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Reflector headache


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Hey all,

I bought a cheshire collimator to try and solve my failure in getting my secondary mirror aligned. No matter what I do I cannot get it to appear as a circle when I look down the EP holder.

The adjuster screws do not help, as getting near a circle involves using the three screws to a point where one is doing nothing and the other two are making the mirror at a crazy angle.

I can supply pics if anyone wants to help, or if anyone in Cambs would like to help me... I am about two headaches away from buying a refractor.

Adam

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The scope if f/5 so may not mean everything looks central. Wasn't there a thread on this a short while back. At low f numbers the secondary appears to be off centre and if I recall has to be off centre. A consequence of the short f number. The small difference between the top and bottom of the secondary from the main mirror is relevant when it comes to small f number scopes.

In effect it seems that you ignore what it looks like and simply go with what the tool says or indicates.

Think about the actual light path and it starts to make sense.

May be imposing on her but Astro-baby seems to be the one to ask maybe a PM. Have you downloaded her guides?

No clubs around you that may help?

Isn't there one at St Neots that advertises viewing and help nights? St Noets is in Cambs and may be close enough.

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it sounds like you are trying to make the secondary look circular with the three adjusters. that's impossible. you need to (carefully) loosen the central bolt a very little (perhaps by loosening the three secondary adjusters a tad) and then turn the secondary mirror with your hand (don't touch the glass). this will make it either round (correct) or oval (not). once round 'tighten' (never really tight, just a little tight) the three secondary adjusters.

you then adjust the tilt of the secondary with the three bolts until the primary and all clips are central and visible.

once you get a round secondary, you should be fine, especially with AB's excellent guide. pay attention to keep the scope at an angle not upright and always be extra careful with tools etc. it would not be funny dropping something.

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After reading this I though I would try Astro baby's tutorial as I haven't collimated my s/hand scope since I got it 3 weeks ago. Firstly, looking through the cheshire the primary was ever so slightly off.

So, on with the tutorial and what an excellent piece of work that is too, hats off to you :)

Checking the secondary mirror legs were not quite centred in the tube so with those adjusted & equal onto the next steps. I found I did not have to make any other adjustments on the secondary ;)

Onto the primary and with the cheshire it was off by one "blob" exactly inline with an adjuster, a little tweak & tighten of the lockscrews & hey presto a perectly collimated f5 scope - now for clear skies to star test it.

I think the questions here sound like you're confusing the round mirror image with the eliptical mount image - as previously quoted follow the tutorial and study the pics as I thought the same as you that it wasn't circular when infact the secondary mirror was!

Chris

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I use the baader laser Colli Mark 3 and it was very easy to collimate but its more expensive. Collimation - Baader Laser Collimator

Ken

You would still be best to go through all of Astro_baby's checks. I have a laser collimator and you can easily get the laser to return to the collimator and your secondary still be slightly off. You will not notice a great degration of the image quality but you would not be getting the best from your scope.

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Wow thanks for the help everyone. As mentioned I have been following Astrobabys guide religiously and any problems I was having I knew were down to my lack of experience.

But one thing has really been clarified for me. The secondary mirror DOESNT need to appear as a circle in the cheshire?? Just for the clips of the primary to be visible (which they also aren't for me).

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The secondary DOES need to appear circular in the cheshire. If it isn't then it's off at an angle:

Telescope Reviews: Useful info about secondary mirror alignment

Telescope Reviews: Is it colimated?

Telescope Reviews: Is it collimated II ?

I am now confused from the two different opinions...? Presumably I have to join the forum in order to view the pictures they mention? Thanks for the links.

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I am not aware of there being two different opinions. The secondary mirror of a Newtonian reflector should appear circular as viewed from the focuser. Period. In the excellent AstroBaby guide it says:

"The first object of secondary mirror alignment is to get the secondary mirror directly under the focus tube and appearing as a circle. The secondary mirror is an elliptical shape but when presented to the focuser at the correct angle will appear as a perfect circle."

You should definitely go to the threads I mention on CN and read them. I thought the images were visible to non-members, but maybe I'm wrong. Join if needed. Jason on CN posts regularly on this topic. He's seriously smart and understands collimation incredibly well. This thread is also useful:

Telescope Reviews: Concise thread about autocollimators+improvements

It mainly discusses the use of the autocollimator, but is useful for understanding axial alignment in general. Furthermore, there's a useful table down at the bottom which tells you which tools measure which errors. It helps to clear up some misconceptions about the use of lasers. Certainly you can quickly achieve very accurate collimation without a laser. Personally speaking, I can now achieve accurate collimation with my Catseye cheshire and autocollimator in well under 5 minutes. The autocollimator provides a wealth of information (more than I need, to be honest) and works well in the dark with a clip-on LED attached to the spider.

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Shane is correct, loosen the centre screw and move your secondary around until it appears circular then tighten centre screw. Then adjust the three scres to tilt until the mirror clips are visible.

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Okay but I cannot even get my secondary mirror to appear within the circle of my focuser or the cheshire. I screw the centre screw in as close to the spider as possible, and it still doesn't get to be near the middle of the view. That is before I even try the tiny adjustments with the adjusting screws.

EDIT: Oh I have a feeling it might be something wrong with the vanes?? If I had put them back onto the central holder all the wrong way round, would this allow not enough sufficient backwards travel for the mirror?

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not sure what's going wrong but it might be helpful to stick 2 sheets of white paper down the tube, one below and one behind the mirror - it makes it a lot easier to look at just the secondary as you don't see all the other reflections. Don't lose heart - I am sure you will work it out:icon_salut:

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ADZ "Oh I have a feeling it might be something wrong with the vanes?? If I had put them back onto the central holder all the wrong way round..." you took the vanes out ??? Anyway, stay calm, as you have figured out, collimating a scope involves juggling a number of variables and it is important to nail down each one before moving to the next. Astro Baby's guide is excellent (thank you AB) also check out FAQ about Collimating a Newtonian telescope another good explanation of the process (from about halfway down the page). So, check those vanes, check they are equal length and then proceed step by step from there. And don't worry, there seem to be weeks of cloudy nights in front of us, so you are not missing anything. It's a process that takes ages the first few times but becomes easier each time.

Richard

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Thank all so much so far, I think this could be it!

Yes I did remove the vanes a little while ago, not for collimation but because one of the joining right angles was so bent by god knows what I had to correct it. When I did this I disconnected all the vanes and just put them back the way I thought was right (stupid I know) which was with the right angled flat side of the vanes pointing out.

I am about to change them all around and if the slightly angled edge of the vanes is supposed to be pointing out of the OTA this could be it. If this is true I feel sooooo stupid right now...! Like one of the people in front of an IT technician that says have you turned it off and on again :)

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Well a quick update as to what has happened today if anyone is interested.

I put all the spider vanes around the right way and hey presto it must have been the problem. I immediately have a lot more play in the centre screw of the secondary mirror.

After getting the secondary aligned absolutely perfectly within the circle through my cheshire I cannot describe the feeling of happyness I had! I updated my facebook and noone had a clue, a ran around the house screaming with my mum used to this sort of thing so she was unphased :)

But then woe is me, what do I do now? I have been following what feels like 6 million guides on collimation and I still cannot work out the next part.

I have to use the 3 fine adjustment screws to get the 3 clips of the primary on the edges of my secondary is that right??!? But as Astrobaby mentions, this isn't possible to see in a cheshire and I don't have a film cannister for this cap idea. :/

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You can make a collimation cap with a bit of creativity and 2mm drill bit. If you have a dust cap on your focusser then you can use that, otherwise search the house for something with a cap that is 1.25" across - preferably something that has a conical profile so that it self centres and can't fall into the scope. Drill a hole dead-centre and away you go. To be honest, I managed to get reasonable collimation just by looking through the eye piece holder (though there are some useful visual clues on my scope to help you look straight down the barrel) and then tweaked it with a star collimation. My first time, so I may have been lucky!

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whaddya mean, "if anyone is interested"?! this is better than any soap :)

Ha ha.

Previously on Heartbeat er... Headaches. Adz sought help from the local community. After a move into their forum...er town he reluctantly asked for help for fear of realising his own stupidity ;)

However after discussion on a thread...err pub. They all made the newcomer feel very welcome and fully happy with his passion right now, Sally. Sally was a strapping young lass, well built at around 8" and she needed a lot of tuning to keep her in place and becoming the village bike :):)

Anywho on a more serious note, after asking my mum for a scalpel and already knowing the answer ha ha she gave me some great advice. I used a heated up needle to make a perfect circle in one of my focuser caps. Now I need to get all three clips in the view right?

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