Oh dear! I fear I may look stupid, but...(help needed!).

[david_r]

After purchasing a second hand refractor to piggyback onto my newt a couple of months ago, I finally got around to fitting it today. Thought I'd mount it and as it's a clear sky have a looky see through the refractor to see what I could see....and I can't; not a thing!!

It just doesn't seem to focus. The refractor is a 102mm F5 Helios.

The focus end looks like this...

There is an adaptor inbetween the focus rack assembly and the eyepiece (the internal bore of the adaptor is threaded throughout - in case that helps you?). This looks like this....

And this is what the adaptor looks like with an EP fitted.

All the covers were removed!! Cant think what else it can be other than something is wrong with the focal path.

I'm sure it's something really dumb, but I just can't get it to focus on anything. Tried EPs from 30mm through to 6mm.

Help (please!)

Any assistance would be humbly received.

Thanks,

David.

[Darth Takahashi]

Sometime you need an extension tube to increase the focus distance to allow it to come to focus. This is done to maximize the back focus distance on refractors. Alternatively, have you tried adding a diagonal to the focuser and then the eyepiece, this will also increase the length of the focus path, try to focus again?

Neil

[tinvek]

im no expert and im sure someone will be along soon with more idea but when you say you couldnt see anything, do you mean no image or just completly black? if you're looking for a star and its a mile out of focus, you probably wont see anything, try looking at a street light or in daylight, ideally try the moon but typically the one time you need a large bright object its not available

as neil says most refractors come with a diagonal these days and need the extra length from one or an extension tube to get the focus

[jgs001]

David, if it's anything like the 80mm F5 frac I have, then you will need a diagonal as a minimum and probably an extension tube as well. A diagonal will make it far easier to view, as you can look down into the ep, rather than squatting under the scope and craning to look up. That adapter you have is to convert a 2" focuser tube to take 1.25" eyepieces and diagonals.

[david_r]

Thanks for the responses. The easy one first, no its not pitch black, there is light in the optical path.

OK, I do not have a diagonal to try. In terms of diagonals, I'm sure they vary in quality, but in terms of optical path length, are all diagonals the same optical length? Therefore to test it 'any' diagonal would be sufficient?

Thanks,

David.

[Parallax]

If you have got a barlow and the lens unscrews, try that.

Stave

[jgs001]

David, something you could try, although it's not going to be easy, hold the EP away from the focuser and move it back and forth, see if you can get focus. It may help using a cardboard tube, something like a kitchen roll tube as a guide and to keep out some of the external light...

[Darth Takahashi]

You need approx 80 - 100mm additional length to come to focus. Diagonals are normally a standard length except when moving up in size from a 1-1/4 to a 2" diagonal.

The barlow suggestion is excellent, if you have one try it.

Neil.

[david_r]

OK. Got a 2x Barlow. I can unscrew the lens, but what I am left with is greater than 1.25" in diameter, so it will not fit into the 1.25" EP orifice.

The scope is back in doors now. From one end of the house to the other (open plan) and panned back and forth along the optical path holding a 30mm EP. It comes into focus when its around 100mm+ away from the EP mount. So I need a diagonal and an extension tube? Do I need to accurately measure this distance and get an extension tube to match?

Thanks again all.

[jgs001]

I've just done a very quick and rough measure of the approx light path through my diagonal and it's about 80mm... As I say, with my 80mm I needed both a mirror diagonal and extension tube, or a prism diagonal (which is what it was supplied with)

[Darth Takahashi]

No a standard diagonal will do, you don't need to be so accurate you just need to bring the point of focus inside the range of the rack & pinion focuser.

Good to hear that your sorted now...

Neil.

[RobH]

I have the 120mm version of the same scope, and it won't focus without a diagonal.

You say you can see nothing at all. In the daylight, can you see light through it?

Cheers

Rob

[david_r]

Super. Thanks folks, much appreciated.