TheSkySearchers

Hi all,

I want to upgrade the 1.25" focuser on my 8" f/7.5 Newtonian to a 2" focuser. My question is, will my current diagonal mirror minor axis size (38mm) be adequate for a 2" eyepiece?

Does anyone know the correct minor axis size given the focal length (60" or 1524mm) and the use of a 2-inch eyepiece? My "L" distance (focal plane to diagonal) is 10.25" (260mm)

Thanks,

JayTee wrote: ↑Fri Jul 09, 2021 1:16 am Hi all,

I want to upgrade the 1.25" focuser on my 8" f/7.5 Newtonian to a 2" focuser. My question is, will my current diagonal minor axis size (38mm) be adequate for a 2" eyepiece?

Does anyone know the correct minor axis size given the focal length (60" or 1524mm) and the use of a 2-inch eyepiece? My "L" distance (focal plane to diagonal) is 10.25" (260mm)

Thanks,

Looks OK to me but there are are several different criteria. Berry and Dakin say it's a touch small. Efberg and Peters and Pike say it's OK.

Here's an online calculator that I've found handy.....

http://www.catseyecollimation.com/designie5.html

I would think it would depend on what you want to do. Your current secondary ratio is 19%, which is going to give you about as good resolution as anyone can ask for from a reflector doing planetary work. Where it will be limited is with wide angle viewing. If you are trying for a wider TFOV, then the outer portions of the field, compared to the field you are currently seeing, might appear to be darker than you want. If so, then a larger secondary mirror can illuminate the outer field better for a possible tradeoff of a little of your resolution. Going to the next larger secondary mirror size would make your scope a little more general purpose, while the setup you have leans more toward lunar/planetary/double star work.

The nice thing about the calculator link I posted is that it produces a table of % illumination in the focal plane. When I enter 7.5 for the f ratio and 10.5 for the distance of the focal plane from the center line I get only 34% illumination pat 2" diameter. For planetary and doubles it's ok at that. I'd want a bigger diagonal for dso work though. A 2.14 diagonal gives 65% illumination.

Sometimes I wish it was easy to swap different diagonals for different targets.....

Looks OK to me but there are are several different criteria. Berry and Dakin say it's a touch small. Efberg and Peters and Pike say it's OK.

You don't make it easy.

PETERS
Peters, William T., and Robert Pike, "The Size of the Newtonian Diagonal," Gleanings for ATM's, Sky and Telescope Magazine , March 1977, pp. 220-223.

EDBERG
-“How to Determine the Size of Diagonals, Draw Tubes, and Dew Caps,” by Stephen Edberg, Telescope Making No. 18 (Winter 1982/83)

DAKIN
Dakin 1962 R.K. Dakin, "Placing and Aligning the Newtonian Diagonal," Sky & Telescope (Gleanings for ATMs), pp. 368-369, Dec. 1962. erratum p. 114, Feb. 1963.

Peters reference found by way of "Practical Calculations for Designing a Newtonian Telescop"e Jeff Beish (Rev. 07 February 2019)
http://www.alpo-astronomy.org/jbeish/Ne ... Mirror.pdf

And Richard Berry ...

Here's an online calculator that I've found handy.....
http://www.catseyecollimation.com/designie5.html

Same but different
https://www.catseyecollimation.com/designns5.html

Thank you for all the great info. I am going to handle this as a two stage process. I'll upgrade the focuser and then if the image looks to be clipped or darker around the edges I will then consider getting a new diagonal which also requires getting a new spider.

Thanks again,

If you decide you need a secondary mirror change and are not familiar with 1800destiny.com, you might check them out. Hans, the owner, is good to work with. I like his curved vane spiders and secondary mirrors.

Along with the helpful link that nFA provided (thanks), I have used that in the past but I find the multitude of inputs to be a bit confusing so I found another old bookmark from Stellafane that is a lot easier to use. For now, it seems that the diagonal currently installed should be sufficient. We'll see.

https://stellafane.org/tm/newt-web/newt-web.html

Cheers

JayTee wrote: ↑Fri Jul 09, 2021 8:08 pm Along with the helpful link that nFA provided (thanks), I have used that in the past but I find the multitude of inputs to be a bit confusing so I found another old bookmark from Stellafane that is a lot easier to use. For now, it seems that the diagonal currently installed should be sufficient. We'll see.

https://stellafane.org/tm/newt-web/newt-web.html

Cheers

That's also a good online calculator.