90mm spotting scope conversion project

Hi Folks. I purchased a cheap 90mm spotting scope recently, it had a built in 30-90zoom. There was a number of things wrong with it
1. tiny FOV
2. lots of false colour on bright objects
3. some flaring
4. unable to collimate the prisms.

So I was now left with a scope which seemed to have a decent looking objective and bad prisms or eyepiece. A scope just sitting there is no good to anybody so I decided on some surgery :twisted:. I whipped out the eyepiece, prisms, housing and all the business end of the scope, leaving me with a 90mm F5 or there abouts coated objective. Needless to say, I had to see if it was worth attempting to do anything with the objective and I knocked up the temporary set up as in the pic. I managed to give it a quick first light before clouds rolled in and with a 32mm plossl, I have about a three degree field of view, with a much reduced amount of CA. there is still a little flaring, but that is due to the eyepiece not in direct line with objective. I turned it on Orion's belt an was pleased to discover that the right belt star is a double. Another gap in clouds appeared and onto Alcor and Mizar. Mizar split after some fidgeting with the eyepiece. I also got to look at saturn and it was very distinct although tiny. I managed a quick look at M35, which was also nice. Anyway, I am happy enough and am now going to try and convert this into a 90mm scope. Wish me luck.

I like your courage Trevor.
I'm all on for modifing and dismantling scopes to improve them,
What are you going to use for a permanent tube ?

Hi Dave

I am not sure what to use yet. The tube in the pic is a hard cardboard tube. I imagine it is like sonotube. It is quite rigid. i have a focuser ordered so I will just use this tubing until I get something better. It is a perfect fit to the hard plastic of the original, and the focuser should be a close fit also.

Interesting project there Trevor. I myself, like what you have done here, like to dismantle the cheap stuff I have, and try rearranging things. By messing with lenses and things on my Tasco 3" reflector one time, I managed to get the image 'upright' instead of upside down (sounds funny when you're looking at it sideways through a Newtonian anyway, but you get the jist!).

Seanie.

Hi Folks

A little update. Focuser has arrived, and i have temporarily installed it. I have yet to give it a whirl at night, but I am having problems with the length of It. I am not sure focuser will stretch far enough to accommodate using scope both with and without diagonal. i reckon that since this scope will be mainly used for quick looks at widefield stuff from suburbia I will design it for use with the diagonal, as it will be much more comfortable to use and the tripod will be more stable at a lower height.... More to follow

Looks nice Trevor, a trip to the car accessory shop for some spray paint is all that is left to do! But double check the focus range for all eyepiece/barlow/camera combinations first.
Looking forward to a dark sky test with it