8 replies to this topic

[Hrothgar]

I have owned a Meade Sarfinder 10 for many years. For various reasons, it never saw much light. Recently I cleaned up the mirrors, changed the focuser and got my hands on a set of mounting rings and dovetail plate which allowed me to mount it on a Skywatcher NEQ6 Pro. These days I mostly do EAA and the instrument worked reasonably well with an IMX585 sensor on the first night out -some coma, collimation not perfect, oversampling etc.

 

Checking the users manual, I see there are two positional settings for the primary mirror: a rear one designated for "visual observation" and a slightly more forward one designated for "prime focus and eyepiece-projection photography". For better or worse, I had the mirror in the 'visual observation' position.

 

I have not noticed this primary mirror positioning distinction on modern Newtonians, though my experience is not extensive. Can someone explain if these Starfinder mirror positions are a throwback to an earlier pre-digital photographic era, and if they still make a difference in our days of CMOS cameras? In short will things be improved with the primary mirror in the forward, 'prime focus photography' position?

 

PS I hope I have posted this in the correct forum. If not please advise.

[ccwemyss]

All it does is move the focal plane further out from the tube. So if your camera doesn't need the extra distance to reach focus, there is no reason to move it. 

 

Chip W. 

[Tom Stock]

SLR Cameras were deep something like 3.5" so moving the mirror forward allowed the film to reach focus.  

 

I use a 10" starfinder also and use the visual position with a crayford JMI focuser and an extender for visual, and I remove it for DSLR imaging.

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Edited by Tom Stock, 16 March 2025 - 05:10 PM.

[M66]

Wow, you got a nice Starliner mount on that scope!

 

Don't see too many of them around.

 

Had  a few of them in my day. Very solid.

[CHASLX200]

Wow, you got a nice Starliner mount on that scope!

 

Don't see too many of them around.

 

Had  a few of them in my day. Very solid.

It was mine and had the shakes bad.

[Hrothgar]

Thank you Chip W and Vanguard.

[Tom Stock]

Wow, you got a nice Starliner mount on that scope!

 

Don't see too many of them around.

 

Had  a few of them in my day. Very solid.

Yeah I was lucky enough to get it from CHAZLK200 since we are both in FL.

 

Had been wanting on old large mount like this for sooo long. 30+ years, but had never found one close enough, and those I did find were already sold or too expensive.

 

Rebuilt it, added setting circles and drive controller. It's very solid now.

 

Tom

Edited by Tom Stock, 18 March 2025 - 10:36 AM.

[Dave Cook]

I ran into the focal plane position issue when going through my 1975 Cave 8".  DSLRs with a T-ring have standard 55mm back focus.  As it came from the factory you couldn't move the focuser in far enough to accommodate that, so I added a second set of mirror cell mounting holes about 2" forward of the original so that the camera would reach focus with the focuser nearly all the way in.  Then you either need a 2" extender tube or a long travel focuser for the eyepieces.  I never thought it was practical to have to switch the mirror cell position so I have left it in the forward position for a long time.  You have to watch the secondary size when doing this...mine was generous.  I never knew that Starliner took care of that in their design - Cave did not!

[Tom Stock]

I ran into the focal plane position issue when going through my 1975 Cave 8".  DSLRs with a T-ring have standard 55mm back focus.  As it came from the factory you couldn't move the focuser in far enough to accommodate that, so I added a second set of mirror cell mounting holes about 2" forward of the original so that the camera would reach focus with the focuser nearly all the way in.  Then you either need a 2" extender tube or a long travel focuser for the eyepieces.  I never thought it was practical to have to switch the mirror cell position so I have left it in the forward position for a long time.  You have to watch the secondary size when doing this...mine was generous.  I never knew that Starliner took care of that in their design - Cave did not!

Idk if Starliner did, the OTA is a Meade Starfinder on a starliner mount.