Refractor rugby ball shaped stars

[Connorbrad98]

Hi I got a skywatcher startravel 120  f5 anyway when I do a star test when I defocus stars they look rugby ball shaped this happens in all me eyepieces even my televue eyepieces anyone know what could be causing these rugby ball shaped stars? thanks

[Cornelius Varley]

Have you compared the star shape with and without the diagonal ?

[John]

Do stars in the centre of the view look like this or is it just those towards the edges ?

[andrew s]

Do you have astigmatism in your eye(s)?

Regards Andrew 

[Cornelius Varley]

I see that you have asked this question before

 

[Gonariu]

@Connorbrad98. If you have recently bought it, it is still under warranty, if you find out that it has any defects, have it changed immediately or return it. If, on the other hand, to do this it is too late, try to diaphragm it with a diaphragm made of cardboard, I did so with the Konusuper 120 (achromatic 120/1000) bought in 1999 which gave me satisfaction in the 2003 Mars opposition but for others wait has never convinced me. It, looking at the planets, exhibits a lateral smear that I did not understand where it comes from, looking intrafocal and extrafocal in one of the two presents the "rugby ball" in the other the blurred image seems circular. Lately I found a chip on the edge of one of the lenses of the achromatic doublet that I never understood how it was produced (surely my mistake was to buy it from Mediaworld, who knows if they did not fall). I then diaphragm to 90 mm to exclude chipping making it an achromatic 90/1000 which on the Moon has become phenomenal, often easily holds 333X (Plossl eyepiece from Celestron 6 mm with a Barlow 2X achromatic also from Celestron), also on Mars has held up well this past fall. I enclose a photograph of the Moon taken this year with the Konusuper 120 diaphragm and with the mobile phone.

[Connorbrad98]

1 hour ago, andrew s said:

Do you have astigmatism in your eye(s)?

Regards Andrew 

Yes I got glasses it doesn't do nothing so I think it's in the scope.

[Connorbrad98]

1 hour ago, John said:

Do stars in the centre of the view look like this or is it just those towards the edges ?

They do look like this a tiny bit in the centre but worse to the edges.

[Louis D]

Have you tried adding a field flattener to the front of you diagonal?  I use a TSFLAT2 with 15mm of spacing in front of my 2" diagonal in my short refractors to do a good job of flattening the curved field.

You can tell if field curvature is the issue by focusing a star in the center, and then moving it to the edge, and seeing if it focuses to a tighter pinpoint by refocusing.  Most inexpensive short tube refractors have field curvature.  The Tele Vue NP101 does not thanks to its Petzval design, though.

[Connorbrad98]

On 12/09/2021 at 20:15, Louis D said:

Have you tried adding a field flattener to the front of you diagonal?  I use a TSFLAT2 with 15mm of spacing in front of my 2" diagonal in my short refractors to do a good job of flattening the curved field.

You can tell if field curvature is the issue by focusing a star in the center, and then moving it to the edge, and seeing if it focuses to a tighter pinpoint by refocusing.  Most inexpensive short tube refractors have field curvature.  The Tele Vue NP101 does not thanks to its Petzval design, though.

Could I also be seeing pinched optics?

[Alan64]

Check the focusser.  You want it to rack in and out stiffly yet smoothly, square and true to the doublet-lens at the front, and with no binding or slop.  The bearings for the draw-tube may need to be added to or replaced to effect an improvement.

As a matter of course, I routinely re-do the spacers between the lens-elements of my achromats.  I simply measure the thickness of the paper ones, and make new ones of aluminium-foil tape, like so...

I also blacken the edges of the lenses.  You must feel comfortable in so doing before attempting, however.

In addition, the doublet may be too loose within its cell, or too tightly held.  There is a retaining-ring that holds the lenses in place.  It should not be tight against the lenses(pinching), nor backed off too much.  I install the ring to where it comes to a dead-stop, gently touching the lenses, then I back the ring off a quarter of a turn.  Afterwards, the lenses may rattle slightly when the tube is gently bumped or shaken, but no harm is done.

This is an example of a refractor's retaining-ring...

Barring all of that, you may have the option of exchanging or returning the telescope.

Edited September 18, 2021 by Alan64