D shaped stars

[alacant]

Hi everyone. I'm almost certain that this is caused by the protrusion of the focus tube into the main tube. Apart from cutting 10mm from the end of the tube, have I any other options?

150-750 reflector with coma corrector: Light-frame-example with D shaped stars to the LHS only. Thanks for looking and TIA for any suggestions.

[cosmojaydee]

This could be caused by flexure , I would check the security of the image train and ensure everything is square 

[alacant]

Hi. OK. Will check tonight. Here's an out of focus star. I think this confirms the focuser? It looks almost identical to looking down the tube. Can anyone advise? TIA

[cosmojaydee]

Have you looked at this , very informative  http://www.astro-baby.com/collimation/astro babys collimation guide.htm , note that on fast scopes F/5 and above there is an off set on collimation HTH

[alacant]

Hi. Thanks for the link. I'll have a read of that. It looks frightening! I collimated using a laser in the eyepiece[1]. First I centred the secondary by putting the red dot in the centre of the primary, the the primary by adjusting the now reflected twice beam to the central hole in the collimator. Is that good enough? Both needed hardly any adjusting. TIA

[1] checked the laser was true by rotating it in the eyepiece.

edit: oh, and the secondary was slightly twisted -clockwise as you look down the tube from the front-. Done that. Not quite sure of this displacement thing though. I stacked ten or so of the light frames. You can also see the D shape in the halos on the left. Also they get more oval toward the left...

[alacant]

OK. Evidently my cheepo unbranded Chinese cc is also a reducer and so requires 12mm more back focus [1]. This causes the focus tube to enter the light path. Time for the big hammer and red black nail varnish.

[1] To avoid the focuser tube protruding into the light path, ES suggested I use their dedicated cc. Twice the cost of the telescope itself!

[Chris Murray]

i would look at you collimation, needs to be a circle your seeing, also i agree chop the focuser tube, i like getting to the mechanics of it all, good luck

[Thalestris24]

Hi

If you don't need the reducer, just get a Baader MPCC - problem solved!

Louise

[alacant]

Yeah, thanks everyone. As you can see I went for the big hammer and cut the end of the focuser tube. All is now well with the stars. I didn't know the cc was a reducer until I compared coma and non coma snaps. It looks a bit like the Skywatcher coma corrector. Clear skies...

[yelsac]

On 02/12/2016 at 23:27, alacant said:

Yeah, thanks everyone. As you can see I went for the big hammer and cut the end of the focuser tube. All is now well with the stars. I didn't know the cc was a reducer until I compared coma and non coma snaps. It looks a bit like the Skywatcher coma corrector. Clear skies...

Hi

I've the same problem with my 150p can I ask how much you cut off the focuser barrel? also have you noticed it effect the performance in any way either in visual or astrophotography? Basically is it worth doing?

thanks

[alacant]

Hi. Yes. With the sw cc, I needed around 10mm more inward focus so it's the difference between D shaped stars and not. With the Baader cc, you need to focus some 10mm more focus away from the tube so you should be OK. Try with the Baader and see how much the focuser barrel protrudes into the tube. It may not enter the light path. To answer your question, I cut around 12mm but YMMV. HTH.