Opticstar Guidescope 60 XL.....but can it take a diagonal ??

[Craney]

Hello evrybody,

Calling upon your collective expertise as I am a novice to all this techie gear.

Thinking about adding  a better finder / guide scope (for the furure)  to my collection of scopes  (  C6XLT, SW200PDS, Opticstar AR80 ).  Siimplicity is the key, so a easy to fit and swap item that I can change from finder duties to guiding duties whilst observing in the dark...... ie without physically taking it all off the primary scope.  This one has come up...

http://www.opticstar.com/Run/Astronomy/Astro-Accessories-Imagers-Opticstar.asp?p=0_10_5_0_3_350

It works with 1.25" EPs as a straight through finder and CCD cameras...GREAT !!.....But   ....  it would be nice to use it with a diagonal, especially with the 200PDS.

It has been suggested that I might need a barlow inserted to achieve this.   Great !! ....but  is this actually achievable ??? ...I don't want to fork out the cash to be disappointed.

Also I don't want the effective magnification to increase dramatically and make it poorer at finding or guiding  (  I have seen a 1.5X barlow for sale which is not too extreme).

 

Anybody had experience with this " problem" on or with similar issues???

Many thanks,

 

Sean.

[Cornelius Varley]

I would assume not. The finderscope's tube length is optimised for straight through use with either an eyepiece or guide camera so adding a diagonal into the equation will position the eyepiece too far away from the point of focus. Adding a barlow either in front of or after the diagonal will move the point of focus farther out but will increase the magnification. You could probably compensate for the increased focal length by using a lower power eyepiece.

[Craney]

Thanks for the reply Cornelius,

I have done a quick web search and it has not come back with a conclusive fix to the issue.   The scope will do the dual roles  (which is great and a big bonus at the end of the day/night(!) ) ....alas it would be easier to use as a finder with the diagonal.   I don't want to go down the line of having a menagerie of scopes hanging off the one mount.....

Ho hum...... such is life.

 

[John]

A 1.25" diagonal uses up around 50-60mm of the light path so you would need to shorten the finderscope tube by that amount to use one without a barlow-type element. Using a barlow type element might work but it seems that the lower the barlow amplification, the less that the focal point is extended. A 2x element, for example, seems to move the focal point outwards by around 40mm, a 2.6x around 55mm and a 1.6x that much less. So it seems that a 2.6x amplifying element might be needed to compensate in full for the light path that the diagonal consumes.

 

[Craney]

Thanks John,

Hmmmm....so not impossible.    With my limited maths.....a 2.6x Barlow would   make the effective focal length   about 560mm...  so with a 40mm eyepiece......14x !!!!

eek.....a bit powerful for a finder.

I heard of extension tubes.....would they help ??

 

Cheers.