Orion Optics Newtonian design question.

[NGC 1502]

Hi all. I have 2 O.O. ( U.K.) Newtonian scopes, a 10" F4.8 and 6" F4.

They both have an upper tube end ring, with a lip that has an

inner diameter the same size as the primary mirror.

It is effectively a baffle at the upper end of the tube.

But will this lip vignette incoming light ? I have done a search

on " Newtonian telescope tube baffles " There were some

complex maths involved, but several places which seemed to

say that the front entrance of the tube should be larger than the

primary mirror. This could be to accomodate correct collimation

that does not coincide with the tube, as correct optical alignment

is possible that is not parallel to the tube walls.

I realise that there is some discussion about Newtonian tube baffles

directing tube currents into the optical path, but that is not my

question here.

So my question is, do the upper tube trim lips, being the same

size as the primary, cause a problem ? Or am I missing some

obvious ( or not so obvious ) point ?

Your comments much appreciated, thanks in advance, Ed.

[andrew s]

Ed - yes it will. The opening need to accommodate the angular field of view you want. Andrew.

[John]

I'm interested in this as I have an OO 10" F/4.8 with the same feature - all the OO newts I've seen have it in fact. The Skywatcher and Meade newtonians that I've owned have a similar feature but the top aperture is wider than the primary mirror diameter.

My take on it is that the lip will not prevent light getting to the primary. I would also be somewhat amazed if a company with the technical experience that OO have would incorporate a feature, for many years, that would in any way reduce the performance on their excellent optics. My 6" Intes Maksutov-Newtonian has a similar lip around the top of it's dewshield and that does not seem to vignette or reduce the light getting into the optical system, as far as I can tell.

I'd be interested in other points of view on this though.

[Nick P]

I am certainly no optician or optics expert but I think you will find the light entering is parallel to the OTA, and when this hits the mirror it is then focussed to the secondary mirror and the eye, hence the shape of the primary mirror.

Any light entering the tube outside of the mirror aperture will be bouncing around inside the OTA reducing contrast.

The lip on the OO OTA should not cause vignetting (just checked on mine and the lip diameter is exactly the same as the primary mirror) because the light travels straight into the open end not at an angle.

The above is just my understanding of the issue and am quite happy to be proved wrong!

Best

Nick

[FraserClarke]

I am certainly no optician or optics expert but I think you will find the light entering is parallel to the OTA

The light from any given object is parallel -- but different objects come in at different angles (because they are in different places on the sky). So typically the tube is wider than the primary to give you a field of view (rather than a 'point' of view).

Is the opening exactly the same size as the primary, or a just little bit larger?

For your 10-inch f/4.8, I guess the maximum field is about 2 degrees (based on min mag and typically eyepieces). So, if the tube is ~45 inches long, which seems about right, you'd need the top of the tube to be ~1.6" wider than the primary to give you an unvignetted 2 degree field of view.

If it is only ~0.5-1" wider, then probably OO are trading some vignetting for improved baffling of scattered light.

Some telescope designs do have the "aperture" defined by the tube rather than the primary mirror -- but I've never heard of this in a normal parabolic mirrored Newtonian.

[astromerlin]

Interesting, Just for comparison my SW 250 F4.8 has a primary of 254mm and the "aperture" is 275mm.

[Peter Drew]

As mentioned above, oversize entry apertures allow for larger unvignetted fields, size for size apertures presumably act as light baffles so there is a trade off to consider. I have a suspicion that Mewlons have an undersize baffle which might explain their legendary contrast.

[sixela]

My take on it is that the lip will not prevent light getting to the primary.

If the entrance is as large as the primary, it will vignette all the light bundles except the on-axis bundle. Which is't a real problem (vignetting by the front is very gradual), but it's not very usual.

The good thing is that it will partially baffle the tube against glare from objects very close to where the scope is pointed.

But are you sure it's exactly as large as the primary?

I would also be somewhat amazed if a company with the technical experience that OO

OO have very good mirrors, but I've seen them do very strange things before with scope mechanics .

[KaStern_Former_Member]

Hello Ed,

if the entrance diameter is precisely the mirror diameter the aperture is not stopped down.

But for objects situated away from the optical axis there will be vignetting.

Antother interesting side-effect ist the following:

If the entrance pupil is placed as far away from the mirror as the focal length is

and so small that all light rays of the off-axis-objects fall onto the mirror

astigmatism is cancelled from the system (!)

But the Orion Optics solution as you describe it is not good. It will hinder the cool down

and tube currents will run through the path of the incoming light.

Cheers, Karsten

[michael.h.f.wilkinson]

There is one way to prevent vignetting with such a baffle, and that is to OVER-size the mirror (various Schmidt cameras have such a design, and I have come across a "rich-field" Newtonian of such a design). Because over-sizing a mirror is rather more expensive than over-sizing the baffle, guess what manufacturers generally do.

The maths behind baffle design is actually not that complex: if the front baffle is at a distance from the primary equal to the focal length, then the unvignetted field at the focal plane is equal to the difference in size between primary and baffle. Thus, for a 2" field stop, the diameter of the front baffle must be 2" larger in diameter. For any baffle at some fraction X of the focal length from the primary: multiply this number by X. So a baffle half way down the tube needs to be 0.5x2"=1" larger than the diameter of the mirror in this example. In practice, the size of the secondary comes into play as well.

[John]

.....But are you sure it's exactly as large as the primary?

OO have very good mirrors, but I've seen them do very strange things before with scope mechanics .

I've just measured mine. The primary mirror is 247mm in diameter and the aperture at the top of the scope is 253mm.

Don't know what that means

But I know what you mean with regard to OO's approach to mechanics at times

[michael.h.f.wilkinson]

I've just measured mine. The primary mirror is 247mm in diameter and the aperture at the top of the scope is 253mm.

Don't know what that means

But I know what you mean with regard to OO's approach to mechanics at times

So that gives you a 6mm unvignetted field by my back-of-the-envelope method:icon_scratch:

[sixela]

I've just measured mine. The primary mirror is 247mm in diameter and the aperture at the top of the scope is 253mm.

Don't know what that means \

It means that you have a fully illuminated field that is roughly 6mm/(250*4.8)*57.3° large, i.e. 0.28° (half a moon diameter), and that beyond that you have some front aperture vignetting.

That's actually assuming that the secondary is offset away from the focuser by the right amount, otherwise that fully illuminated field could well not be centred.

Not the way I'd build the scope, but not a catastrophe either.

[John]

It means that you have a fully illuminated field that is roughly 5mm/(250*4.8)*57.3° large, i.e. 0.24° (half a moon diameter), and that beyond that you have some front aperture vignetting.

That's actually assuming that the secondary is offset away from the focuser by the right amount, otherwise that fully illuminated field could well not be centred.

Not the way I'd build the scope, but not a catastrophe either.

Thanks for the feedback. Not disastrous but not too encouraging either, especially considering that these OTA's cost well over £600 new !.

In an earlier thread I pondered why OO scopes depreciated rather quickly - perhaps this sort of thing explains that

[Moonshane]

very interesting thread. I have two OO scopes too. my 300mm scope has an 'aperture' of 310mm and the 150mm of 152mm.

I am more concerned about the tube currents than the vignetting in truth.

Although I am not going to worry too much!

[NGC 1502]

Hi everyone, thanks for the feedback.

Earlier today, I emailed OO to ask if the upper tube trim lip, being the

same size as the primary, would cause vignetting. The prompt reply

I recieved said that :-

"Yes that is correct. All telescopes have a trade off and a compromise

in their design"

So maybe I'll leave it at that for now, or perhaps discuss it with them

in person, when I visit Astrofest in London next Feb.

Thanks again, and best regards, Ed.

[Moonshane]

I might try removing the trim off my 12" and see if it makes a difference I can see - if it doesn't then I'll put it back on!

[sixela]

Not disastrous but not too encouraging either, especially considering that these OTA's cost well over £600 new !.

Vignetting by the front is very gradual. At e.g. the left edge of a 1° field, you're going to lose between 10mm (on the side) and 0mm (on the top and bottom) on one side of that precious 250mm mirror. I had an Excel calculator for it but I lost it, but that's certainly less than 4% illumination. At the other side of the mirror the secondary is probably going to clip roughly 25% of the mirror.

Which leads me to the next thing: some Orion Optics scopes have the focal plane thrown way out of the tube, making you require a 2" extension to get eyepieces to focus (the good point, of course, is that without the tube you can usually get a DSLR camera to focus). That's less visible but usually far more important for edge of field illumination.

OO does, most of the time, compensate that with a a humongous secondary, but a slightly smaller secondary and different focal plane placement would make more sense.

The problem isn't that it's all a compromise: it indeed is, as you find out once you design your own Newtonian. The issue is that OO makes very different compromises from what others would make at times, and some that may make sense for others (e.g. using the scope as an astrograph) but not for you.

You can actually convince OO to implement other compromises, by the way, but usually that will require some negotiation and some tinkering (a friend of mine bought an OO with a slightly smaller secondary but OO didn't move the focal plane placement accordingly, and then replaced the mirror supports from the cell with something shorter to move the mirror back).

[russ]

Which leads me to the next thing: some Orion Optics scopes have the focal plane thrown way out of the tube, making you require a 2" extension to get eyepieces to focus (the good point, of course, is that without the tube you can usually get a DSLR camera to focus). That's less visible but usually far more important for edge of field illumination.

Their fast newts, the ones more likely to be used as an imaging tool for deepsky, do have the focal plane well outside the tube? I need an 80mm extension for the 150 f5.

But the 150 f8, more of a planetary scope, had the focal plane setup perfectly for eyepieces and required no extension.

Not sure i would want to remove the rim though. OO tubes are pretty flimsy even with the rims in place. Is it really worth removing for a 4% gain?

[NGC 1502]

Hi again. Some further thoughts on what has been said in your comments -

I had thought of removing the front tube trim, and making a replacement that

fits outside the tube, (to remove the same size as primary baffle) and retain the

strength that the original trim provided. Or perhaps just remove the lip from

the trim. Not at all sure I'll do that now, on either of my OO tubes.

Yes OO will do "specials" a clubmate had them make a 200mm F4.5 SPX Newt

with thicker tube walls, and a secondary of his chosen size.

My 150mm F4 has a 59mm secondary (39%) with the focal plane 140mm outside the

tube. I'm no optics expert, but I'd have preferred a smaller secondary, with a

bit longer tube, and the focal plane closer to the tube. However, it does mean

that the OTA I have is a super portable 535mm long. It walks all over the

Edmund Astroscan that it replaces, partly because it is collimatable. The other

main problem was that the dovetail was bolted directly to the tube, and the tube

wall flexed alarmingly when on my AstroTech alt az mount. I cured that by using

tube rings, and fixing the dovetail to that, in conventional fashion. BTW, OO made

150mm F4 Newts a few years ago, not made now. (My 250/4.8 doesn't have these

issues)

Anyone care to come up with an ideal Newtonian solid tube design for visual use ?

Apart from homemade, does any company make such a product ?

Cheers & best regards, Ed.

[sixela]

Their fast newts, the ones more likely to be used as an imaging tool for deepsky, do have the focal plane well outside the tube? I need an 80mm extension for the 150 f5.

Exactly what I meant.

But the 150 f8, more of a planetary scope, had the focal plane setup perfectly for eyepieces and required no extension.

Which is why I said "some" scopes. The trouble is that they're not exactly telling you these things, and while you can indeed buy a different size for the secondary, if you're unaware of these things the scopes can be a bit weird for the intended usage.

Not sure i would want to remove the rim though. OO tubes are pretty flimsy even with the rims in place. Is it really worth removing for a 4% gain?

No. It's also only 3-4% at the edge of a quite wide field, and less everywhere else.

[Moonshane]

I have to use 50mm extensions on both of my scopes. the point of focus (i.e. without an EP in) is about 165mm (just over 6 inches!) outside the scope in both.

my 150mm has a 25mm secondary (16.7% obstruction).

[sixela]

I have to use 50mm extensions on both of my scopes. the point of focus (i.e. without an EP in) is about 165mm (just over 6 inches!) outside the scope in both.

my 150mm has a 25mm secondary (16.7% obstruction).

I hope you meant 165mm from the scope axis, not the tube edge, because otherwise it's quite simply insane. With a 150mm f/11 and a 25mm secondary, you want the focal plane 250mm from the axis of the scope just to get a point sized fully illuminated field!

Easy to check: you have to see the entire secondary from the middle of the focal plane. If you don't, then either the secondary is undersized or the focal plane needs to be closer to it. And that's just for the minimally sane "planetary" confgiuration where you don't care about edge of field illumination for a large field at all!

[Moonshane]

hi Alexis

the measurement from the tube to the 'bare' 2"-1.25" adapter (i.e. where the eyepieces are focused but with no EP in) is indeed 160-165mm plus about 75 to the axis = 235-240mm.

when I put my eye to the drawtube at this position, I can see the whole of the primary but it is a tight squeeze literally completely filling the secondary.

I ran the distances through 'NEWT' and it seemed to suggest that it was within acceptable tolerances but really I am not expert on such matters.

it is certainly designed for planetary use and gives very contrasty and detailed views of luna and Jupiter at least.

[sixela]

I ran the distances through 'NEWT' and it seemed to suggest that it was within acceptable tolerances but really I am not expert on such matters.

As 240mm is less than 250mm it's not completely insane.

But something a bit saner would have allowed either a much smaller secondary (17% is quite a lot for an f/11 150mm, normally you should be able to get away with 19-20mm or 13% or even less with a low profile focuser) or yielded a much larger fully illuminated field (with a 25mm and the focal plane 175mm from the axis, you can get a 20mm field that's 70% illuminated).

One thing's for sure, though: the baffling will be excellent because there's no way for light from the front to reach the focal plane even if the tube is shortish.