John

Still pretty light here but the scope is in action on the Moon anyway

The 5mm Pentax XW gives 318x with this scope and really, really sharp views this evening

Anyone remember reviews which commented on scopes that "snapped to sharp focus" ?.

I suspect the reason was that the scope was fast so the depth of sharp focus was shallow.

It looks more Questar than Meade now. That is a compliment

I'd be amazed if todays manufacturers could not produce very good quality small aperture long focal length achromatic doublets. If these are anything other than extremely good performers, something has gone rather awry.

Just now, johninderby said:

Think I need to get this T-shirt. 🤪

I would need a few - it seems to be more or less permanent these days

I saw Saturn and Jupiter visually at around 3:00 am a few nights ago. They are still very, very low down from here

Having observed them often over the years when they were high in the sky I'm having trouble getting motivated to get a scope setup because the views are likely to be not as satisfying

If you have not seen them before I can understand why it is worth making the effort though

Another good but older (out of production) refractor is the Vixen 102mm F/9.8 achromat. Japanese made and fine optics often for quite a low price if you can find one on the used market.

52 minutes ago, andrew s said:

Visually, at the same magnification  and aperture the size of the airy disk should be the same. 

If you take a prime focus image the linear size of the airy disk will increase in proportion to the focal length.

That's it period.....

 

42 minutes ago, Dave1 said:

..From my understanding. Comparing like for like aperture, at the same magnification the slower telescope has a bigger airy disc. Which is definitely an advantage for planetary viewing....

 

There is the confusion, right there !

Can both assertions be correct ??????

When I was a kid and sci-fi / space mad, I painted the walls of my bedroom and most of the furniture matt black and then painted star fields all over them. To the brighter stars I added a set of diffraction spikes even though at that age I had never looked through a telescope. I just thought that was how stars were supposed to look !

When I moved out my Dad complained that re-decorating that room was the hardest DIY job that they had ever done

All so that I could listen to Hawkwind more "immersively"

3 minutes ago, andrew s said:

There should be no difference as focal ratio does not affect t the angular airy disk size. There may be second order effects due to the shorter fl telescope having larger aberrations,  field curvature being the most obvious one.

Regards Andrew 

Well I am confused now

Probably was before as well, so no change there

These crazy refractor people !

You can't rely on what they say of course - they are smitten after all

Of course I have not fallen into their trap - I only have 5 refractors ........

12 minutes ago, andrew s said:

I agree aesthetics is important that's why I love the diffraction spikes from the point spread function on my ex Newt and current ODK. Just as much an "image" of the star as that produced by a refractors circular aperture! 

Regards Andrew 

Do refractor / SCT / mak imagers use processing to add in those spikes to their images do you think ?

Thanks for the answers to my question. My assumption about airy disk sizes did assume that the magnification used was consistent. I ought to have said that.

I also thought (probably incorrectly) that the reason that a larger aperture scope can potentially (important P word) resolve closer pairs of stars than a smaller aperture one was because the airy disk in the larger aperture was smaller ?

I didn't realise that focal ratio affected the airy disk size. I can test this I suppose by setting up my 102mm F/6.5 and my F/9 100mm refractors, using them at the same magnifications and seeing if I can see differences in the airy disk size

38 minutes ago, mih said:

is a refractor a good tool for DSOs? Or is it better for planet viewing?

Aperture and dark skies are the best tools for observing DSO's.

The planets and double stars are more the refractor strong points.

The newtonian can do those as well though but the crispness of a refractor image is particularly nice for splitting double stars.

With the aperture the same, the airy disk should be the same apparent diameter in both scopes. Have I got that right ?

So if the F/7.5 has a similar figuring accuracy to the F/15 and care is taken to achieve accurate focus, I would have thought that the actual resolving power would be pretty much the same ?

Or are other factors at work here ?

The ISS pass was very bright from here. Went just below Spica in Virgo. The Dragon capsule was supposed to be around 4 minutes behind it and a bit lower but I could not see it. It was predicted as about magnitude 2.5 which might have been a bit faint with that much light still in the sky. My "window" between trees in that direction is not that large either so it would be easy to miss it.

Just now, mih said:

So you reckon i get a refractor?

Think through what you want it to do for you. Then work out if a refractor is the tool you need.

Getting these things to work right for you is all about experimentation

It's difficult to judge if you have got it "right" until you actually use the thing under the stars.

Pay £1000 for an Orion Optics dob and you will still need to tweak and adjust things.

Just for information, a newtonian reflector is also an apochromatic telescope

I think the Super 2.5 is a Super 25 really so 25mm - that is your low power eyepiece and the one to use as you get used to the scope.

Don't worry - the scope is a good one and very popular on here. These things are scientific instruments though so some time is needed to get used to how they function, what they can do and also what they cannot do.

I've said "bye bye" to Venus for this apparition now.

I got it down to 1.3% illuminated which beats what I managed last time around so I'm happy

6 minutes ago, mih said:

Thanks! I wanted to buy an eq mount, so i was thinking maybe i could get a refractor on an eq and put my reflector on it as well. Is that an option?

Yes, that is quite possible.

Probably best to get a refractor that compliments what your 150mm newtonian can do so you get a different experience when you use each scope. They will have different strengths and weaknesses.

Refractor optics cost more per mm to make because there are 4 or more optical surfaces to figure, polish and coat compared to just the two surfaces for a newtonian. A 150mm refractor is expensive and sizes above that comparatively rare whereas reflectors can easily be made in 200mm, 250mm, 300mm and up to around 500mm.

So if you want a larger aperture for deep sky objects, a reflecting telescope is the way to go.

Refractors have a lot of fans though for the purity of their image, their relatively robust collimation, their lack of a central obstruction etc, etc.

The subject of these different designs of telescope is the subject of many books, articles and, many, many posts, on forums

Here is a piece by our own Steve Richards ( @steppenwolf ) with a contribution from Paul Money, on these designs:

https://www.skyatnightmagazine.com/advice/buyers-guides/first-telescope-reflector-or-refractor/

A great session !

The Blinking Planetary has become a favourite of mine over the past few weeks. I see similar tints in it to you. Nice central star as well !

A wide field of view is also quite important - it is a huge object in total - 3.5 degrees across. Each of the major elements is around 1.5 degrees in extent. The eastern segment is easier to spot than the western part although the latter does run right by the star 52 Cygni so it is easier to locate.

This chart and also the composite image below, give an idea of the scale of the thing:

Apologies to @Whistlin Bob for hijacking his thread slightly.