Don't play with your secondary if you can help it - Forums

Topic starter

Sep 6, 2022 · Edited: Sep 7, 2022

I've learned from bitter experience that adjusting your secondary mirror on a Newtonian when you don't know what you are doing is a big mistake

To any newcomers, I would suggest not touching it for a couple of years if you can help it. It is very sensitive to adjustment and getting it wrong will cause all manner of issues.

It moves in so many degrees of freedom and as such you can lose control of it quite quickly.

I have learned from my mistakes with it and am now happy that I know how to control it, however it is not for the fainthearted.

[Update] I should have said don't tinker with the middle screw that ends up with you adjusting the angle of the secondary and the height up and down the tube; they should have been set quite well at the factory. The other three are fine to use as you can learn how to align the secondary to the primary etc..

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jewzaam

Perfect post

Sep 7, 2022

I got a 150PDS and made a point to adjust things, including secondary, before I ever got the scope out for first light. Seems to have turned out OK, but I also have no baseline to come from. So it's only up from here! On those lines though of "if it ain't broke.." I changed the bearings in my EQ6-R Pro back at the beginning of the year and have regretted it. Got it working well enough but it took some courage to finally take the hit last weekend and crack it open and put old RA bearings back in. I hope it fixes a pretty bad PE issue that my fiddling introduced. But I've learned a lot in that process. First of which is don't adjust all the things at the same time!

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Ian McIntyre

Sep 7, 2022

Andy Wray:
I've learned from bitter experience that adjusting your secondary mirror on a Newtonian when you don't know what you are doing is a big mistake

To any newcomers, I would suggest not touching it for a couple of years if you can help it. It is very sensitive to adjustment and getting it wrong will cause all manner of issues.

It moves in so many degrees of freedom and as such you can lose control of it quite quickly.

I have learned from my mistakes with it and am now happy that I know how to control it, however it is not for the fainthearted.

I need to add a dew heater to the secondary of my RC real soon and I grit my teeth just thinking about it. Also needs a cleaning. Not sure how different a Newtonian is.

Matthew Proulx

Sep 7, 2022 · Edited: Sep 7, 2022

Ian McIntyre:
Andy Wray:
I've learned from bitter experience that adjusting your secondary mirror on a Newtonian when you don't know what you are doing is a big mistake

To any newcomers, I would suggest not touching it for a couple of years if you can help it. It is very sensitive to adjustment and getting it wrong will cause all manner of issues.

It moves in so many degrees of freedom and as such you can lose control of it quite quickly.

I have learned from my mistakes with it and am now happy that I know how to control it, however it is not for the fainthearted.

I need to add a dew heater to the secondary of my RC real soon and I grit my teeth just thinking about it. Also needs a cleaning. Not sure how different a Newtonian is.

Very easy. Nothing to it. They simply glue on without disassembly. The difference between RC and newt is you shouldn't touch a Newts secondary after you adjusted the primary, with an RC you should. I find them very easy to collimate, I currently run 2 RC8.

Newtonians are also easy to collimate provided you know the process.
Center secondary under focuser > Align secondary to primary center dot > adjust primary so it reflects back into eyepiece. Only ever adjust primary on star test. Any other problems after this is likely focuser tilt or mirror shift.

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Tim Hawkes

Sep 7, 2022

Not sure about that Andy. I would say better to develop confidence by learning to take it all apart and put back together again. The secondary can move a bit and it is good to monitor if that is happening to check laser spot still centred on primary mirror as the scope moves around. Usually only very subtle adjustment needed - but good I would say to get confident about being able to do it? Tim

Michele Campini

Sep 7, 2022

If u use an OCAL for the collimation u can play with secondary one time for week. In 5 minuter u perfect collimate it without any problem if it's a F5. If it's a F4 10 minutes (included primary).

Fabiano R. Maioli

Sep 7, 2022

Well, I would say that in life, if you don't know what you are doing, think twice if you really want to do it.
Empty advices apart, this is why I prefer to use a cheshire collimator instead of a laser collimator. It can give you much more informations about the position of the secondary. And yes, the central screw usually does not require often adjustments, almost never even…

Good luck!
Fabiano.

Jacob Heppell

Sep 10, 2022

With a concenter (this one especially), cheshire and autocollimator, the job of correctly positioning a secondary becomes just about trivial. To make it a little easier, it's best if you put a metal washer between the collimation screws and the central stub. Makes rotating the secondary so much easier as you don't have to completely loosen the collimation screws first.
For the fast newt people (F5 and lower), always make sure your secondary is fully offset. It's the only way to center the secondary with the focuser without introducing heaps of cone error.

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Ferenc Szabo

Sep 12, 2022

I had to play with my secondary, right after I received my telescope (this was last year) . Took some flats and immediately noticed that the vignette I had was larger on one side than the other. Looking into the telescope with Cheshire, I noticed right away that the secondary wasn't centered below the focuser.

andrea tasselli

Frequent contributor

Sep 12, 2022

There are 3 possible geometrical configurations for the 2ndary mirror placements, namely;

1. Geometrically centred in the telescope main tube thus having its centre laying on the optical axis of the telescope (but skewed when looked at form the focuser)
2. Offset away from the optical axis of the main mirror (and thus away from the focuser/tune open end and towards the main mirror) but would look centre in the focuser from the sight tube
3. Geometrically centred but with all the optical components at more 90 degrees from each other where relevant. It would look centred but not perfectly circular (although you would be hard pressed to note the difference)

All of them would produce corrected, on axis performance but would look different form the POV of a sight tube/Cheshire and would also yield different illumination of the focal plane.

Andy Wray

Topic starter

Sep 12, 2022

andrea tasselli:
All of them would produce corrected, on axis performance but would look different form the POV of a sight tube/Cheshire and would also yield different illumination of the focal plane.

OK, having hopefully now aligned the secondary (an offset variety I believe) under the focusser and done basic collimation I ended up with a field illumination as per below. So, just getting the secondary to look circular and bang under the focuser yielded what I think is a pretty even illumination field. N.B. I obviously stretched it greatly to exaggerate the vignetting which was a fall-off of about 20% in reality.

Ferenc Szabo

Sep 12, 2022

Also how close are you to star focus when you took the flat? It's important, because the vignette may not appear the same way . I had very good looking flats with the focuser turned farther in, and the tilted secondary (or tilted focuser) only started to show itself when I focused on a star and then tried to make flats.

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andrea tasselli

Frequent contributor

Sep 12, 2022 · Edited: Sep 12, 2022

Also how close are you to star focus when you took the flat? It's important, because the vignette may not appear the same way . I had very good looking flats with the focuser turned farther in, and the tilted secondary (or tilted focuser) only started to show itself when I focused on a star and then tried to make flats.

That's very true. I collimate with the collimation tool at around 55mm offset from the flange, which I never touch (the focuser position). Doesn't need to be more than within 1 mm for collimation purposes. As for the illumination it is what it is and don't fret over it.

Alan Brunelle

Sep 12, 2022

I think if you are going to use a Newtonian, you need to learn how to collimate from front to back. It's not that hard. To expect to rely on the factory set is already on the path to a low probability of success. Often a newly shipped Newt will have lost optimal collimation. You need to know enough about collimation to see that. Second, it's a bit much to ask a Newt to stay in collimation indefinitely.

To expect to simply send it back for colimation from time to time strikes me as someone who has too many $ to spend and only cares about the end result. There are other telescope designs that would be a better fit.

I see the ability of swapping out my secondary as an advantage. I have a large one for wide field work, and a small one to do planetary or very narrow field. I even have a third because TS sent me the wrong one with the OTA. They are easy to swap. Easy to collimate. Practice, gain competence in collimation and stop worrying.

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Andy Wray

Topic starter

Sep 12, 2022

Alan Brunelle:
To expect to simply send it back for colimation from time to time strikes me as someone who has too many $ to spend and only cares about the end result.

I'm not sure where this comment came from. I'm someone who is trying to make the most of a $400 scope and has never sent it anywhere for collimation. I'm definitely one of the most frugal people on this web site (Total spend to date around $4000 for all my astronomy gear).

I must admit: My initial post was misleading (not deliberately).

I must also admit that following very helpful feeback on here I am much more comfortable with aligning the secondary to the focusser and would also urge people to learn how to do it. Moving from the grub screws in my cheap Newt to Bob's Knobs has also made life a hell of a lot easier for secondary to primary alignment.

I would still advise newcomers to the hobby: Once you have your focusser aligned to your secondary and got the secondary rotation right, then leave it alone in general. Check it by all means, but it shouldn't need adjusting that often.

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Alan Brunelle

Sep 12, 2022 · Edited: Sep 12, 2022

Sorry if I came across too strong. Certainly throughout this thread you evolved your position. My comment was meant to be a generalized response.

You have to admit that your original post was a bit of a bait. Suggesting as a broad rule (even the title!) that newcomers to newts not touch their secondaries for a couple years… Certainly this is no useful advice for a newcomer if their new scope arrives out of collimation. Of all scopes to make such a suggestion, the Newtonian is the one scope where an owner almost certainly will be required to collimate themselves not long after acquisition. Better to state to new owners or ones who might want to get a Newt that it is the one type of scope most likely to need collimation on at least a periodic basis and if you get one, learn how to do it as soon as possible so that you don't put off doing it when it is required.

In my opinion, there is no adjustment screw that is scarier to turn than another on a Newt. They all have their functions. The secondary center screw moves the secondary fore and aft along the optical axis. It can also be used to tension (tighten) against the three tilting set screws. If you need to translate the mirror along the axis, I.e. fore or aft to properly align with the focuser, you will have no choice but to use it. I admit that loosening this screw can scare a new user if at first it causes the secondary to suddenly flop and even dangle from the spider, but once that lesson is learned, it's either avoidable or not an issue.

Bottom line, if your choice of scope is a Newt, learn to collimate. It's not hard to do. It is also a great way for any telescope user to learn a bit about optics. They are also wonderful regarding their being amenable to modification.

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John Hayes

Frequent contributor

Exemplary contributor

Sep 13, 2022

Alan Brunelle:
Second, it's a bit much to ask a Newt to stay in collimation indefinitely.

Alan,
Why is it unreasonable to expect a Newtonian to remain in alignment indefinitely? Perhaps there are so many poorly designed telescopes on the market that it has normalized the notion that a telescope can't be designed to hold alignment. In fact, a Newtonian is far less sensitive to alignment errors than almost every other two-mirror telescope. I've run Cassegrain-type telescopes for years on end without ever having to touch the alignment. As I recall, one of my scopes was last aligned in 2016 and it has since been on airplanes, transported in cars over dirt roads, used continuously through both very cold winters and very hot summers and it shows no sign of any optical misalignment. The alignment stability comes from the ultra-stable, pseudo-kinematic mirror mounts that I designed and retrofitted. If the mechanics are good, there is no reason that a well designed Newtonian should go out of alignment--short of dropping it.

John

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Alan Brunelle

Sep 13, 2022 · Edited: Sep 13, 2022

John,

I think for the discussion here, we are talking about the true Newtonian reflectors, as generally available through vendors or home built (as many are). With secondary spider, set screws, primary mirror cells, which hold the optic rather loosely, etc. Also for 99% of owners, not mounted on a permanent pier.

Not, Cassegrains, Mak newts, Edges, etc.

Most Newtonian visual users, who travel to sites, or even who set up at home will at least check alignment prior to use. Takes 2 minutes. So why not? And once that activity is initiated, the picky among us will make a quick tweak here or there. Especially after a quick star test.

Reasons why these scopes don't stay in alignment? You may be right regarding poor mechanics. But unlike the fixed cells of refractors, or Edges, etc, the multiple points of failure probably make newts more prone to misalignment for various reasons. Heck just thermal expansion and contraction of spiders, mounting screws, primary cells, and the various set screws and the focuser is likely to cause loosening (as you have suggested in your post on mechanics). That doesn't even make mention of how all this is affected by moving and transport issues for most.

But for these design problems, solutions may technically exist, but at what price point? Do not forget that the benefits of newts come at a remarkably inexpensive price point. If one wants to spend a fortune on a Newt that has solved all collimation issues, then I am sure it can be had. Or one can save the small fortune and get as good a result by spending the 5 minutes doing a collimation check/tweak just prior to use.

Your solutions to these problems could be extremely valuable. If implemented commercially, how much would you estimate the added cost to a scope would be? Would you make these designs available to the Newtonian user community? After all, these users are among the most resourceful when it comes to home built telescopes. But many might find these solutions useful for retrofits as well.

What I find amazing is the quality of images posted here from users of simple Newtonians from the typical vendors. Given the price points of these they can easily match the imaging performance (and also exceed) of much more expensive rigs when set up correctly. However, I cannot argue that they are relatively uncommon here on AstroBin. I wonder why ;)

My reason for setting up my Newtonian, however, is only because I wanted a purely reflective optic for doing photometry in the vis/NIR part of the spectrum. But I may take some aesthetic images as well.

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andrea tasselli

Frequent contributor

Sep 13, 2022

Frankly, the current crop of spiders and mirror cells from most reputable manufacturers/sellers are perfectly capable of holding collimation well for extended period of time mounted and in use. The main discriminant is relative speed of the primary. My f/6.3 stayed in collimation for years on a stretch despite being mounted and unmounted and occasionally dropped (!) but it's a 10" and made in astrositall. The 12" f/4 is another beast altogether, both thicker (and larger) but crucially also made in Pyrex which makes it more prone to issue related to differential thermal expansion. It needs more careful collimation and more recurrent checks but having used for extended period of time I I can say that the mirror cell is doing the job decently well but could be made better in terms of tolerances (and probably are, just not this one). Design-wise all reasonable cell configurations have been tried and tested by the AA community in the past 40-odd years and the poor ones been weeded out so it is not an issue of how (to make a good one) but how much (I'm willing to spend to get a good one with the right tolerances).

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Andy Wray

Topic starter

Sep 13, 2022

Also how close are you to star focus when you took the flat? It's important, because the vignette may not appear the same way . I had very good looking flats with the focuser turned farther in, and the tilted secondary (or tilted focuser) only started to show itself when I focused on a star and then tried to make flats.

I always take flats at the focus point; as you say it is important as they won't be correct otherwise. Also, my TS Concenter Eyepiece arrived today and it made centering and squaring the secondary to the focuser an absolute doddle. Having also added Bob's Knobs recently I could actually make the adjustments whilst looking through the eyepiece at the same time

Matthew Proulx

Sep 13, 2022

Alan Brunelle:
John,

I think for the discussion here, we are talking about the true Newtonian reflectors, as generally available through vendors or home built (as many are). With secondary spider, set screws, primary mirror cells, which hold the optic rather loosely, etc. Also for 99% of owners, not mounted on a permanent pier.

Not, Cassegrains, Mak newts, Edges, etc.

Most Newtonian visual users, who travel to sites, or even who set up at home will at least check alignment prior to use. Takes 2 minutes. So why not? And once that activity is initiated, the picky among us will make a quick tweak here or there. Especially after a quick star test.

Reasons why these scopes don't stay in alignment? You may be right regarding poor mechanics. But unlike the fixed cells of refractors, or Edges, etc, the multiple points of failure probably make newts more prone to misalignment for various reasons. Heck just thermal expansion and contraction of spiders, mounting screws, primary cells, and the various set screws and the focuser is likely to cause loosening (as you have suggested in your post on mechanics). That doesn't even make mention of how all this is affected by moving and transport issues for most.

But for these design problems, solutions may technically exist, but at what price point? Do not forget that the benefits of newts come at a remarkably inexpensive price point. If one wants to spend a fortune on a Newt that has solved all collimation issues, then I am sure it can be had. Or one can save the small fortune and get as good a result by spending the 5 minutes doing a collimation check/tweak just prior to use.

Your solutions to these problems could be extremely valuable. If implemented commercially, how much would you estimate the added cost to a scope would be? Would you make these designs available to the Newtonian user community? After all, these users are among the most resourceful when it comes to home built telescopes. But many might find these solutions useful for retrofits as well.

What I find amazing is the quality of images posted here from users of simple Newtonians from the typical vendors. Given the price points of these they can easily match the imaging performance (and also exceed) of much more expensive rigs when set up correctly. However, I cannot argue that they are relatively uncommon here on AstroBin. I wonder why ;)

My reason for setting up my Newtonian, however, is only because I wanted a purely reflective optic for doing photometry in the vis/NIR part of the spectrum. But I may take some aesthetic images as well.

The thermal expansion here is negligible. If you do the math, it’s microns. Far more likely to cause the derotation of threaded hardware is uneven tensioning. 3 set screws shouldn’t come loose if one knows how to torque them properly. Nor should one expand at a different rate than another enough to cause a measurable error. Especially in this case lol you are not rotating at high speed like many applications in the world that still use the 3 screw alignment. Likely the alignment of the attachments is the error which caused collimation to be inaccurate in the first place. There’s no reason even the cheapest scope should lose alignment simply being. If you were ever concerned about threads loosening, applying a low strength thread locker to the screws would suffice. There’s no reason to accept something as inevitable.

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Alan Brunelle

Sep 13, 2022 · Edited: Sep 13, 2022

Matthew Proulx:
The thermal expansion here is negligible. If

I wish I could agree. Let me repeat, if your rig is nicely nestled on a permanent mount in a shelter, sure. No problem. Such situations are a small fraction of amateur astronomers.

My OTA is carbon fiber. Known for low expansion. (Let me remind those who go that direction that this does not mean "no expansion".) On my non-permanent mount, one evening the OTA caught the sun for a short while. It was amazing how far out of collimation the scope went, clearly bending by heated expansion of one side of the OTA. Sure, the effect mostly reverses upon cooling, but what happened to the differential forces on the spider attachment points? Did the OTA, just slide past the tube ring felt on one side? I have large dove tails on top and bottom tube rings, so I'm sure they did not flex. Many leave their scopes out under a tarp. On a sunny day, just putting a hand under even a reflective tarp will show one just how hot and uneven the scope will get. My counter weights will remain hot for hours after the sun sets. So yes, I check my collimation and not uncommonly make adjustments. Easy stuff!

To the original point of the post, can it be true that those who buy a Newtonian actually expect it to arrive perfectly collimated and not have to collimated it for years? And expect to not have to learn how to do so? The fact that most all such scopes come with easily accessible adjustment knobs and there is a highly competitive industry based on collimation devices, etc. would suggest I'm not alone in my experience.

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Clint Lemasters

Sep 13, 2022

Rule 1: Topic titles like this should be avoided.

Rule 2: Learn how to use your equipment. Find a good thread on Cloudy Nights or look at Catseye website for how to collimate.

Finally, if you're not comfortable collimating a Newtonian you shouldn't be doing astrophotography with one. Astrophotography builds upon fundamental skills of telescope and mount use.

In the Coast Guard, we get comfortable hovering our helicopters before we try to hoist someone off a sinking boat. Technique only…

andrea tasselli

Frequent contributor

Sep 13, 2022

Newtonian primary cells, by and large, are adjusted by a system of 3 push-pull spring loaded threaded rods. In addition to these most of manufacturers have a set of 3 locking screws to try and prevent the movement under different loading conditions. In addition we have 3 lateral holders (for smaller, GEM mounted telescopes) with a nylon screw each to keep the mirror centred (for larger, thinner primaries different lateral support arrangement exist, such a sling type, but let's ignore those).

The typical aluminum alloys have 22 microns per meter per degrees centrigade. Typical steel 14 micron/m/degC. Float glass ~6 microns/m/degC, Pyrex ~3 microns/m/degC, nylon ~70 microns/m/degC.

There is ample scope for 20 degrees variation in temperature to affect the balances of forces and relative stiffnesses of all the various components to reach and surpass the tolerances for tilting for fast systems, such as a f/4 scope, which is 0.03 mm.

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Andy Wray

Topic starter

Sep 13, 2022 · Edited: Sep 13, 2022

Rule 1: Topic titles like this should be avoided.

Rule 2: Learn how to use your equipment. Find a good thread on Cloudy Nights or look at Catseye website for how to collimate.

Finally, if you're not comfortable collimating a Newtonian you shouldn't be doing astrophotography with one. Astrophotography builds upon fundamental skills of telescope and mount use.

In the Coast Guard, we get comfortable hovering our helicopters before we try to hoist someone off a sinking boat. Technique only...

OK, I admit the title was wrong and was probably driven by my frustration at my problems with my secondary alignment.

I apologise profusely to all on here!

Since I did the original (bad) post I have learned a lot thanks to the feedback I received on here. I now know (thanks to all that constructive feedback) how to align my secondary under the focusser in just a few minutes. It's also taught me the right order to do collimation and what tools to use. It has also taught me that having the optical path aligned to the coma corrector is critical and as such how well aligned and attached the coma corrector is to the focus tube is critical.

To anyone who is interested: I am now using/doing: TS Concenter Eyepiece to do the secondary positioning with respect to the focusser, laser to center the secondary to the primary, laser to align the primary and then fine tuning using a cheshire. I know I could use a barlowed laser for more accuracy and maybe an autocollimator.

Thank you SeaLint for your supportive comments though.

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