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4" f4-5 telescope help needed

Started by RoDnSuE, 12/18/2004 05:06PM
Posted 12/18/2004 05:06PM | Edited 12/18/2004 05:07PM Opening Post
I have a homebrew telescope that I was given several years ago. I haven't really used it since I don't have any 1.25" eyepieces. It has a solid aluminium screw in type of focuser instead of a rack and pinion like most scopes I have seen.
I've been reading some of information on the telescope building sites and was wondering how useful this size telescope might be. With a ROUGH measurment the mirror is 4" and the FL about 18-19". This mirror should be about f4-5. There is some writing (in felt tip pen) on the back that say's 34" R.C. , 80% parabolic.
Please correct me if I'm wrong but from what I have read, wouldn't this be considered a wide field telescope?
What should I expect with this type of telescope? The mirror is not perfect but money is very tight right now so it will have to do.
I just want to know what I should consider before I spend any money on eyepieces. Also what eyepieces would be best to start with?
Any help is MUCH appreciated,

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RoDnSuE's attachment for post 20887
Posted 12/18/2004 06:09PM | Edited 12/18/2004 06:18PM #1
OK, Thanks for the info. That helps some. At least I have a better idea of what I have smile I took the mirror cell out and measured closer and it is 4.25"
Would this be a good telescope for planet viewing? Any idea of what I should expect to be able to see with this limited apeture?

James R. Chastain
Posted 12/19/2004 07:09AM #2
You should enjoy some wide fields--not too much magnification.

It should work. But don't expect the sharpest views. One thing that you mentioned-- 80% parabolic--indicates to me that an amateur made the mirror, and got it about 80% parabolized before stopping work. It is not wholly unusual that a mirror maker would write that on the back of a mirror, although usually he would come back and finish figuring it. (Although, technically, a true optician is never "finished" with a mirror, you just finally have to steal it from his Foucault tester and get it aluminized or he'll go on forever trying to tweak it just a bit more.)

Why don't you find a star party, and take it, and borrow some eyepieces and see how it works.


Posted 12/19/2004 08:18AM #3
Thanks I will take a look. I'm not really sure which type of ep (plossl?) to get but 17" focal length is 431.8mm so a 12.5mm ep should give me about 34.5 power. Then a x2 barlow should double that. (see...I have been reading) 8)
I don't want to spend a fortune on ep's but I would like something that will be useful when I do upgrade to a better telescope.

Thanks for the reply,
Posted 12/20/2004 06:05AM #4

I put together a 4-1/2" reflector with approximately the same focal length back in the 1970s.

I use this mostly with a 25mm Kellner eyepiece that I bought at the time. You should be able to get a new Plössl eyepiece for between $25 - $35. Check at

Like one of the above posts said, I hold the tube under my arm and scan around the sky. I see wonderful wide-field views of the Milky Way, open clusters and even bright nebula like the Great Orion nebula.

I also epoxied a little tapped plate to the tube which allows me to mount the scope on a photography tripod. With this and 12mm and 9mm Plössl eyepieces, I have viewed Saturn and Jupiter very nicely indeed.

I think you can make some great observations with your scope, at very little investment for an eyepiece. Best wishes and merry Christmas,

Rick Wiseman

8" Zhumell Dob
90mm f11 achro refractor
4-1/2 f4 rich field newt reflector
Posted 12/21/2004 12:28PM | Edited 12/21/2004 12:33PM #5
Very many folks have newts in this size range, as their first, second and even third scopes. Low power, wide field and portable. Simple and inexpensive possl's will work fine for eyepieces. They are very affordable right now in the classifieds here.

What can be seen in this scope is certainly dependent upon it's optical quality. But it is just as dependant upon your growing experience as you use the scope. There is tons of stuff within the range of a 4" newt this size, particularly from dark sky (and this scope is portable sir!). Beyond the Messier list, there are hundreds targets from beautiful open clusters in the NGC to double, multiple and variable stars, emmission and absorbtion nebulae, just to note a few things. As well as the show pieces of our own solar system.

Do a search on the forums here for "pipe mount", for some very affordable ways to mount your scope. You need not be limited to table top or handheld observing.

My 4" f/4 newt has a peep sight. With a little experience it is easy to bullseye planets at +100x with a pretty simple no-power finder. Batteries not even required.