phenix d203 spherical reflector

Technical Specification

203mm (8-inch), f=750mm (f/3.7) Newtonian

Primary Mirror : Spherical

Aperture : 203mm (8-inch)

Focal length (f) : 750mm (35.4-inch), f/3.7

Equatorial Mount : EQ4(ii) German Equatorial

Finderscope : 6x30mm

Tripod : ST2 Heavy Duty Steel tube

Standard Accessories : 31.75mm (1.25") Plossl Eyepiece PL7.5mm, PL32mm

Angular Resolution (Resolving Power) : 0.564 arcsecs

Maximum Useful Magnification : 400X

Limiting Magnitude : 15.4
my question is how would i get the best from this telescope, i have tried and tried without success. to see saturn and other planets such as venus... but when i look through it with either eyepieces at the planets they appear just like blurry blobs no matter how much i try to focus, by the way im just a newbie and havent a clue about telescopes.. do i need other eyepieces, or a barlow lens etc... and which ones would you recomend for there are so many to choose from any help would be much appreciated so please if anyone could explain in laymans terms for i dont understand all the tecnical stuff...lol btw my m8 collimated the scope for me as best he could.

hmm,

I *really* hate to be the purveyor of bad news here but alas I think you are never going to get the kind of views you would hope from that scope I'm sorry but I don;t want to lead you false hope. The problem is the spherical mirror. In order to focus objects correctly, reflector mirros must be parabolic, not spherical. At f3.7 this is even more important. The mount is probably pretty good, but unfortunately all you will ever see of the planets is blobs.

The problem is this. Parabolic mirrors focus all of the colours of an object such as a planet in one place so you can examine with your eyepiece. Spherical mirrors do not do that and all of the colours can never be in focus at the same time. So you will always see coloured frings which swamp out detail and make it look so bad.

I *really* wish you had come here before buying that scope anyone here would have told you to steer a mile from it. It *really* annoys me that shops are still selling this sort of scope. Damn moneygrabbers don't care about astronomy, heck they don't even know about astronomy.

Anyway onto the positive advice. The scope may work, to a point, on star clusters and nebulas where fine resolution isn't so important. Keep the power low (use the 31mm eyepiece) and you will get the best view.

And don't mind that rubbuish about 400x useful magnification, that would only apply to a well corrected parabolic mirror not a spherical one. Limiting Magnitude 15.8, trust me you will never see that faint even in a well corrected scope. Come along to your local club meeting and see what the view through a properly made scope looks like and you will see exactly what I mean.

Oh I hate it when people are treated like this. Sorry Marty but I can't be any more positive than that.

One thing that might be of use. Did by anychance the sales assitant tell you that this was a high quality telescope?? If so you might be able to get a refund under the trade descriptions act.

Sorry mate!

You might have SOME use for it.

It might be useful for very low power wide angle views of the milky way under dark skies. These can look marvellous through fast optics at low power, where raw aperture is more important than spot-on optics.

For detail on the planets...forget it.

What Dave has said is correct. Spherical mirrors are not capable of producing a sharp (diffraction limited) image unless they are at least f/12... and there is the solution:

You can turn your 8" f3.7 telescope into an f/12 telescope very easily by using an off-axis stop. Unfortunately, you will also be reducing the aperture from 8" to just 2.5". To do this, get a piece of stiff cardboard and cut out a circle that just fits inside the front opening of the telescope. Now mark and cut out a 2.5" diameter circle that has its centre about half way between the centre of the cardboard disk and its edge. Now place this "off-axis stop" at the front of your telescope and tape it in place. You should get nice sharp images. Note, however, that you will probably not be able to use magnifications higher than about 125x.

If this works well, then you could try making an off-axis stop with a larger opening (say 3" for f/10) and see how that performs.

Look on the bright side - Galileo did most of his work with about a 1" lens of even worse quality than your spherical mirror!

Hope this helps,

Phil.

Stop it down to 2.5" Phil?

Preposterous.

Better to throw it in the bin.

Peter.

Hi Marty,
Unfortunately, what the guys above say is all true, so forget it for planets and look at clusters, that scope will give a workable image at low magnifications on clusters, however, you were basically taken for a ride by been given a spherical mirror, go back to the shop and get a refund if you can, tell them its not working, make up some excuse.