Does anyone know of a practical method of aligning the focuser on the sharpstar 15028hnt? The one I recently bought has horrible alignment, rotating the camera angle results in substantial collimation offset. There are three pairs of push/pull screws at the base of the focuser tube. How do I get the focuser square to the OTA? I have a collimated laser (I sure hope it is at the price I paid).
Sharpstar 15028hnt focuser tilt
Well Written Engaging
Blaine Gibby:
Does anyone know of a practical method of aligning the focuser on the sharpstar 15028hnt? The one I recently bought has horrible alignment, rotating the camera angle results in substantial collimation offset. There are three pairs of push/pull screws at the base of the focuser tube. How do I get the focuser square to the OTA? I have a collimated laser (I sure hope it is at the price I paid).
First off, is it truly collimated. Use a V-block and point the laser to a far-away (say 20 mt) away and slowly rotate the laser pressing down so that it doesn't tilt whilst doing it. If after a full rotation it never moved then it is really collimated. If it did and described a small circle than it ain't and you chucked money down the proverbial drain.
Secondly, assuming that the first pass is fine, insert the aforementioned laser in the focuser. Start rotating it slowly (gently pressing it down while keeping it just a little clamped down, enough to avoid tilting but not to impede rotation) and see whether it describes a circle on the primary mirror. If it does then your focuser is tilted and the way to correct is to mentally visualize the virtual circle it is describing on the mirror and a virtual diameter it insists on. You want to operate on the collimation screws on the focuser so that the bright spot of the laser moves half way in to the virtual center of circle moving on such a diameter. Since any point on a circle is one end of a diameter chose a point opposite to the set of push-pull screws you want to adjust. After your best stab at moving the bright laser spot repeat the rotation exercise and see whether the spot still move in a circle. If it does and you are doing the right thing than it must be smaller one and you repeat the above procedure with the next set of push-pull screws and so on. Normally it takes 2-3 iterations to square the thing as a first pass.
Now you'll have to collimate the secondary on the primary and then use an auto-collimator to square the focuser on the reflected image of the bull's eye attached to the primary (hopefully you should have one). Once this is done you should see no movement at all on the reflected laser beam once you rotate it around the focuser.
Loads of caveats in doing this but at least this should get you started in the right direction.
Helpful
Thanks you made some good points that filled in the gaps of my tinkering!
@Brian Boyle Brian, did you get into this one? I remember you had one of these I think?
andrea tasselli:Blaine Gibby:
Does anyone know of a practical method of aligning the focuser on the sharpstar 15028hnt? The one I recently bought has horrible alignment, rotating the camera angle results in substantial collimation offset. There are three pairs of push/pull screws at the base of the focuser tube. How do I get the focuser square to the OTA? I have a collimated laser (I sure hope it is at the price I paid).
First off, is it truly collimated. Use a V-block and point the laser to a far-away (say 20 mt) away and slowly rotate the laser pressing down so that it doesn't tilt whilst doing it. If after a full rotation it never moved then it is really collimated. If it did and described a small circle than it ain't and you chucked money down the proverbial drain.
Secondly, assuming that the first pass is fine, insert the aforementioned laser in the focuser. Start rotating it slowly (gently pressing it down while keeping it just a little clamped down, enough to avoid tilting but not to impede rotation) and see whether it describes a circle on the primary mirror. If it does then your focuser is tilted and the way to correct is to mentally visualize the virtual circle it is describing on the mirror and a virtual diameter it insists on. You want to operate on the collimation screws on the focuser so that the bright spot of the laser moves half way in to the virtual center of circle moving on such a diameter. Since any point on a circle is one end of a diameter chose a point opposite to the set of push-pull screws you want to adjust. After your best stab at moving the bright laser spot repeat the rotation exercise and see whether the spot still move in a circle. If it does and you are doing the right thing than it must be smaller one and you repeat the above procedure with the next set of push-pull screws and so on. Normally it takes 2-3 iterations to square the thing as a first pass.
Now you'll have to collimate the secondary on the primary and then use an auto-collimator to square the focuser on the reflected image of the bull's eye attached to the primary (hopefully you should have one). Once this is done you should see no movement at all on the reflected laser beam once you rotate it around the focuser.
Loads of caveatsin doing this but at least this should get you started in the right direction
Hello Blain,
I agree with your procedure but how can you tell the 2nd mirror is tilted when the laser point describes a circle on the primary when rotating the laser beam ?
That would only happen if the first step of your instruction wasn't good enough.
Otherwise no idea how this should happens ? Concentric aligned laser beam deflected on a plan mirror !!
br
Franz
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