So, 10" Newtonian tube.

What's the focal length?

I would shoot for having a range of magnifications up to approx 150x-200x. You'll need to know your focal length to figure that out.

So how do I calculate all that? what to measure? I'm a newby

If the scope is an unknown, ie it's not a XYZ model from Brand 123, then I'm not sure. I'm sure there's a way. Mike/Pazazu would probably know. I don't think it's going to be an easy linear measurement, but maybe I'm wrong. I don't know if it's going to be the distance from the primary to the secondary or primary, to secondary to focal point of the EP. I'm not sure.

Without that info, then I would probably get the 26mm and use that a bit, then maybe shoot for something in the 21-17.5 mm range.

Another question is how much are you looking to spend on EP. You can get an EP for $20 or you can get one that's $200 or you can spend $600.

So maybe a budget would help with recommendations.

Ah, so the focal length from the primary mirror to the focal point behind the secondary mirror. There's probably a way to tell by measuring the distance between the primary and secondary and then measuring the shorter diameter of the secondary and then doing some math.

https://www.bbastrodesigns.com/optic...20callouts.jpg

Thanks, I was googling and your image diagram is the best solution.!

EP choice may be influenced by what your viewing targets will be. 10" will let you view planets nicely and some of the brighter DSOs.

Register at Cloudy Nights forum, there's a lot of info there and a forum for beginners.

Absolutely, Cloudy Nights is the Pelican Parts of astronomy forums. I tried several many years ago. A few were good, but I think Cloudy Nights is the best.

https://www.cloudynights.com/index

What measurement is 10 inches? The tube itself, or the mirror? Did you check?

Based on the era (1970s), that would be an 8 inch f/6 or 8 inch f/8. Probably not a 10 inch, but it's possible. If so, 10 inch f/6. How long is the tube (most Newt tubes are very close in length to the focal length, the secondary is set back from the edge the same amount as the tube diameter. You only need to be within 2 inches or correct to know.

The secondary is small, which means a tightly shaped light cone, which means an f/8 system. I bet it's 8 inch f/8, with a tube diameter of 10 inches, and a length right around 60-65 inches.

Mike's the man!

I too noticed that the tube seemed long and the secondary seemed small (based on my experience with an 8" f5.9).

yup, 8" mirror, 57" tube length.


For $100 I couldn't say no. especially with a clock drive.

That's a nice score. Love classics like that with a simple clock drive.

At f/8, the optics will be essentially perfect, it was easy to make an f/8 mirror back then. Also, the heavy german mounts like that usually had some top notch, hand ground optics.

Now...the rings that hold the tube...they're probably rotating rings, which are PIMP.

Also, I don't recognize that drive assembly, but based on the size of the worm wheel and the 3 bolt clutch assembly, it might very well be a Byers gearset. One way or the other, it's MUCH higher quality than many of them (Parks, Meade) at that time, they all had much smaller gears.

www.cave-astrola.com????

Les Schwab ain't the only thing in Prineville any more!

https://www.oregonlive.com/travel/2021/05/prineville-reservoir-certified-as-oregons-first-dark-sky-park.html

Cool. Oregon has some of the darkest skies in the country...SouthEast Oregon, that is.

More for Prineville.
https://www.oregonlive.com/silicon-forest/2021/03/facebook-will-add-two-more-data-centers-at-2-billion-prineville-complex.html#:~:text=Facebook%20plans%20to%20add%2 0two,its%20latest%20expansion%20in%202023.&text=Ea ch%20of%20the%20two%20new,on%20par%20with%20their% 20predecessors.

I stumbled across this listing a while back. I thought it was pretty cool.

https://www.shopgoodwill.com/Item/124393116

https://sgwmscdnimages.azureedge.net...143114Neas.JPG

https://sgwmscdnimages.azureedge.net...143115Neas.JPG

https://sgwmscdnimages.azureedge.net...143115Neas.JPG

It seems like it would make, if nothing else, a really cool conversation piece once cleaned up a bit. Seems like a reasonable price at $61+$10 for shipping.

Last year as a birthday present, the missus approved the purchase of a telescope. I got a Celestron Nextar Evo 8 from Mike. I've used it a few times and it works well. It took a while but I finally got around to ordering some EP. Yesterday I received a TV Panoptic 24mm and Baader Morpheus 12.5mm. I still have 2 others on order, a TV Delite 15mm and a Baader Morpheus 17.5mm. I was surprised at how small the TV Pano 24 was. I was expecting it to be bigger.

And it was cloudy yesterday and last night. I did get a peak at Venus in Mars in close proximity with binos after sunset. The dog got me up at 4am with an upset stomach so he could go eat some grass. It was not too humid and very clear. I'm sure it'll be cloudy every night this week.

Saturn SE with Jupiter blazing further E, Antares shining brightly red and the teapot with M8 S on a warm summer night.

YO PAZ! What are the chances on rebuilding the drive on an original ETX 90?