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low profile focuser?


Rusty Strings
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What are the pros and cons of low/ high profile focusers. The 2" R+P fullerscopes focuser, that is part of the 12" newt I'm rebuilding, stands 5" tall and I'm considering reusing it as it's built like a tank with very little perceptable backlash and slop. The only real negative is it doesn't have compression rings. I can't, on the face of it, see why a low profile focuser would be better other than maybe better back focus for imaging (I'm visual only).

Would I be right in thinking that the lower the profile of the focuser, the further from the end of the OTA it has to be to prevent stay light entering?

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I think the main advantage is that you can push the secondary further away from the primary, and then maybe have a smaller secondary too. You may also prevent vignetting.

If you run a few tests with Newt it should show what difference you could expect.

/callump

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If the mirrors are set to work with a focuser 5" high if you added a low profile focuser would you then need to add extension tubes to get the EP to the same focal plane? Or would you have to move the mirrors further apart to allow for the EP being closer to the secondary?

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To be honest, there's not much point in switching to a low profile focuser unless you are going to move the mirrors further apart.

If the mirrors are set to work with a focuser 5" high if you added a low profile focuser would you then need to add extension tubes to get the EP to the same focal plane? Or would you have to move the mirrors further apart to allow for the EP being closer to the secondary?
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To be honest, there's not much point in switching to a low profile focuser unless you are going to move the mirrors further apart.

Or if you have some gear that requires too much focuser in-travel with your current focuser, of course. That can be an eyepiece like a Speers-Walers or more commonly a photosensor (webcam, DSLR,...)

One point that people sometimes forget is that it's much easier to make a focuser baffle work well (or to make the focuser draw tube itself act as an efficient baffle) with a focal plane that is not too close to the scope axis, which makes a "high profile" focuser more practical (unless like me you have a low profile focuser but simply mount if further from the axis, but that's pretty much functionally equivalent).

A low profile focuser (mounted close to the scope axis) does allow you to have a larger fully illuminated field with a given secondary size (provided you also don't overdimension the tube or UTA diameter), but it's more difficult to ensure no stray light can reach the focal plane (or if you still baffle everything correctly, may mean you need a much larger light shield opposite the focuser, something that can become quite impractical).

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If the mirrors are set to work with a focuser 5" high if you added a low profile focuser would you then need to add extension tubes to get the EP to the same focal plane? Or would you have to move the mirrors further apart to allow for the EP being closer to the secondary?

Either solution would work, but the first one is a bit pointless (unless, of course, you have some stuff that only works without the extension tube, in which case the second solution won't work).

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The original position of the mirrors isn't an issue as I'm replacing the old rusty steel truss tubes with ali to adjust the balance of the scope, if I'm going to replace the focuser with a lower profile jobby, I'll just alter the new truss length. After looking at the specs of other R+P focusers the one I have does seem especially high.

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If you have a secondary already, try to compute what the fully illuminated field and the edge of field illumination will be for your largest field eyepiece using

Mel Bartels' Diagonal Calculator

Note: the radius for the objects at the edge of the field is, of course, half the field stop size of the eyepiece. And the tool works with off-axis distances, i.e. radii.

If you get a reasonably sized fully illuminated field (a couple of millimetres) and more than 70% at the edge of the widest field eyepiece, then there's no pressing reason to change focusers, unless you also want to use a smaller diagonal.

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The diagonal I have is 3" minor axis, 25% of the primary diameter. The scope was used for DSO imaging back in the 1970's so I suspect the oversize secondary was to ensure max light to the camera with slightly reduced contrast. I want the scope to be a better all rounder so I plan to get a 2.25 or 2.5" secondary. This puts the obstruction in the >20% area. Newt says this will still ensure 100% illumination with the existing focuser.

I think I also need to learn about some of the terminology used to describe this dark art.

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Newt's terminology about vignetting is very confusing. If I were you I'd just use Mel Bartels's diagonal calculator. I'd give it s spin (assuming a 14" tube, 2.25" secondary and 5"+1" from the tube to the focal plane unless you'd tell me otherwise), but I don't know the f/ratio of your scope.

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I've used:

12" mirror

f/5

12.5" from diagonal to focal plane (7" for tube, 5" for focuser, 0.5" from shoulder of focuser to focal plane -- that's pretty aggressive and will e.g. prevent the use of a Paracorr)

The smallest "sane" size is the 2.6 inch mirror, but that's really borderline, with a very small fully illuminated field. 3.1 inch is a lot more comfortable. Get a low profile focuser with 2" less height and then the 2.6" mirror becomes fairly comfortable, with the 2.14" mirror decidedly borderline.

If you have a truss Dob you can consider making the UTA ring smaller than 14" and mounting your focuser closer to the axis, but the 2.14" diagonal is still not on the safe side. If you are planning a tube scope I wouldn't go there, because a 13" tube for a 12" mirror gives you tube currents in the light path with an almost 100% guarantee.

In other words, if you want to keep your current secondary or go to a 3.1 inch then the current focuser is fine, but if you want to go to a 2.6 inch secondary then you'd better think hard about getting a lower profile focuser.

If you want to go lower, your best bet is an Orion Optics 63mm secondary (roughly 2.4"). But that will mean a lowish (but still normal) profile focuser and a UTA/tube size that isn't too liberally oversized, perhaps accepting some front vignetting for very wide fields (not a problem, as front vignetting is usually not that severe unless you take it too far, and it helps avoid the more severe vignetting by a small secondary).

If you want to go to a 2.14" secondary, then you need something really low profile like a Kineoptics HC-2, mounted so that the draw tube fully racked in just misses the light path of the least favourable object in your widest field eyepiece.

Edited by sixela
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Cheers mate, that helps a lot.

With the 3" secondary, doesn't that present a larger contrast reducing obstruction

Depends on what you mean by "contrast". It does make some low contrast planetary features a tad less visible (notably on Mars, Jupiter and the subtle colour and luminance variations in the Moon maria), but to be frank, even 25% isn't that large a central obstruction. It also does not affect small high contrast features that much.

See Thierry Legault - What are the effects of obstruction ? for a discussion.

You have to note that on larger scopes it's even less important for most mortals (who don't live in Florida or Barbados), as on a 12" you'll need to be working at around 400x before you'll really be able to see the difference oh so slightly. And for that to happen, you have other more important gremlins to fight first: collimation and cooling.

Also, it's possible to be penny wise but pound foolish: if you get a small sized secondary, it has to be an excellent one (Protostar, Antares Optics, NY --not Antares Optical-- or Orion Optics), because if you use a cheap one and its edge is so-so you're going to lose more than you gain by having a central obstruction that is 3% smaller : on a large secondary you won't be using the edge at high power, on a really small secondary you'll be using the edge even pretty close to the axis.

For deep-sky observation the central obstruction is almost completely irrelevant; it's much more important for your scope to be well baffled (and to have a Paracorr if you want to use the whole field and want pinpoint stars).

Personally? I'd get a 2.4" secondary and a lower profile focuser if you're going to be looking only at planets, a 2.6" secondary and a lower profile focuser if you are going to look at planets for a considerable fraction of the time, and a 3.1" secondary if it's mainly a deep sky scope with the odd planet thrown in (they are still going to be breathtakingly detailed if the seeing cooperates; 12" is a lot of aperture unless you're blessed with extremely good seeing all of the time!)

Edited by sixela
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