Coma cause ?

[Davey-T]

Stars all have coma in one direction, does this indicate the the secondary is rotated or some other cause ?

Got various collimation aids which don't agree with each other

Dave

Bit of image 200% enlargement ZWO ASI2600MC

 

 

[vlaiv]

That looks more like astigmatism to me than coma. What scope is that?

Third one is astigmatism and if it is off axis - cross tends to be shifted to one side.

Could be due to coma corrector or perhaps mirror is too much tightened in mirror cell. How many clips are there, 3 or 4?

[Davey-T]

5 hours ago, vlaiv said:

Third one is astigmatism and if it is off axis - cross tends to be shifted to one side.

Could be due to coma corrector or perhaps mirror is too much tightened in mirror cell. How many clips are there, 3 or 4?

It's the Sharpstar 150HNT F/2.8 so proving a trial to get right, been playing with the mirror cell so will loosen it off a bit, three mirror clips but not clips as such, felt pads in front of the mirror.

Dave

[vlaiv]

8 minutes ago, Davey-T said:

It's the Sharpstar 150HNT F/2.8 so proving a trial to get right, been playing with the mirror cell so will loosen it off a bit, three mirror clips but not clips as such, felt pads in front of the mirror.

Dave

Ah, it has corrective optics. That is probably tilt of some sorts.

I had something similar with refractor and focal reducer / field flattener - but not as clear. It was confined to one corner (other was just tangential astigmatism and rest were relatively fine).

This is one corner - it shows tangential astigmatism - elongation in direction towards the optical axis.

But other corner showed this:

 

Each star started showing a little cross.

In my case it was problem that FF/FR and sensor were not squared on optical axis of the scope (poor 2" connection - clamping type not threaded).

Your telescope has fixed corrector inside focuser if I'm not mistaken, and my guess is that focuser unit is ok. This leaves collimation as likely cause. Possibly secondary, but I think primary could be the issue as well.

Do you have any means to remove corrective element from focuser and do collimation of secondary by looking down the focuser tube?

 

[Davey-T]

6 minutes ago, vlaiv said:

Do you have any means to remove corrective element from focuser and do collimation of secondary by looking down the focuser tube?

Yes it's designed to collimate with coma corrector removed, I've set the secondary central under the focuser and measured the gap between the back of the mirror support and spider centre with micrometre in three places and all the same within fractions of a millimetre, I've spent a while getting the primary centred in the tube but still having trouble getting the primary and secondary lined up with the centre spot which results in the secondary being rotated slightly or the gap behind secondary not being equal.

None of the mirror adjustments are very smooth all a bit twitchy metal on metal so thinking of getting some nylon tipped screws to make them a bit smoother.

Dave