DIY 5" REFRACTOR

[steelfixer]

Hi all

Thought you might like to see my home made 5" refractor I am hopeing to use as a guide scope.

I purchased a 5 inch '3 diopter' lens from fleebay for £3.00.

To make the body of the scope I have used the following.

1 number 350 mm x 125 mm plastic extractor fan duct pipe.

1 number 125 mm to 100 mm plastic reducer.

1 number 350 mm x 100 mm plastic extracter fan duct pipe.

4 number 100 mm diameter plywood rings (machined on a lathe).

3 rings with varing sized center holes for the baffles the last is for the ep holder + focuser.

1 can of matt black spray paint.

Method.

Lens cell

Cut 3 pieces from the 125 mm tube.

1 number at 20 mm.

1 number at 90 mm.

1 number at 80 mm.

Split the 20 mm and the 80 mm lengthways and remove just enough material so it will close up and be a tight fit inside the 90 mm bit.

These 2 pieces will be the lens holders and allow enough to overhang the end of the 90 mm bit for glueing into the reducer.

Glue the 80 mm piece inside the 90 mm bit leaving enough overhang to fit the reducer.

Then glue on the reducer.

Spray the tubes with matt black paint inside and out.

When the paint is dry push the lens into the end of the 90 mm x 125 mm using the 20 mm piece as a depth gauge.

When it is fully home remove the 20 mm piece and then glue it back into position making sure that it has fully secured the lens.

Lay out the finished lens cell with the 100 mm tube pushed fully home onto the reducer.

Measure the focal length from the lens down the inside of the 100 mm tube and then cut it to length.

By drawing out the 'image cone' on a sheet of paper you can then determine the position of the baffles depending on the hole diameter in each plywood ring.

After spraying them with the matt black paint slide each ring into position and fix with screws from the outside of the tube .

The EP holder should be a nice tight fit inside the 100 mm tube to allow it to be slid up and down to gain focus.

When focus on a suitable guide star is achieved either fix it into position with screws or clamp it with a jubilee clip.

I did a rough test with this yesterday in daylight and it worked better than I expected.

I managed to focus it on an Ivy covered tree some distance away and you could see every single ivy leaf in detail.

All I need now is for the clouds to clear so I can do a proper star test.

So there you have it a 5 inch f 2.6 refractor for less than £15.00.

Ok I know it will not win any prizes for optical quality but if it works as a guide scope I am going to be chuffed to bits.

[MjrTom]

Very interesting project

[swamp thing]

Looks great.

Be interested to hear how it performs.

Regards Steve

[steelfixer]

You and me both Steve.

I will do a follow up when I get a chance to try it out.

[MjrTom]

What have you in mind for a focuser?

Helical?

[steelfixer]

Hi Mark.

As I am going to use this purely as a guide scope I figured that I could gain an acceptable focus just by sliding the EP holder up and down inside the 100 mm tube.

If this fails I do have an old focuser from a 60 mm frac I can use.

[angusb1]

Nice work Graham. I'm using the same kind of ducting for a small reflector I'm making.

[steelfixer]

Nice one Angus be sure to post up some pics of it.

[EBU]

Nice Job steelfixer !

How did You aligned (colimated) lens in the lens cell?

[Merlin66]

EBU,

You can use a laser collimator to check the positioning and alignment of the focuser, and a LED torch and a Cheshire eyepiece to confirm the alignment/ collimation of the objective.

[steelfixer]

Merlin has summed it up perfectly.

I must admit I assembled it without checking the collomation.

Logic says if all parts are kept square within a circle then it will not be to far out.

As this is for a guide scope and not for visual work I was not overly worried and as it has turned out the collomation is pretty good.