squaring the focuser

[stoodscissors]

hello all,

after a post on th equipment board it was recommended to me that my current issue(chromatic aberation/reflector - see equipment board) may be caused by my focused not being in line/true/square. I'm afraid I'm not too sure exactly what to check. I've done what I can with a rule and spirit level and it all looks ok to my untrained eye, but as I said I'm not sure what I'm really checking. How can i know for definite that my focuser is looking directly at the secondary and is not 'off'. In my mind, if the focuser is out of place to begin with I should never be able to get primary/secondary collimation correct. Is that the case or am I complicating things? Any help is greatly appreciated.

[MarkVIIIMarc]

I am fairly new, I would assume if my focuser was out of place the return beam on my laser collimator would be off.

[Jason D]

10 hours ago, stoodscissors said:

hello all,

after a post on th equipment board it was recommended to me that my current issue(chromatic aberation/reflector - see equipment board) may be caused by my focused not being in line/true/square. I'm afraid I'm not too sure exactly what to check. I've done what I can with a rule and spirit level and it all looks ok to my untrained eye, but as I said I'm not sure what I'm really checking. How can i know for definite that my focuser is looking directly at the secondary and is not 'off'. In my mind, if the focuser is out of place to begin with I should never be able to get primary/secondary collimation correct. Is that the case or am I complicating things? Any help is greatly appreciated.

That is completely incorrect. It is recommended to square the focuser and center the secondary to ease collimation including field illumination optimization. If other words, to ensure the best brightness distribution at the eyepiece. The worst that could happen is reducing brightness and possibly resolution; however, chromatic aberration is completely unrelated to focuser squareness. Chromatic aberration is causes by lenses -- not mirrors or focusers. The issue must be related to your eyepiece or/and any built-in lens in your scope.

Jason

  

[stoodscissors]

Hi Jason,

thank you for the response. As I have said above this started originally as I am experiences chromatic aberration in all of my eyepieces (>5 eyepieces) and it has stumped me as you rightly say that it is impossible to have in a reflector. I was merely following up on a suggestion from another SGL'r. It would seem then that I have been unlucky with my eyepieces I will have to buy a new one and see if the problems persist .

regards,

Greg

[Dave In Vermont]

Could you post what exactly the 'chromatic abberation' looks like? And what model/size telescope do you have? As well as which eyepieces you're using.

Thank you!

Dave

[paul mc c]

Also what were you observing when you seen this as observing low down can give you the effect you describe,Mars is a good example at the moment.

[N3ptune]

I asked Skywatcher about squaring the focuser for a reflector and it may not be a big deal:

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Actually it is a misunderstanding that users have to "square" the focuser accurately first. There is nothing wrong with doing it, of course, but the secondary is optically flat, and the angle of reflection is not critical. The important thing is to make sure the focuser and the secondary are lined up, as seen from the focus. As it is a little bit difficult to explain though emails, I am going to share a video and hope that will help you : www.youtube.com/watch?v=8G98RTP6jbY

Please let me know if you have any other questions. Thanks!

Clear skies,

Sky-Watcher Support

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I asked that question to SW because I thought I had a comatic aberration issue (stars having a comet tail) turned out to be a tube current problem (has discovered here with the SGL people), telescope was hotter then the surrounding air, messing up all the view seriously. It's a basic error causing major troubles.

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I also found a method on the internet to square the focuser using a mirror. I tried to align the mirror perpendicularly with the vanes (which are on the same axis has the focuser in my case) and using the front plastic hoop of the scope to sit the mirror at the right place (with office blue paper gum). I took a lot of precise measurements around my instrument to figure out the center position of the focuser then I drew a dot on the mirror indicating this center. Having the center on the mirror I used a cheshire to aim at the dot and it was possible to play with the adjusting screws on the focuser to move it slightly, right on spot.

--> It may not be perfect but it's not bad at all and it has to be done while looking thought a Cheshire.

My telescope is 203mm x 1000 and I can watch the moon at 425x getting a really clean image if the atmospheric conditions are good. It's working.

 

[Dave In Vermont]

That was some good support you acquired! May I inquire where you bought your Skywatcher?

I wish more people would understand that shopping for a good dealer is as important as shopping for a good telescope. And it's the aftercare that's often the acid-test of this fact.

Good work -

Dave

[N3ptune]

Hello Dave, I asked the question to Skywatcher directly via their website (Canada or US I don't recall, possibly the US customer service). For a question like "squaring the focuser" I wanted to talk with a skywatcher technician directly because I thought it was to specific to do otherwise.

 

[Dave In Vermont]

Their quickie fix with a laser was accurate and well done. But I did/do have one problem with it: They didn't tell people to level the OTA before playing with a screwdriver or wrench. If you do drop a tool while working on such with the tube tilted upwards, guess where it's going to land? Yup! Right on your primary!

If you're bored - call 'em back and tell the tech to change their suggestion to one that includes such a nugget of advice.

Murphy's Law,

Dave

[N3ptune]

The most valuable information SW gave me was the fact that squaring the focuser is not an issue has big has I thought it was in reality. The video on collimation using the laser was totally useless to me, I should have cut that youtube before publishing the SW message really.

[stoodscissors]

wow, thanks guys for all the info. Ok so squaring the focuser is not a big issue then. I have an Orion xt10 Newtonian, which I forgot to mention earlier. Also since my Las post I have acquired a brand new Meade 5000 series super plossl. Using an artificial star I have checked that collimation is ok, all 4 clips are visible when waaay out of focus and airy disc is concentric. Now, when at dead center of the eyepiece last night there was no chromatic aberration of Jupiter. However as Jupiter moved away from center the chromatic aberration became gradually worse until the planet was an orange and blue blob near the edge of field. Is this a normal effect? It seems I have all the gear and no real idea lol. This photo was taken when just off from center (it will still have been in the 100% zone from my calculations).

[stoodscissors]

also just to add that this photo is a couple of weeks old, but its just to demonstrate what I'm seeing. It was the same last night with new eyepiece

[Jason D]

What you described is caused by the eyepiece. Not all eyepieces are equal. Distortion and Chromatic Aberration for off-center targets differ between eyepieces.

Jason