I don't see diffraction "Airy Discs" at all

[Buqibu]

Hello, I've read that stars through a telescope show up as small discs with diffraction rings around them. I have a 4 inch achromatic refractor and have never been able to see this. Could it be the optics are sub-par, or has the seeing never been good enough? The second seems unlikely, as I've tried it on multiple occasions, through high magnifications, but always just saw a deformed smudge, not a defined disc. Thanks for any advice! 

[vlaiv]

What do your views look like? Maybe one of these images:

These show "progressive" improvement in seeing conditions. Only good and very good seeing will show definitive disk with either broken or full rings around it.

If your view resembles - first three images, then I'm afraid, it is the seeing. Check out some of the guides for optimizing planetary views (cool down, choice of location, time of the night, target near zenith, etc ...) - same things apply here

[Buqibu]

5 minutes ago, vlaiv said:

What do your views look like? Maybe one of these images:

These show "progressive" improvement in seeing conditions. Only good and very good seeing will show definitive disk with either broken or full rings around it.

If your view resembles - first three images, then I'm afraid, it is the seeing. Check out some of the guides for optimizing planetary views (cool down, choice of location, time of the night, target near zenith, etc ...) - same things apply here

I see the first three, but not that bad, its more like a triangle looking shape. Could it be the optics? It really only bothers me because of some close double stars, doesnt't seem to affect other observation. 

[vlaiv]

3 minutes ago, Buqibu said:

I see the first three, but not that bad, its more like a triangle looking shape. Could it be the optics? It really only bothers me because of some close double stars, doesnt't seem to affect other observation. 

What scope are you using. If it is triangle shape - it could be pinched optics.

Try slight defocus on a bright star and see what the pattern looks like. If it is still triangular, it could be pinched optics.

[Buqibu]

It does kind of look like that, if it is pinched optics, can it be fixed?

[NGC 1502]

A triangular shaped infocus or slightly defocussed star could mean a pinched objective lens. With a refractor objective try carefully undoing the retaining ring around the front lens a small amount. Test again at high power when you can.

When slackening the retaining ring be ultra careful to not damage the lens.

Hope you sort it, Ed.

[Buqibu]

Thanks!

[Pixies]

Hi @Buqibu 

What scope do you have? 

[Buqibu]

17 minutes ago, Pixies said:

Hi @Buqibu 

What scope do you have? 

Its an AstroMaster 102 refractor, 660mm fl

[vlaiv]

6 hours ago, Buqibu said:

It does kind of look like that, if it is pinched optics, can it be fixed?

Have a look at this video:

https://www.youtube.com/watch?v=Ylb7xnc_03U

It shows whole process of disassembling ST80 refractor telescope and blackening lens edges to improve contrast. Interesting parts are re seating the lens back in lens cell - tapping to get it to seat properly. This part is at around minute 5-6.

Like Ed pointed out above - you can just slightly loosen retaining ring - and then tighten it as instructed in video - each time gently tapping / shaking scope to seat lens properly - and just slightly touch up on retaining ring without forcing it.

[Buqibu]

7 minutes ago, vlaiv said:

Have a look at this video:

https://www.youtube.com/watch?v=Ylb7xnc_03U

It shows whole process of disassembling ST80 refractor telescope and blackening lens edges to improve contrast. Interesting parts are re seating the lens back in lens cell - tapping to get it to seat properly. This part is at around minute 5-6.

Like Ed pointed out above - you can just slightly loosen retaining ring - and then tighten it as instructed in video - each time gently tapping / shaking scope to seat lens properly - and just slightly touch up on retaining ring without forcing it.

Thank you!

[Pixies]

Hi. I was going to make the same suggestion as @vlaiv - but on further inspection, I'm not sure your objective cell is configured the same. I think older 102 models even had collimation screws. I may be wrong though.

Edited July 27, 2021 by Pixies

[RobertI]

My Tal 100mm refractor has a dew shield which is held in place with three grub screws which sit on the circumference of the lens cell. When I first bought the scope I over-tightened the screws and got that exact problem. It took me ages to work out the problem but on slackening them off, immediate perfect airy discs! 

Edited July 27, 2021 by RobertI

[John]

I have seen this issue with refractors a few times and each time an over-tight lens retaining ring was the cuprit.

Following the method that @vlaiv explains usually improves things a great deal. The lens retaining ring just needs to be tight enough to stop the lens elements rattling but no more than that.

It should also be noted that you need decent seeing conditions, a cooled scope and a reasonably high magnification to see the airy disk and diffraction rings clearly. Poor conditions will cause them to be ill defined. Trying to interpret what you see when you star test under poor conditions can be very misleading.

 

Edited July 27, 2021 by John

[Alan64]

This is the lens-cell from a 90mm achromat that I have...

You can see the retaining-ring there.  What you want to do is loosen the ring, then screw it back in slowly until it comes to a dead stop, then back it off just a quarter of a turn.  That will allow the doublet to expand, room to spread out.  You may hear a slight rattling of the lenses on occasion afterwards, but no harm done.

[Voyager 3]

What magnification were you using ? You have to use quite high magnifications ( around 1mm exit pupil even for the brighter stars and smaller exit pupils for dimmer ones ) to notice the airy disk . Some may be able to notice it at 1.5mm exit pupil for the brightest stars but they are experienced in seeing the disks and other diffraction patterns thousands of times . 

[ollypenrice]

It's common for pinched optics to be worse in cold weather when the metal parts of the cell contract. Does this ring any bells for you?

Olly

[Buqibu]

Thanks for the help