Building a refractor with Durst enlarger head

[Leo.A]

This may have been asked before and may be a silly idea, but I have an old Durst photographic enlarger which has a 130mm primary lens. The head off the unit consists of two lenses with an air gap between them. The top lens (nearest the light source) is convex on both sides and 130mm OD (5.12") and the lower lens has one flat side nearest the image being processed and a convex inner surface, also 130mm. The two lenses are separated by an air gap of approximately 55mm.

Would it be possible to use this to make a refractor telescope and if so, how exactly would I go about determining the correct focal length for the required tube?

I understand if it were possible, the scope wouldn't have near the optical quality of a professional unit but surely it would give acceptable viewing of the moon and planets.

I'd love to hear any thoughts on this matter, or even better, hear from anyone who has attempted this or something similar.

I'm assuming, being from a quality photographic enlarger the glass would be of good optic quality. No where near the scopes available now but for my purposes and finances (or lack thereof as a disability pensioner), it could give slightly better magnification than my 80mm Tasco with Vixen ED lens (better magnification, no where near the quality I guess).

I was contemplating using the cardboard tube used for concreting for a temporary housing till I determine the correct focal length of the head.

Just a guess here, but since the original enlarger had limited travel, this would also mean it has a very short focal length, making it totally unsuitable for my purposes.

Anyone want to buy an antique Durst enlarger (freight from Australia would be a killer though)??

[Naz]

I think you have summed it up, the lenses are short focal lenght and are designed to focus the light from the bulb on to the film plane in the negative holder. The CA I suspect would be horrendous and very short focal length. Try setting them up and using an eyepiece in daylight, I suspect you will give up after ths test.

[Islander]

Actually they're designed to focus onto the film plane - the light from the passes through a condenser or diffuser onto the film plane to illuminate it and the lens focuses on the negative image. You're right about the focal length though - for a 35mm enlarger it would typically be around 50mm. With a bigger enlarger designed for large sheet or plate film, the lens would typically be around 135-200mm or so. The focal length should be printed/engraved,stamped somewhere on the lens assembly.

[Islander]

Actually, after re-reading this are you sure you're not talking about the condenser rather than the lens?

The condenser sits between the lamp and the negative carrier, the lens sits between the negative carrier and the baseboard and is attached to a rack and pinion focuser.

[Leo.A]

It may very well be the condenser I'm talking about. I didn't pay a great deal of attention to the unit, it's under my house in relative darkness. I'll go down tomorrow with a torch.

[Islander]

From the size of the glass involved, I'd say it is. A condenser isn't designed to bring light to a tight focus, it's designed to evenly illuminate the image stage (negative holder).

[Peter Drew]

Hello Leo.

I'm afraid the condenser lens project for an astro telescope is a non starter for the reasons given by others, the nearest you could get to anything vaguely astronomical would be a giant Ramsden eyepiece. Your 80mm scope is the best you've got.

[Leo.A]

Thanks everyone, all I know is when I sat it on a magazine it made a good magnifier.

I have found another good use for it though, it will go into my camping kit as an amazing fire starter when the sun is shining. I used it today to engrave some plastic tool cases and the pin point focus of light puts out so much heat within seconds. I found out about it's magnifying properties the hard way, I had it sitting in the palm of my hand for a few seconds when I walked out in the sunshine (one of those rare days of sunshine we've had this year).