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secondary mirror woes

lhuchison

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lhuchison

lhuchison

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This is driving me crazy! just spent another 2 hours, or was it 3? trying to get the secondary mirror perfect alignment .

I can get the perfect position by holding the body of the mirror in place but dont have enough hands to tighten up the screws, soon as i move 1 screw , it all goes astray.

The primary mirror i found quite easy, ( hope i,m doing it right, ) Then i saw the Hotech laser tool and i thought thats my prayers answered! But now i read that its no good for the secondary mirror as theres too many variables and what can appear as perfectly centred could be well off.

Any help / advice would be much appreciated .

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JamesF

JamesF

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That sounds to me like you're not doing it the right way.

Screwing each of the three screws in or out moves the mirror in a different plane. If you have the laser in the focuser then you need to adjust the screws so the laser hits the centre spot on the primary.

I usually start by putting in the laser and waving my hand over the open end of the tube so I know where the laser is pointing and that I'm not going to stare straight into it when I look down the tube. I then look at the laser on the primary and see which direction it needs to move in to get it closer to the centre spot. Turn the screw on the opposite side of the secondary housing one way or the other until it moves inwards. If the screw doesn't turn easily, slacken the other two off to give more play. Once the reflection of the laser has moved as close as possible with that screw, check which direction it needs to move in next and repeat.

James

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JamesF

JamesF

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maybe i,m over fussy, but it reminds me of building a tower of playing cards, trying to add that extra card on top and it all comes tumbling down!

Will the Hotech laser be a good buy?

Ah, you don't have the laser yet?

James

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malc-c

malc-c

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There are loads of threads on colimating a reflector, and there are lots of arguments for and against using Hotech or other brands. For me I use one of Orioin's self centering adapters, so using this with the hotech gives me precise positioning of the laser. Others say that as you use the thumb screws then the hotech will be slightly off axis due to the self centering function. I can't say the difference would matter that much IMO.

One thing to watch is that you don't over tighten the screws on the back of the mirror.

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lhuchison

lhuchison

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i,m using a standard cheshire sight tube/ collimator. i think the sky watcher was way off , as stars started to appear defocused just not right, so i ,m attempting a complete re- collimation.

I,ve read all the posts and guides but i find the secondary so fiddly . i understand the concepts, but what a pain! i must be going wrong somewhere.

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malc-c

malc-c

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Leon, trust me, a full colimation can seem to take an age, especially after a complete strip down such as after flocking the inside of the tube. The combination of a cheshire and laser collimator is the ideal pairing for best results - juts take your time

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WaveSoarer

WaveSoarer

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I'm quite used to aligning other pieces of scientific kit that require the tweaking of screws with Allen keys. The trick is to always use tiny tweaks and to compensate the movements so that the screws are nearly always as tight as you need them. For the secondary tilt alignment I try to keep them all nearly fully tight and then, after deciding which way I need to adjust, tweak one slightly less tight and then compensate with one of the others to tighten things up again (as there are three screws tightening one of the others should make the two others, including the one you just loosened, equally tight). You shouldn't need to hold the mirror as there should always be some tension in the screws to hold the mirror in place provided you use tiny tweaks. The primary adjustment screws have springs to hold the tension and, consequently, the mirror cell doesn't need held. The secondary adjustment doesn't have springs and you need to retain a little tension in the screws so that it's held reasonably firmly but not too tight so that you can still adjust it. If you do tiny tweaks then the last adjustment should be sufficient to lock it in position. If you need to hold the secondary during the tilt alignment then this suggests to me that you are letting the screws get too loose. You don't make the screws too tight either and bear in mind that the secondary is quite light and shouldn't shift unless the telescope gets quite a knock.

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lhuchison

lhuchison

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Quick update, i still have to manually move the secondary in relation to the focuser, so its central . then i use my new hotech laser collimator to align the secondary to the primary and finally primary.

Using a star test, i found the alignment perfect! very happy with my new hotech laser.

I probably wont have to touch the secondary again,( hopefully )and just occasionally use the laser to align the primary.

Looking clear again tonight as im writing this , so off to the garden to try and find some DSO,s

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