Need help please to collimate new scope

[mswhin63 (Malcolm)]

I have one of these scopes and it is frustrating to remove the lens. I checked my scope and the FL spec did not match the length of the tube I suspect the lens in mine was used to compensate for short tube to give better spec. Again another trap to get into buying cheaper scopes although wasn't really cheap. (me)

Apart from that I hate having to take out the lens all the time to use my laser colliminator.

Hence I am looking at another scope.

[erick (Eric)]

Originally Posted by mswhin63

I suspect the lens in mine was used to compensate for short tube to give better spec.

Sort-of yes - the corrector lens (a 2x barlow usually) allows the physical tube length to be around half the optical focal length.

My understanding is that the design achieves two things:-

1. Shorter tube length, hence more compact as well as less material used (= less cost)

2. Allows the use of a spherical mirror - cheaper to produce?

[Shano592 (Shane)]

Yeah, there is no way that my tube is 700mm. Actual length is 517mm, tip to tip. I reckon that it is about 350-355mm from primary to secondary, which with the barlow in the focuser (which was found easily enough), would give close to 1400mm, f/9 characteristics.

Now ... if I were to get an F/5 Bintel Parabolic primary ...theoretically, I should be able to modify this scope with minor minor adjustments, albeit halving any magnification in he process. I'm talking about adjusting the position of the secondary by a few mm, to accommodate for the new focal length.

This would then allow me to do away with the in-line barlow, and have a short-tube F/5 scope.

Any holes in my argument so far?

[Glenhuon (Bill)]

Sounds good so far, but apart from the fun of doing the mods (which is something I like too). In money terms it would probably be a better option to look around for a good s/h 150 F5 to replace the OTA.

Just checked the lengths on my 150 F5 newt, centre of focuser to mirror 500mm, overall tube length 675mm. Looks like the tube is too short to accomodate a parabolic 150 F5 mirror.

Bill

[Shano592 (Shane)]

Yeah, I thought that, almost as soon as I posted.

Bunnings should have some good stormwater pipe that i can spray black inside. When I get the chance, I may take the spider off the top, take it on a roadtrip ...