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Gregorian Mak


Tweedledee

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Both nice scopes Mick. The bigger Maks are usually more expensive than the SCTs.

 

I actually like the idea of a c925 as it has a bigger (well, intermediate) size baffle tube between the 8" and 10" SCTs. Would therefore lend itself better to use with big eyepieces. Would like one with an add on dual speed focuser to supplement the mirror moving focuser. Moving the mirror changes the focal length in an SCT according to strict formulae as well as sometimes shifting the image slightly. 

 

It's all costly stuff this astro gear. :)

 

 

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Remember by putting an extra focuser on a SCT, then a 2" diagonal and a big eyepiece,

you will be adding to the focal length further, possibly turning the scope in to an F11 or F12.

 

Not necessarily a big problem and it effects binoviewing use more really, but you will find

you may lose some aperture as well, as the moving primary has to be racked all the way in 

to achieve focus. This has the effect of the secondary mirror getting too close to the primary

which results the outer part of the primary being clipped off. 

 

Having said that as you are using an extra focuser anyway, this may not be an issue,

as the primary will not be moving. I'm not sure how it effects the aperture loss issue.

 

Its definitely best avoided though.

To compound the situation further, aperture loss from the outer edge of the primary

will result in the secondary obstruction getting bigger by percentage, and will result

in a big loss of contrast and a rather dim over magnified view at the eyepiece.

Also you will be introducing spherical aberation into the mix (as much as 1/5 wave i'm led to believe)

which will degrade things further.

 

As i said though, more an issue with binoviewing as this can add inches to the lightpath.

But with any moving mirror scope its always best to keep the lightpath as short as possible.

 

 

 

 

 

 

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6 hours ago, tuckstar said:

Isn't Cazz selling her orange c8?

 

 

Cheers Andy.

 

Cazz's scope looks pretty good and worth considering. :thumbsup:

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5 hours ago, Bino-viewer said:

Remember by putting an extra focuser on a SCT, then a 2" diagonal and a big eyepiece,

you will be adding to the focal length further, possibly turning the scope in to an F11 or F12.

 

Not necessarily a big problem and it effects binoviewing use more really, but you will find

you may lose some aperture as well, as the moving primary has to be racked all the way in 

to achieve focus. This has the effect of the secondary mirror getting too close to the primary

which results the outer part of the primary being clipped off. 

 

Having said that as you are using an extra focuser anyway, this may not be an issue,

as the primary will not be moving. I'm not sure how it effects the aperture loss issue.

 

Its definitely best avoided though.

To compound the situation further, aperture loss from the outer edge of the primary

will result in the secondary obstruction getting bigger by percentage, and will result

in a big loss of contrast and a rather dim over magnified view at the eyepiece.

Also you will be introducing spherical aberation into the mix (as much as 1/5 wave i'm led to believe)

which will degrade things further.

 

As i said though, more an issue with binoviewing as this can add inches to the lightpath.

But with any moving mirror scope its always best to keep the lightpath as short as possible.

 

Thanks Rob, all very useful considerations.

 

There are always compromises to be made.

 

The back focus issue would probably be as bad as with a binoviewer because using the Amici prism requires extra light path over and above a mirror diagonal even though on the outside they both look the same size. So introducing a focuser, Amici and a big eyepiece might not even work. Even though SCT's and Maks usually have lots of back focus to spare, subject to the limitations you have pointed out, it would certainly require some experimenation to get it to work as I'd like it to work (with a correct image).

 

I'm just never satisfied and like to push things to the limit. If it doesn't work then I will have learned a lot in the process. :)

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