Mods to el-cheapo 4.5" reflector

[asimov (John)]

Hi guys.

I'm in process of doing a few mods to a little reflector I inherited off my dad years ago. I replaced the .965" focuser with a 1.25". After fitting it, I pretty well knew I wouldn't be able to come to focus because of the extra height of the new focuser. I needed the primary closer to the EP 25mm.

I very nearly just cut an inch off the primary end of the tube, but decided to move focuser/diag/finder towards the primary instead. {I couldn't just move the primary up the tube cos' the cell is built into the tube reinforcement ring}

After I did all that, I collimated, took it out for a test. The trouble I'm having is kinda hard to explain. When I de-focus heavily, {outside of focus} on a star, It looks like I'm not receiving the full light from the primary, I see like a cresent....like a 3/4 full moon.

It's probably crappy collimation causing it, but I'm just wondering if it was the right thing to do by adding the bigger diameter focuser without adding a slightly bigger diagonal mirror?....The diagonal I'm using is 25mm diameter.

TY in advance.

[rmcpb (Rob)]

It sound to me that you have changed the distance from the secondary to the eyepiece. In this case you would need a larger secondary as the cone has to be larger at the base to come to focus within your eyepieces.

I thought about doing the exact same mod but chose not to because I was lazy and put it in the too hard basket.

Try a larger secondary using the following formula, it will give you a fairly good idea of the size necessary.

eg. Main Mirror Diameter 200mm (8")
Focal Length = 1000mm
Height of Focuser = 70mm
Outside Diameter of Tube = 230mm divided by 2 (half) = 115mm
Total distance from Secondary to Eyepiece = 70mm + 115mm = 185mm

Formula

eg. 200mm divided by 1000mm times 185mm = 37mm diameter Secondary Mirror.

Good luck

[asimov (John)]

Thanks rob.

According to those calculations, I need a secondary diameter of 19.25mm
Seeing I've got a 25mm that cant be the problem.

[Starkler (Geoff)]

What is that straight edged obstruction to the upper right seen in the secondary ?
The obstruction on the left of course is the focuser drawtube. Any obstruction is going to affect the star test and the quality of focused views.

[asimov (John)]

It's actually a piece of white paper that I stuck inside the OTA behind the secondary.....one corner of the paper must of 'flopped' over. Yes, Iv'e always wondered about draw-tube's that protrude like that.....On the .965 focuser I had on there, I simply cut the rotter off with me trusty hack-saw..

[RAJAH235]

After enlarging the image, The collimation appears slightly off. Have you run your hand or something around the edge of the tube while looking thru the f/tube, to try to pinpoint the area? Have you tried shining a torch down, while looking thru the f/tube? Boy it's hard doing it this way.
Do you have 'NEWTWIN' to help with the calcs? Don't know what else. You didn't leave the 'hacksaw' in there? L.
ps.Have you adjusted the focuser tube to be in the centre of the optical path? A sheet of paper behind the 2ndary, helps with this. (In the image it's hard to make out, but it looks like the focuser is off centre). From the inside of the f/tube to the outside of the main, it's different from right side to left side. Why is the spider vane, (on right), not pointing towards the centre of the main??
The whole 'darkened' area around the main is skewed off centre, (5oClock to 11oClock positions). HTH. L.
pps.BTW, a 114 & a 6mm ortho, was all I had when Shoemaker/Levy 9 hit Jupiter. Nice, small views tho!

[ballaratdragons (Ken)]

John,

A really simple trick I learnt for checking 2ndry size is when sighting down the drawtube with a film canister etc is that the Primary should just fit inside the circle of the 2ndry. If you cannot see all of the Primary in the 2ndry then the 2ndry is too small.

Even if the 2ndry is only a tiny bit too small quite a bit of light from the primary will travel back out of the tube passed the 2ndry, and the passing light can illuminate the underside of the spider which in turn will be visible in the primary. Yuk!

EDIT: And don't forget that the larger the 2ndry, the less the light gathering capacity on the primary.