APO Scopes

I am thinking about purchasing an APO scope. This will be my first "serious" retractor. My current scope is a mish-mash dob made from an old Mead 10" starfinder OTA. I have thought about a SCT, but they seem to be a compromise. So I think I will eventually settle on a good retractor of modest size for the planets and the moon (and doubles although I've never got into these much) and a good dob. I've noticed most APO are in the medum or short focal length. Why? Vixen makes an F9 APO, is there a disadvantage to this?

Thanks!

Darryl

Darryl Simmons said:

I am thinking about purchasing an APO scope. This will be my first "serious" retractor. My current scope is a mish-mash dob made from an old Mead 10" starfinder OTA. I have thought about a SCT, but they seem to be a compromise. So I think I will eventually settle on a good retractor of modest size for the planets and the moon (and doubles although I've never got into these much) and a good dob. I've noticed most APO are in the medum or short focal length. Why? Vixen makes an F9 APO, is there a disadvantage to this?

Thanks!

Darryl

Darryl: There are quite a few 4 inch APOs in the F/9 range, the Orion ED-100, the TeleVue 102 are a couple of em. But APO's generally have faster focal ratios for a number of reasons. One is simply that the best APOs are aimed at astrophotography and a fast focal ratio means shorter exposures as well as a wider field of view.

Another reason is that it makes for a more portable scope and manageable scope. A 102mm F/7 can be used as a "grab and go" scope that is not much bigger than an ED-80. A 102mm F/9, at least for me, is on the other side of the line, takes a real mount and has a narrower maximum field of view. It falls into the "serious" to setup catagory.

Another advantage is that the shorter focal ratio/shorter tube minimizes the eyepiece height change as you move from the horizon to the zenith. A longer focal length requires a taller mount to avoid the awkwardness of getting down your your knees to view at higher elevations.

If you have a "good" dob, it is likely you will discover that it will be your scope of choice for planetary as well as splitting doubles, at least the tight doubles. The increased aperture buys you increased resolution as well as image brightness and contrast.

Jon

To me, the real advantage to the smaller apochromats, such as a 4", is their versatility. In the shorter focal lengths, they can show wide true fields, yet also provide fabulous high power viewing. With the longer focal lengths, you give up some of the wide field advantage.

While they can provide surprising lunar and planetary views for the aperture, they are still limited by their modest aperture. A 10" Newtonian with reasonable optics and proper cooling should provide considerably better lunar and planetary views.

Clear skies, Alan

Longer focal length APOs, like the f/8.8 Vixen I have and the f/8.6 Televue 101 are "doublets" - they use 2 lense elements. The shorter focal length APOs, like the f/5.4 TV NP101 and related scopes have much more complex optical systems - using "triplet" arrangements or in the case of the NP101 a 4 element "Petzval" design. With more elements, the harder it is to manufacture, making it more expensive than the simpler "doublet" designs.

Check out these comparisons of the TV 101 & 102 series:

http://www.company7.com/televue/telescopes/tv102.html

http://www.company7.com/televue/telescopes/tv101.htm

K. Michael M.

Darryl Simmons said: I've noticed most APO are in the medum or short focal length. Why? Vixen makes an F9 APO, is there a disadvantage to this?

Thanks!

Darryl