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Parabolic mirror


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Barring removing the mirror from my scope how can i tell if its a parabolic mirror?

Looking down my short tube Newt it looks 'flat' could it be the secondary mirror is parabolic?

Is it cost effective to replace the primary mirror with para version?

sorry for all the questions

Justin

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The secondary will be flat.

Any curve will definately be on the main mirror.

The fastest and easiest way to check is to use a Ronchi screen at the focuser (no eyepiece) and look at a reasonably bright star high in the sky. If the mirror is not parabolic the lines will appear to bend...with a parabola they will appear straight.

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No you cant tell just by looking at the primary.

This would be a lot simpler if you gave us some basic info on the scope, make, model, type, anything really.

However, short tube newts usually have spherical primaries(so NOT parabolic), but the clue is that they then also have a lens down inside the eyepiece tube. You may not even have noticed this, they are almost invisible, try inserting a pencil or some soft skinny object down the focuser tube and see if it stops befor it gets to the diagonal.

Another clue, the actual tube length is much less than the focal length of the primary.

Is it cost effective to change to a parabolic mirror? No.

Barry

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No you cant tell just by looking at the primary.

This would be a lot simpler if you gave us some basic info on the scope, make, model, type, anything really.

However, short tube newts usually have spherical primaries(so NOT parabolic), but the clue is that they then also have a lens down inside the eyepiece tube. You may not even have noticed this, they are almost invisible, try inserting a pencil or some soft skinny object down the focuser tube and see if it stops befor it gets to the diagonal.

Another clue, the actual tube length is much less than the focal length of the primary.

Is it cost effective to change to a parabolic mirror? No.

Barry

An expert optician can tell by looking at them (I have one to hand!) but I'm blowed if I can do it...

Olly

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Then that's hopeful, when I said the tube length would be much shorter if spherical, I really meant MUCH shorter. It would have been in the order of 250-300 mm with a spherical primary.

So it appears you have a 6" f/5 para newt.

Regards

Barry

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Ah got you :)

Sounds good at least I think that's good :) sorry still a newbie to this.

If I could work out what's going on with my lack of focus basically with my webcam the image begins to come into focus but I run out of INward motion on the focuser.

Thanks for the help Barry :(

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Yep! very common problem, they now make versions of the popular newts that are shorter so that webcams can get close enough to focus!

How to make yours work? if you can afford, buy a replacement focuser, look out for those that are low profile.

Use a barlow lens, they focus further out anyway, but the extra magnification might be inconvenient.

Disassemble the webcam and make it mount IN the focuser tube.

Finally, take a hacksaw to your focuser and shorten it!

I did it to a cheap 3" newt, you might not be so keen!

Or remount the primary further up the tube.

These are serious mechanical mods that need a fair degree of skill and resources.

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Thanks again.

I'll try my BarlowX5 tonight would that be better over the X2 which was better but still no quite there? putting the extra mag to one side that is.

I'll take a look for a replacement focuser, dont fancy the hack saqw route.

Cheers,

Justin

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Then that's hopeful, when I said the tube length would be much shorter if spherical, I really meant MUCH shorter. It would have been in the order of 250-300 mm with a spherical primary.

The focal length is shorter if the mirror is parabolic not if it is spherical.

Spherical mirrors have a longer focal length to compensate for the aberations caused by the mirror be spherical.

If f/5 then I would expect a parabolic if the intention was to supply a usable instrument. However there is no reason why the mirror cannot be f/5 and be spherical. Just it would be useless in the majority of cases.

If it does not state Parabolic I would play safe.

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You have a manual?

Then why are you asking all these questions?

What does the manual say the scope make, type or model is?Could you post pictures of the scope and manual for us to look at?

because the manuals useless, I had already looked for the info but hadnt expected it to be 3/4's of the way through rather the front or back!

The manual doesnt say anything about the make/model just that its a Short Tube Newtonian 150/750

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If it has no small lens in the focuser tube and the scope gives a sharp image then it must be parabolic. A F5 spherical mirror, without any secondary optical "help" would be hopeless. :).

Not necessarily, there are 150/750 reflectors on ebay that have spherical mirrors. The fact that the primary is only described as "concave" suggests that it is not parabolic.

One way to confirm the mirror is spherical and not parabolic is to compare the image sharpness in the centre of the eyepiece and at the edge. A parabolic mirror will give a constant sharpness across the FOV whereas a spherical mirror will only be sharp near the centre and become less focussed near the edge.

Peter

Edited by Cornelius Varley
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I am aware that there are scopes out there, ebay and elsewhere, that use spherical primary mirrors, these scopes then have a correcting/barlow lens in the eyepiece tube. This is a Jones-Bird design scope.

I do not believe there are any scopes that are trying to use ONLY a spherical mirror, it just would perform so badly no one would accept it.

The corrector/barlow arrangement increases the focal length. So you have scope that physically much shorter than its focal length.

That is the aspect that suggest to me this scope has a parabolic mirror.

Barry

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you can be pretty sure had it been parabolic, they would have said so rather than concave.

Totally agree. Even the budget scope vendors have latched onto the term "parabolic" and use it in their marketing, if the scope actually has a parabolic mirror that is. If not then they tend to flannel using terms such as "precision" and "highly accurate" which mean nothing in reality of course :)

I guess it will soon become clear whether this scope has a decent parabolic mirror or not as the owner uses it on a variety of objects and compares what can be seen with reports of observations made with other 6" F/5 scopes.

I find tight double stars a good test of optics, collimation, observer and, most of all, observing conditions !.

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I am aware that there are scopes out there, ebay and elsewhere, that use spherical primary mirrors, these scopes then have a correcting/barlow lens in the eyepiece tube. This is a Jones-Bird design scope.

I do not believe there are any scopes that are trying to use ONLY a spherical mirror, it just would perform so badly no one would accept it.

The corrector/barlow arrangement increases the focal length. So you have scope that physically much shorter than its focal length.

That is the aspect that suggest to me this scope has a parabolic mirror.

Barry

A year or two ago sky at night did a group test of 6" reflectors which also included a 150/750 telescope with a spherical mirror. I can't remember the supplier but could have been Skys the Limit but not certain. They commented on the fact that the telescope had a spherical mirror compared to the others (Orion, Skywatcher, Celestron) which had parabolic mirrors and cost less than them.

Peter

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