Posts Made By: Alexis Cousein

Is there anything technically wrong with this approach?

There's nothing wrong with it, but it isn't as insensitive to errors in pointing the reflected focuser axis towards the centre of the primary as the more cumbersome approach.

For best insensitivity to focuser axis errors, you want the virtual point source made by the laser plus barlow to be at the other side of the focal plane to the reading screen (and to have both at the same distance to the focal plane).

The rear view barlowed laser is, in this regard, probably unbalanced, though racking the focuser in completely will probably reduce the sensitivity to focuser axis errors.

If the laser itself is well collimated, though, that shouldn't be a problem, given that the focuser axis should be well set after having used the unbarlowed laser to set it.

5. In order to be properly collimated, first both mirrors must be collimated by laser, then they must be star-collimated.

Wrong. First of all, there are accurate tools that don't have a laser (and actually, many cheap laser collimators arrive miscollimated).

Secondly, some protocols advocated by the instruction manuals (using the return beam of an unbarlowed laser for setting tilt on the primary) yield results that aren't as good as those you obtain with a passive and cheaper tool (or a barlowed laser and screen).

Thirdly, tool collimation often reveals errors well below the tolerances needed to see coma in the star test, especially if you use a good autocollimator, but even if you use a very good Cheshire.

Finally, star testing basically only does what a Cheshire or barlowed laser does - make sure the focuser axis and optical axis intersect at the focal plane. It can't help you place your secondary optimally and it won't ensure your focal plane isn't tilted. So it's more of a sanity check of your Cheshire or barlowed laser routine than anything else (in particular, a sanity check for the placement of the centre spot on your primary).

6. Star-collimation: Focus a star with your highest magnification eyepiece. Next, gradually take the star out of focus. The star's image should then display Airy rings, which should be concentric. If they are not, then you need to collimate your mirrors until they do.

Someone else has already pointed out the difference between a Fresnel pattern and an Airy disc (what you probalby don't realise yet is that you hav eto work at very high powers and only defocus very little).

Also, in this step you don't "collimate the mirrors", but just set the tilt of the primary, because of what I said above. You can only tune one parameter with this, and you usually pick the most convenient one (and the one that will only move the optical axis).

Paulo martin said:

Hello,

Which is best for a newton telescope with small focal lenght(F/6)? Is the Powermate an instrument very similar at barlow lens?

Yes.

Advantages of the Powermate:
-no vignetting at all of some (rare) eyepiece types, like eyepieces with a field stop close to the limit of the barrel or TV Plössls. Mind you, the effect is usually very mild even with a regular barlow.
-keeps eye relief constant - a barlow will increase the eye relief of long focal length eyepieces considerably, rendering them uncomfortable.
-throws the new focal plane out just enough (as much as the height of the part sticking out of the focuser) without requiring a large insertion depth, like e.g. the excellent TV 2x barlow. That means you rarely require extra focuser in- or out-travel; if things come to focus without the Powermate, it will with the Powermate.
-on some models, the magnification does not depend critically on the placement with respect to the focal plane (and thus indirectly on where the focal plane of your eyepieces lies with respect to the end of the eyepiece barrel). That's usually a more important consideration for astrophotography, though.

If none of this matters, then a regular barlow, something like a TV 2x barlow, is usualy more interesting (all these things don't come free: there are more elements and the powermate is more expensive).

But often, one of these things does matter, in particular if you are planning to barlow longer focal length eyepieces (over 20mm) or can't stick the longer TV barlow in the focuser in some conditions.

Lee Spain said:

I've heard that a long focal length helps an achromat reduce false color issues. For lunar and planetary viewing, is there a point at which a long focal length would allow the performance of a regular achromat telescope to meet or exceed the performance of a shorter focal length APO or semi-APO?

Yes. Where that lies depends on the quality of the APO, obviously, and what spectrum of light you're looking at. The trouble is that the necessary f/ratio increases with aperture, and much more drastically than you'd think, making very large achromats with negligible colour error really very impractical.

Less coma, yes. But if it's a long f/ratio scope it's tolerant to collimation errors anyway (and the difference between a paraboloidal and spherical and if it's a short f/ratio scope it's got so much spherical aberration the coma and collimation are the least of your worries.

And once you introduce correctors answers vary.

Talking about on-axis designs, if you have a spherical mirror in a Schmidt-Newt, collimation is less critical but you still want to collimate well. If you've just designed a Lurie-Houghton, then you have less issues. Grab a copy of Rutten and van Venrooij's "Telescope Optics" and start reading.

If you have an off-axis design with a spherical mirror, everything hinges on the corrector and/or mirrors you design behind the main mirror, obviously.

I used it to do my collimation however, my eye pieces do NOT fit snugly, and require a setscrew to hold them in which of course "presses" the eyepiece off center against one side of the focuser.

The diffraction-limited field of your scope may be small, it's not that small.

I know that if I use a laser (tightened in place with the set screw)the laser beam is definitely off to one side of center and if I "center" that, then the collimation cap is shows the collimation to be off to one side.

You're not using the tools for which they're good. I'll assume, for the sake of the argument, that the laser collimator is itself collimated (if you rotate it in the focuser and retighten it, does the laser spot on the primary move?).

The laser collimator is good for pointing the reflected focuser axis at the primary's centre (by changing the tilt of the secondary).

The collimation cap (placed close to the focal plane) is good for making the reflected optical axis cross the focuser axis at the focal plane (by setting the tilt of the primary).

Given that they essentially serve two different purposes, they can't "disagree".

By the way, the return beam of a laser can be used to set the tilt of the primary *roughly*, but it's much less precise than the mundane collimation cap for doing so, because the return beam is only precise if the forward beam hits the primary's centre perfectly -- much more perfectly than is necessary for pointing the reflected focuser axis accurately enough.

To answer you last question: well, if the focuser draw tube is wide enough to make eyepieces "rattle", there is not really such a thing as a stable focuser or eyepiece axis, so part of the collimation becomes a fuzzy concept at best (even though the most important thing to do, i.e. make the optical and focuser axes cross at the focal plane, is still achievable for a given position of the focuser, pointing the focuser axis to the primary with a laser collimator becomes well nigh impossible). The end result can be a tilted focal plane (troublesome at low powers) or collimation that can only be good for one focuser position (if the eyepiece axis isn't parallel to the focuser travel axis, different eyepieces "want" a different tilt of the primary depending on where their focal plane lies in the eyepiece barrel).

Frequently, the fault lies not in the focuser but in the poor quality of the 2"-1.25" adapter (assuming you have 1.25" tools and are talking about 1.25" eyepieces); getting a better one may already help.

Finally this: collimation tolerances on an f/6 scope are rather generous, so do the best you can and enjoy the view while you're figuring out a way to chase the mechanical gremlins.

Too many unstated variables. If you're in Barbados so that the SCT is in thermal equilibrium and you have good seeing and the SCT is well collimated, it will spank the 6" Newt, assuming equal optical quality (but that's a big assumption!); more so on high contrast detail and less so on (larger) low contrast detail because of the larger central obstruction.

However, in some climates it's really hard to make an SCT perform well unless you have something like a Lymax cooler. A 6" Newt cools down easily, especially if it has a fan.

First of all, there's no need to SHOUT. The CAPS LOCK key isn't for writing plain text.

Secondly, be patient. An APO will not turn these dim smudges into anything but even dimmer smudges - work on your averted vision, dark adaptation (shield yourself from stray light, even in light polluted environments) and take your scope to a dark site if you can.

If you have lots of light pollution and want to look at emission nebulae, get an OIII filter and observe close to zenith.

As far as a wavy Saturn is concerned, wait until it's high in the sky. Work on your collimation skills. Make sure the mirror is cooled, and possibly equip your Dob with a fan.

Analyse the problem. Then only solve it. Throwing hardware at it blindly won't fix anything.

There are many reasons for a 6" APO, but you haven't convinced me yet any of them apply to you. And one of those doesn't tend to be easier to set up than a 10" Dob.

Robert Howe said:

Setting up an observatory, I have been comparing a lot of telescopes and EPs. A question arises. For a 1200-1800 FL telescope, I find that I am using all 2 inch EPs--the new 2/1.25 inch Ethos (13mm),

Uhm -- that's a 1.25" eyepiece. If a skirt is all you need to call an eyepiece a 2" eyepiece, then you can simply buy dedicated 2"-1.25" adapters and call everything a 2" eyepiece...

David Martin said:

I'm building a new telescope and don't yet have a Paracorr. But since it's a 12/5"f/5, I want to be sure that I'm buidling it to be "Paracorr-ready" in case I feel the need for one.

The shoulder of the Paracorr's 2" barrel wants to be roughly 18-20mm below the focal plane (in other words, if you design the focal plane to be 1" above the focuser draw tube's shoulder when it's fully racked in, you have roughly 5-7mm of focuser in-travel left once you've focused).

But you usually need very little travel if you are using a Paracorr, because differences in eyepiece focal plane are handled via the tunable top (when changing eyepieces, you use the tunable top for rough focusing and then only your focuser for fine focusing, if you want the tunable top to remain at its optimal position for all eyepieces).

If you've designed the focal plane to hover only 0.25" or 0.5" over the focuser draw tube when it's fully racked in, tough luck, unless you can compensate that by moving the mirror on its collimation bolts (this reduces the fully illuminated field size, but the Paracorr makes every eyepiece use a field with a diameter that's 15% smaller because of its barlowing effect, so that's usually a wash).

And yes, the Paracorr is long; a reason to make the focal plane lie a bit higher than 1" over the fully racked in position if the focuser is a very low profile focuser, but even if it does clip off-axis light bundles, vignetting by the Paracorr is usually very mild for scopes with some aperture.