Massive cross-shaped glare from Jupiter

[PeterM]

The last three or four times I've observed Jupiter with my 200p I've been plagued with what looks like a a huge cross of light emanating from Jupiter. The glare isn't as bright as the planet but it's not far off - probably about a fifth as bright, but highly noticeable and it's spoiling the view.

Jupiter itself is almost completely washed out.

I used a variety of eyepieces (Nirvana, 6mm TMB clone, 20mm Celestron plossl etc) and it's always present. A moon filter helps to reduce the light but doesn't help with contrast on Jupiter itself. The glare is not present when I observed through my refractor and binoculars.

There was no cloud cover on any of the sessions but relative humidity was high - from between 95% and 99% according to my weather station. Primary and secondary as well as the EP were dew free and the scope had been cooling for at least an hour on all occasions.

Does anyone have any advice as to what's causing this please? I'm reasonable sure collimation is OK stars look sharp and when I defocus on a star the resulting doughnut is symmetrical but could be wrong. Is this just how bright objects look through a newt?

[oldfruit]

I am not sure but sounds like diffraction spikes?

[Peter Drew]

I agree with OldFruit. The diffraction spikes on Newtonians become more obvious as the brightness of the object in view is increased. Try the scope on Vega or Sirius and then compare with, say, Polaris to see the difference.

[Rossco72]

If you can, try using a filter to reduce some of the glare, Jupiter is very bright and I get diffraction spikes on my 350p.

[perrin6]

Is the cross shape a lot brighter than it used to be with that scope on Jupiter? Cross-shaped diffraction spikes probably get more troublesome if the spider loses some of it's paint perhaps ?

Alan

[NGC 1502]

It certainly sounds like diffraction spikes caused by the secondary spider.

It's one of the downsides of a Newtonian reflector. The brighter the object

the more noticable the spikes are. After a while, you tend to notice them less.

Thicker spider vanes give more noticable spikes. Some use curved vane spiders

to lessen the effect.

Regards, Ed.

Edit, more info here : http://www.nightskies.net/scopetest/accessories/destiny/spider.html

[PeterM]

Thanks very much guys, it looks like a secondary holder with curved vanes can help* so I may well check those out (or use it as an excuse to buy a long, slow refractor).

*As NGC points out just above.

[Doc]

I get this on my dob as well, on really bright stars or Jupiter. After a while you don't notice it, well I don't anyway.

[PeterM]

Now I know what it's called I've been able to find a few images of examples of planetary diffraction but there are hundreds of images out there of Jupiter taken with newts with no evidence of diffraction - do people edit them out or something?

[Breakintheclouds]

There was a thread on here quite recently (I'm afraid I couldn't find it just now, but it IS here somewhere!) and the consensus was that curved spider vanes don't actually reduce the amount of diffraction glare at all, they just 'spread it around' the entire view so you don't notice it as much. IIRC, most people in that thread seemed to prefer having whatever diffraction glare was going to be present in one place, where they could keep an eye on it. I think I'd agree with this, and would use a filter rather than change the spider. I tried a medium blue filter with Jupiter recently and it can help bring out some extra surface contrast, so given that it also helped reduce the glare, using a filter was a win-win.

[Dangerous-Dave]

I am 100% sure this is a diffraction spike due to the secondary vanes. I used refractors and SCTs for years then switched back to a Newt with 150P and it showed a visual diff spike on Jupiter at low magnifications. You can simply stop the tube down using the small hole in the OTA's cap or ramp up magnification. Both will reduce the brightness of Jupiter and the diff spike.

According to my learned friend Dr Jurgen Schmoll, curved spider vanes create a strange dot diffraction pattern around the star.

[xboxdevil]

I know what your problem is and I know how to fix it....Until yesterday I used to get 4 large search lights coming from Jupiter on my Skywatcher 300p, now I get a much smaller and less obtrusive diffraction spikes.

What's the problem? It's the stray light being scattered from your secondary mirror, I blackened the sides of mine yesterday and it has made a tremendous difference.

[John]

I know what your problem is and I know how to fix it....Until yesterday I used to get 4 large search lights coming from Jupiter on my Skywatcher 300p, now I get a much smaller and less obtrusive diffraction spikes.

What's the problem? It's the stray light being scattered from your secondary mirror, I blackened the sides of mine yesterday and it has made a tremendous difference.

Spot on - the spider and secondary mirror on the Skywatchers are very close to the top of the tube and any stray light that gets in there will create these sorts of issues and a reduction in contrast.

A dew shield projecting 10" or so above the top of the tube will also help to keep stray light away.

[peteftm]

I had this problem with my 250PX on jupiter.This was with using the 25mm eyepiece but was much reduced with the 10mm.

xboxdevil , how did you blacken the sides of your secondry?

[xboxdevil]

I took out the mirror for a clean, after it dried I just took a black indelible ink pen and coloured the sides. It sounds scary, but it isn't really as there's about a 1mm edge to the side of the mirror. So as long as the mirrored surface is pointing away from you and you use the side edge of the pen you should be fine.

I had this problem with my 250PX on jupiter.This was with using the 25mm eyepiece but was much reduced with the 10mm.

xboxdevil , how did you blacken the sides of your secondry?

[Astro_Baby]

Dew shield should help a lot - it may be diffraction spikes though. Something else you could try is fit the front dust cap and remove the small cap in it. The dust cap has two small projections on it - one of these is removable (the other one is what you hang the removable one on).

Make sure the empty hole isn't directly over a spider vane.

That will (a) reduce glare somewhat and ( increase the focal ratio of the scope which may give you more detal. This may help.

[PeterM]

I'm not sure I'm brave enough to remove the secondary mirror just yet but I'm definitely going to try a home-made dew shield. I also don't know why I didn't think of using the apperture reduction hole in the cap before but will try it and out and report back.

Thinking about the last few observing sessions I have been pretty careless with telescope placement and because I've been viewing a bright object haven't bothered too much with putting my scope in a dark bit of the garden (in fact I've been sitting right next to the patio doors when the wife's been in the sitting room with all the lights on) so the opprtunities for stray light at ground level have certainly been there.

Again, thank you all for the time you've taken to reply, it's really much appreciated.

EDIT PS forgot to say it's quite a relief to know it's a feature & something that can be mitigated rather than a dodgy scope.

[John]

... I've been viewing a bright object haven't bothered too much with putting my scope in a dark bit of the garden (in fact I've been sitting right next to the patio doors when the wife's been in the sitting room with all the lights on) so the opprtunities for stray light at ground level have certainly been there...

You may get a nice surprise if you can get the scope to a darker part of the garden and use a dew / light shield. Stray light entering the top of the tube really kills the contrast even on bright objects like Jupiter and renders deep sky objects virtually invisible I've found.