Focal length issues?

[Zed (Zeljko)]

I managed to put together my scope and collimate the mirrors, but when I tried to observe the Moon ( using a chair and some phone books as a mount ) I could not get a clear image. Image starts getting sharper toward the bottom limit of the focuser travel - focuser fully retracted. But then I run out of adjustment and cant get clear image.
So I concluded that the focal length of the primary mirror is shorter than the length used to design the scope. I then moved the primary mirror closer to the secondary about 20mm and managed to get clear image of the moon. However image still clears closer to the bottom limit of the focuser.
I'm using 20 and 12.5 mm eyepieces.

Is it possible that whoever built this scope originally got the focal length wrong ? I received the scope in disassembled state, primary has been re coated and it is in top condition. I doubt that this scope has been used at all...
I guess I should get clear image with focuser about half extended ?
If it turns out that the primary mirror has been mounted on the wrong position should I permanently reposition it ?

Thank you
Zeljko

[erick (Eric)]

Odd. I guess "yes" to all your questions. I haven't done it before but I guess you can roughly measure the focal length of the primary by setting up a torch shining on it a bit of axis, then see how far from the surface the torch light comes to a rough point. Might be worth doing as a check, then compare with measurements on the tube unit.

There is absolutely nothing else in the light path - no lens or similar at the bottom of the focusser draw tube?

Mounting the mirror several centimetres up the tube shouldn't be a problem, as long as you can get your fingers onto those three wingnuts which will be used for collimation.

I guess a question arises as to whether the secondary is now doing a good job of capturing all the reflected light - ie. is the outside of the light cone from the primary missing the secondary all together? OK, we need the experts' views now.

However, before driling holes and cutting tube to length, best to stop and think and get some other eyes/brains on the job in case something is happening to cause this.

[Zed (Zeljko)]

It is definitely weird problem. I will not drill or cut anything yet. I mean, if someone decided to build a scope focal length of the primary would be the most important measurement on the whole scope. To get such an important dimension wrong seems like a big mistake even for a beginner.
It is out by at least 20 mm !!
I have some basic knowledge about optics, so the focal point of the primary must fall on the secondary within a couple of mm....

Erick, thanks again for your idea to center-spot the primary. It worked good, except for finding binder reinforcement ring. I went to officeworks to get them but the only kind they stock now is transparent rings !!! Kinda bad feature if you need to clearly see them.

I will wait for some more opinions about the focal length issue...

[erick (Eric)]

Originally Posted by Zed

I have some basic knowledge about optics, so the focal point of the primary must fall on the secondary within a couple of mm....

When you rethink that, you'll realise that that the focussed image actually has to fall close to the eyepiece. The eyepiece allows you to magnify that image or the bit of it you want to see.

(aside - leave an eyepiece out of the focusser and put a partly opaque cover over it and point the scope at the Moon. Adjust the focusser up and down and you should be able to see an image of the Moon focussed on that cover!)

[Blue Skies (Jacquie)]

Hi Zed,

Did you solve your problem?