Am I supposed to be seeing the airy disk in an 8" SCT?

[Saturninus]

I've had my 8" SCT for awhile now, and I have been generally satisfied with it. But I have a secret that I have been keeping to myself this whole time...I don't think I have ever really seen the airy disk with it.

I've seen it with a 127mm mak-cas, and in a 110mm refractor, but I never quite get there in my 200mm SCT. I know that the larger the aperture of the telescope, the smaller the airy disk. Is it the case that at 200mm aperture, atmospheric blurring will prevent me from ever getting to a high enough magnification to see the airy disk under normal conditions?

How else can I evaluate the optics of my telescope? Planetary and lunar details seem sharp to me, globular clusters resolve pretty well, but the bright stars are always a bit fuzzy...

[Mike Hawtin]

This will undoubtedly bring forth howls of outrage but welcome to the world of sct's.  I have had four over the years, two 11" and one each 9.25" and 6" all Celestrons and I've never once seen the Airy disk through any of them, plenty of fuzzy stars and some so so lunar and planetary views but that's it.  I think you need to be quite lucky to get one that performs anywhere near its theoretical limits.  I'll stand well back now and wait for the storm to break.

Mike.

[LukeSkywatcher]

We're talking collimation here, right?. Ive had my 8" SCT for about 3 yrs now and have never collimated it and the views are lovely. 

Either that, or there is a DSO called "The Airy disk" and in that case i have never seen it either. If you point it at a star and the star is not a pinpoint of sharp focused light then the focus is slightly off. The focusing on the 8" SCT can be tricky and the standard focusing knob on the scope doesnt help much. Its very finicky and can be out every so slightly most of the time. There are easy free DIY jobs you can do to fine tune the focusing. They involve plastic bottle tops or even plastic peanut butter lids. I dont have any links, but it has been discussed on SGL before. 

[russ.will]

Storm to break? It's not like you've suggested there's something you don't like in a Televue.....

You need a lot of magnification to see the airy disk in an 8" scope. I can see the airy disk in mine when I Barlow (well Focal Extender) my 15mm Plossl, which gives 266x and it's still tiny. That said it's also got to be a good, stable night (the ones with a tiny bit of high haze and no wind) AND the scope has been out for at least 2 hours. In other words, exactly what's required for top notch planetary viewing.

I imagine that in Honolulu (lucky sod) that the wind is pretty much always blowing on or off shore. I'm guessing, that you might be able to go for it if all the above holds AND you wait for that point when the wind switches from on to off-shore in the evening. I stand to be corrected on that though.

Russell

[Saturninus]

Thanks for explanations! Sometimes I do feel as if the Airy Disk is some kind of 15th magnitude planetary nebula that everyone else has been able to find except for me...

But if I need to get above 250x to see it, then that explains why I never have. Too much atmospheric blurring at that magnification to make out something as delicate as an airy disk. I am usually limited to 225x or below. Occasionally, I can get above 300x on Saturn specifically, but gather that planets behave very differently than point sources of light such as stars.

To be clear, most stars are pinpoint sharp. It just the really bright ones, like Arcturus or Sirius, that look like asterisks. Come to think of it, I'm not even sure how those stars are supposed to look at 400x under perfect conditions in an 8" SCT

[Dave In Vermont]

One of the wonders of a CAT (SCT) is their low-maintenance. If you are getting pinpoint starts, you should be fine. If you need to collimate your scope, their are good instructions out there. Here's one:

http://skywatch.brainiac.com/collimation.pdf

That one is from Rod Mollise. He's one of the Guru's of the SCT.

So don't stress. An 8" SCT is a wonderful instrument.

Clear & Sharp Skies,

Dave

[ianm2k4]

To be clear, most stars are pinpoint sharp. It just the really bright ones, like Arcturus or Sirius, that look like asterisks. Come to think of it, I'm not even sure how those stars are supposed to look at 400x under perfect conditions in an 8" SCT

Glad you mentioned this I was having the same thoughts last night. I was under the impression that some of the bright ones were in fact asterisk shaped.. Thought my scope was out of calibration but the other stars were dots so I wasn't sure.

[Scorpius]

A while back I purchased Bob’s knobs for my Celestron 8” SCT because I suspected it was in need of collimation due to several articles stating SCT’s need to be collimated on a regular basis - especially if they’re transported a lot from one place to another. One individual even said they check their collimation each time out so I usually go ahead and do a quick check initially just to make sure it’s not been knocked out of collimation during the trip from home to my dark sky site.

Are you able to see the doughnut shape from an out of focus star as you intentionally bring the star further and further out of focus? I have found this works best on a brighter star. It takes a lot of twisting to get the doughnut shape to appear and once you get that just keep going in the same direction as the “doughnut hole” continues to open up. Then after a while, the diffraction rings around the edges of the out of focus star will begin to show and I’ve found - as other have stated - that the seeing needs to be excellent in order to get a stable airy disk. Point is - I don’t think you can see the airy disk unless you see the doughnut first and then just keep going in the same direction until the diffraction rings around the edges begin to appear. Good collimation does make a difference on an SCT (especially for planetary) so good luck and clear skies...

[ollypenrice]

This will undoubtedly bring forth howls of outrage but welcome to the world of sct's.  I have had four over the years, two 11" and one each 9.25" and 6" all Celestrons and I've never once seen the Airy disk through any of them, plenty of fuzzy stars and some so so lunar and planetary views but that's it.  I think you need to be quite lucky to get one that performs anywhere near its theoretical limits.  I'll stand well back now and wait for the storm to break.

Mike.

It won't break from my direction because I agree. My Meade 10 inch is a decent scope and I'd call the optics good or even very good but I cannot call them excellent. There is always something just very slightly soft about them. Collimation is good. A visiting professional optical engineer confirmed this. I'm not being negative about my SCT, I'm just not being effusive.

Olly

[Grotemobile]

I have just used this video to sort mine out.

One of the members posted it the other day.

Like a different scope now.

Steve.