Drawtube issue with 152mm Refractor

I recently purchased a TS152/990 refractor and there appears to be a problem with the draw tube length interfering with the light cone from the objective.
This problem was widely discussed with regard to the Antares version of this telescope on the Cloudy nights forum a year ago and I'm somewhat annoyed to see that it still an issue with current telescopes. :evil:

My calculations based on measurements taken last night are as follows
Overall Scope length ( from Collimation screws to end of visual back)=900mm
Focal length = 990mm
Distance of visual centre of lens from collimation screws =25mm
putting the focal point 115mm (990+25-900) behind the visual back.
Overall length of draw tube including visual back 270mm
ID of draw tube 50mm
This limits the image seen at the focal point to at 385mm (270+115) is 50mm . Using the law of equal triangles this will limit the diameter of lens seen at the focal point to
50*990/ 385=128mm
effectively stopping the telescope aperture down from 152mm to 128mm.

I've asked Telescope Services for their proposed solution to this issue, but I would like any input I could get here also. Even as I newbie I wouldn't be entirely adverse to start chopping up new telescope with a hacksaw to get it operating to its specs but I have no workshop or suitable tools to carry out any mods, so before I start anything:

Are my calculations correct, or have I missed something?
Even if my calculations are correct is there really a problem e.g. once I rack the draw tube out by 60mm it should leave the light cone intact?
Is there anyway to calculate how far i need to rack the draw tube out for a given eyepiece to focus?

Any help you can give will be greatly appreciated.

PS I have a sketch of this that would make everything clearer but I can't figure out how to attach it.

Even if my calculations are correct is there really a problem e.g. once I rack the draw tube out by 60mm it should leave the light cone intact?

What you need to do is make sure that the eyepiece reaches focus with the draw tube more the 60mm out, if not then you have a problem ( I didnt check your figures), which is easily fixed with a hacksaw, no kidding!.

For the solar eclipose in Turkey, I had to remove over 5 cm off my 4" refractor as the canon could not reach focus, as focus for the camera was "in the telescope tube", in fixing that I had to cut off some of the inner focuser tube also or else (like with your scope) I'd end up stopping down the scope, the modification worked like a charm .

I just the suave way you talk about performing surgery on your scope, no doubt while drinking a Martini (stirred not shaken)

If I were to :shock: cut back the drawtube, which means removing it from the refractor, would I have to recollimate it after I put it back into the telescope?

I just the suave way you talk about performing surgery on your scope, no doubt while drinking a Martini (stirred not shaken)

They don't call him Double-Oh-Dave for nothing you know!

You know my middle name is McGuyver :lol: leave me in a shed full of bits'n pieces and I'll make you up a RCOS.

As for collimating the scope, i dont think there would be a need, the entire cell (usually) screws off the front end of the scope while the focuer end comes off when you take away a few screws, at least in mine it did, do not remove the lenses from the front cell.

If you are going to cut the tube get it done at a local machine workshop, thats what I did to be honest, the reason been that it is critical that the cut is square with the tube, also cut the end of the tube with no threads on it as they are hard to re-make.