trap in the cold air, then bring the cases inside and let them slowly warm up over 12-24 hours. This is to prevent frost from forming on a lens surface (even on the inside of the tube

If I'm bringing my gear in from a hot, steamy environment to a drier, air conditioned environment; then yes, I do keep everything uncapped to let the excess moisture escape and recap everything later in the night or in the morning.  However, the above is what I've always done in the winter.  Otherwise, you'll end up with condensation on everything and you have to wait to put everything away just like in the summer.

[Louis D]

My favorite quote of the entire article:

you don’t need 8-12” to have fun!

4” or 5” is enough to keep you busy for decades!