Is a laser collimator too accurate for a Heritage 130p?

[the lemming]

I have a Cheshire collimator for my heritage 130p telescope, which I think I am using as accurately as I can when fine tuning.

Today I borrowed a laser collimator from a friend to see how good they were and how out of sync my mirrors were.

When I used the laser tool I discovered that the primary mirror was off by a small amount. Also, the dot of the laser never once went into the very centre of the primary mirror, marked out with a small circle etched onto the glass. The dot skirted around the etched out marking but never hit dead centre.

I next discovered that every time I repositioned the laser collimator and the adjustment screws on the focusing mechanism then the laser dot would move about on the target display of the collimator. Even folding down and extending the telescope itself produced different results.

By the very nature of a small portable telescope that retracts and expands as part of the design feature, is a laser collimator too accurate for so many movable and flexible parts?

And am I asking too much for the laser dot to hit dead centre of the primary mirror when using the etched out marking on it?

Cheers

[Caelus]

Presuming the collimator is itself correctly collimated, I think the issue you're having may be with the design of your telescope.

I believe many people who have a "flextube" type scope leave them permanently assembled because each time you collapse and and extend it, things will never be aligned quite as they were before. There'll always be miniscule differences in the focal length and the relative position of the mirrors. If this isn't practical that it might just have to be something you live with

Of course, I might be talking out of my bottom, but I think the main thing is that the collimation is close enough that you get good views and you enjoy using your scope

[Steep]

It probably is a case of using a sledge hammer to crack a nut but Caelus is right too, my own laser collimator was out of whack when it arrived and had to be adjusted before I used it on the scope.

[Carbon Brush]

In a lot of scopes, gentle pressure on scope parts, especially the focus tube will move the spot.

The usual recommendation is to have the focus tube upright when collimating.

A laser check is very good for highlighting unwanted flexiblity in scopes.

Hope this helps.

[Oily]

The focuser on the 130P isn't the best,there's a bit of 'slack' in it,try winding some PTFE tape on it to take up some of this 'lack'.Plus the fact the laser collimator won't be 100% 'central' in the focuser every time you use it unless you get a self centering one.

[cosmojaydee]

The focuser on the 130P isn't the best,there's a bit of 'slack' in it,try winding some PTFE tape on it to take up some of this 'lack'.Plus the fact the laser collimator won't be 100% 'central' in the focuser every time you use it unless you get a self centering one.

I agree totaly

[AndyWB]

Presuming the collimator is itself correctly collimated, I think the issue you're having may be with the design of your telescope.

At the risk of asking a noob question, how would one collimate the collimator? Do you need some sort of laser to collimate the laser collimator? Oh, wait, now I'm really confused.

[Guest]

hammer 4 nails into a piece of wood so that you have two pairs of nails, each pair being like a cross. Now you can rest the laser collimator in the Xs and rotate it. The spot should remain in one position as the collimator is rolled in it's "cradle"

Hope that makes sense?

[AndyWB]

Thanks! That makes sense. It's always the simple things. It doesn't even matter then if the Xs are particularly square to anything - the dot should stay in the one spot. Neat.

[umadog]

By the very nature of a small portable telescope that retracts and expands as part of the design feature, is a laser collimator too accurate for so many movable and flexible parts?

A well-aligned laser is about as accurate as a good Cheshire/sight-tube combo tool. The first pass of the beam (equivalent to the cross hairs) is probably easier to work with than the cross-hairs. Equally, however, that alignment is the least critical.

The return beam of the laser is generally considered less trustworthy than a Cheshire (the "reflection" part of the combo tool) because it more prone to minor errors in the laser's collimation. In addition, this second alignment (i.e. that of the primary mirror) is more critical. It is for this reason that most people barlow the laser and use the shadow of the centre spot rather than the return beam. This is more accurate.

I next discovered that every time I repositioned the laser collimator and the adjustment screws on the focusing mechanism then the laser dot would move about on the target display of the collimator. Even folding down and extending the telescope itself produced different results.

Par for the course. That's why you have to tweak the collimation on each night.

And am I asking too much for the laser dot to hit dead centre of the primary mirror when using the etched out marking on it?

Cheers

No, it should do that.

[cs1cjc]

To work accurately, a laser collimator needs all telescope parts rigid and consistently aligned, beyond that necessary for good viewing. A passive collimator like your Cheshire would probably be better in this case.

[umadog]

To work accurately, a laser collimator needs all telescope parts rigid and consistently aligned, beyond that necessary for good viewing. A passive collimator like your Cheshire would probably be better in this case.

I don't understand, can you expand on that?

[cs1cjc]

Only the central part of the image of a Newtonian is sharp, and this is particularly the case with faster instruments. For example at F/4.5, the diffraction limited area at the focal plane is only 2mm in diameter, only about half the field of a 6mm Ortho. The object of collimation is to align the mirrors so this sharp area of the image is centred in the eyepiece. Any slackness or small tilt of the focuser is not significant to viewing, but will prevent satisfactory collimation using a (unbarlowed) laser.

[nicks90]

dont waste your money on gadgets.

i find a bright star and centre it in the fov - slap in my 4mm ep and unfocus it. If the doughnut of the secondary is more or less in the middle of the unfocused star - its good enough for visual! If its off centre, then a quick twiddle with the primary adjusters so its near-as-dammit , jobs a good'un.

I am sure its possible to get it more accurate with a collimated laser collimator ( the irony is not lost on me!) or using x300 mag for a proper star test....... but on a short focal length smallish aperture reflector for visual use, it aint worth the hassle and wont give you any real noticeable viewing differences (in my opinion)

nick

[umadog]

collimation is to align the mirrors so this sharp area of the image is centred in the eyepiece. Any slackness or small tilt of the focuser is not significant to viewing, but will prevent satisfactory collimation using a (unbarlowed) laser.

Sure, if the laser is loose in the focuser that would be a problem but it wouldn't it also be a problem for a sight-tube? The sight-tube basically works the same way as a laser. I don't think tilt of the focuser matters, though. The whole point of collimation is to get the image plane perpendicular to the focuser axis and also centred in the focuser. So when you adjust the mirrors you're basically "de-tilting" the focuser.

[umadog]

dont waste your money on gadgets.

i find a bright star and centre it in the fov - slap in my 4mm ep and unfocus it. If the doughnut of the secondary is more or less in the middle of the unfocused star - its good enough for visual! If its off centre, then a quick twiddle with the primary adjusters so its near-as-

A Cheshire/sight tube is not a waste of money and it needn't cost the Earth. The star test is fine for adjusting the primary (if you do it correctly). It won't help you with centering and rounding the secondary or adjusting the secondary tilt (IIRC). It's quicker and easier to use a combo tool and it's no less accurate, assuming the center spot is well placed and the tool is easy to read.

[Naemeth]

A Cheshire/sight tube is not a waste of money and it needn't cost the Earth. The star test is fine for adjusting the primary (if you do it correctly). It won't help you with centering and rounding the secondary or adjusting the secondary tilt (IIRC). It's quicker and easier to use a combo tool and it's no less accurate, assuming the center spot is well placed and the tool is easy to read.

It can also easily be done in daylight, even indoors. I use a Cheshire, and do get collimation pretty much spot on .