Focus and Image Shift...

[tony4563]

What exacty is meant by "Focus Shift" and "Image shift"? What should you look out for when testing a scope?

[GordonCopestake]

these usually refer to CATS that focus by moving the primary mirror, this can make the image shift out of the FOV when you are using high magnifications and can be VERY irritating. I think all Celestron and Meade CATS suffer from this to some degree. Testing is easy, crank the scope up to a high mag and focus on an object. Defocus and if the object moves out of your FOV then the scope suffers from image shift. Some are worse than others even in the same batch. A crayford stuck on the back will fix it though

[tony4563]

Thanks for clearing that up Gordon. I'm thinking about a Mak to add to my telescope arsenal. It's gonna be either the Skywatcher 127 or 150. The 150 apparently does have some shift whereas the 127 has been reported to have none. Would the 127 Mak be a good purchase for planetary viewing considering I have a 100ED already, or should I go for the 150 (but that's about £200 extra on top!) The 180 is out as I have an EQ5 mount and don't want to go bigger to support this size scope.

[GazOC]

The 127mm is a great little scope with no image shift that I can see but the only advantage it has over an ED100 is that its easier to get higher magnifications due to the longer focal lenght. If you are hoping to see a difference in planetary views then you'll need to go to at least the 150mm and probably the 180mm for a substantial improvement.

[tony4563]

..Why is my wallet weeping?

[GordonCopestake]

Just buy a 5x powermate instead and use your ED100

[Jamie]

What exacty is meant by "Focus Shift" and "Image shift"? What should you look out for when testing a scope?

Only to add to what Gordon said....

IMAGE SHIFT = This is when your scope has a mirror cell inside at the back of the tube

and it moves due to the rotation of the scope whilst tracking.

It has to do with how the mirror is held in place.

FOCUS SHIFT = This can be caused on a number of types of telescope's focusing devices.

With the likes of an SCT it has to do "again" with how the mirror is held

in place and the fact that the mirror is moved to focus.

With any scope that uses a rack and pinion type focuser focus shift can

occur when the focusing barrel moves in and out but it does not move

perfectly straight kind of like one end moving to the side slightly.

Hope this helps.

[tony4563]

Thanks for all your replies. Gordon, won't a powermate just make the image bigger but dimmer in the 100ED?

[GazOC]

Yes, you won't see any more detail then if you'd have got to that magnification using a Barlow or a staight eyepiece.

[GordonCopestake]

I would rather have the ED100 and a barlow/powermate than a MAK127 personally or even a MAK150 thinking about it.

For bright objects like planets you dont need more light grasp but you do need more resolution (which aperture provides) however the crisp views of the APO (might) outweigh the central obstruction and lack of contrast in the MAK's. Personal preference though and a topic all on it's own!

[GazOC]

I'd guess a 4" apo would outperform a 5" Mak on the planets and is also probably a better all round scope as well.

[tony4563]

OK, thanks guys.. (scribbles 127 Mak out) 8)