ScopeTech 80mm Fraunhofer Telescopes

[andrew s]

9 minutes ago, F15Rules said:

Exactly Stu, well put.

And much as I like my views in the 80/F15, I won't be letting my FS128 go anywhere, anytime!😁

Dave

I have never quite understood why small refractor owners wax lyrical about looking at the point spread function of their telescopes aperture stop. I guess it's because they can where as with large apertures it's too small to see. 🙃

Regards Andrew 

PS only joking, honest.

[Stu]

4 minutes ago, andrew s said:

I have never quite understood why small refractor owners wax lyrical about looking at the point spread function of their telescopes aperture stop. I guess it's because they can where as with large apertures it's too small to see. 🙃

Regards Andrew 

PS only joking, honest.

Because it’s aesthetically beautiful Andrew. If I could see the same perfect airy disk and diffraction patterns regularly with a larger refractor or dob then believe me, I would be waxing lyrical about them even more. Whilst I like to try to see doubles like Zeta Herc, I don’t find it a particularly rewarding experience looking at an often unstable mess trying to pick out the secondary in moments of good seeing. Call it laziness or just an appreciation for a particular way of viewing stars.

[Trikeflyer]

1 hour ago, Stu said:

I wasn’t aware of the link with focal ratio before reading this thread; I thought airy disk size was only related to aperture so that’s very interesting to know.

What I’ve been seeing lately when using my little Telementor and Alkor recently, is that they do produce much larger airy disks than larger scopes, so they show them much more easily. Your FS128 will show them under the right conditions, they are just smaller and harder to see if conditions don’t support high power. That’s the reason larger scopes show more detail.

Strangely I do actually enjoy the larger disks in the smaller scopes; they easily present textbook star patterns and I guess this in some way accounts for doubles like Izar looking so neat in them, you are just looking at two disks and a diffraction ring around each. In a larger scope if the conditions aren’t so good then it can look a little messier, but that’s just because you are seeing more of what is going on.

Hey Stu - I know no Astro pun was intended and I know what you are saying but I couldn’t help but do a double take on your last sentence - just a thought  - Is a little messier something like m71 😉🤣. Compared to a big messier like M39 😜. 
Steve 

[andrew s]

25 minutes ago, Stu said:

Because it’s aesthetically beautiful Andrew. If I could see the same perfect airy disk and diffraction patterns regularly with a larger refractor or dob then believe me, I would be waxing lyrical about them even more. Whilst I like to try to see doubles like Zeta Herc, I don’t find it a particularly rewarding experience looking at an often unstable mess trying to pick out the secondary in moments of good seeing. Call it laziness or just an appreciation for a particular way of viewing stars.

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 

[John]

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

 

[John]

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 ?

 

 

[andrew s]

3 minutes ago, John said:

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, using them at the same magnifications and seeing if I can see differences in the airy disk size

 

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 

[andrew s]

2 minutes ago, John said:

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

 

 

I have seen some examples where they do and some where spikes were removed! 

There have been magazine articles on how to add them for Xmas cards if I recall correctly. 

It's a free choice,  enjoy what you will.

Regards Andrew 

[Stu]

5 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 

I have no knowledge about this Andrew, but that’s not what the link Johninderby posted to said. Now confused!

 

[John]

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

[Stu]

21 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 

Yep, each to their own 😁👍

[John]

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"

[andrew s]

1 minute ago, John said:

Well I am confused now

Probably was before as well, so no change there

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. 

Trust me I am a physicist .

Regards Andrew 

[Dave1]

12 hours ago, johninderby said:

Some useful info on the subject here.

http://www.astropix.com/html/i_astrop/focus/defs.html

I've read a few different websites on this subject. That has to be one of the best well written easy to understand explanations I've ever read.

Great share John, thumbs up from me. 👍

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.

 David

[johninderby]

For some reason this discussion  reminded me of this.🤔

https://en.wikipedia.org/wiki/How_many_angels_can_dance_on_the_head_of_a_pin%3F

Just my warped sense of humour I suppose. 🤪🤪🤪

[John]

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 ??????

 

 

[Stu]

8 minutes ago, John said:

 

There is the confusion, right there !

Can both assertions be correct ??????

 

 

They are both saying the same thing I think John, but planetary viewing benefits from smaller airy disk size surely? Confusion reigns here too!

[johninderby]

Think I need to get this T-shirt. 🤪

 

Edited May 31, 2020 by johninderby

[John]

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

 

[andrew s]

Yes I am confused now and I think I was wrong . Can't work on it now but will do the sums and get back with the results

Regards Andrew 

 

 

[JeremyS]

Now, where's my plot gone.....? 🙂

 

[andrew s]

Ok here is my logic having checked the equations more carefully!,,,,

The angular size of the airy disk "a" is a = l/D for wave length "l"  objective diameter "D" 

The apparant angular size of an image "b" in an eyepiece is such that b/a = M = F/f where M is the magnification F and f the focal length of the objective  and eyepiece respectively. 

Thus the angular size in an eyepiece is 

b = l*M/D so at constant magnification the image has the same angular size.

You have to use angular size as the eyepiece image is at infinity. 

So I was right after all.

Regards Andrew 

PS equations from "Telescope Optics" Rutten & van Venrooij

Now corrected.

Edited May 31, 2020 by andrew s

[Stu]

19 minutes ago, andrew s said:

Ok here is my logic having checked the equations. 

The angular size of the airy disk "a" is a = l/D for wave length "l"  objective diameter "D" 

The apparant angular size of an image "b" in an eyepiece is such that a/b = M = F/f where M is the magnification F and f the focal length of the objective  and eyepiece respectively. 

Thus the angular size in an eyepiece is 

b = l/(DM) so at constant magnification the image has the same angular size.

You have to use angular size as the eyepiece image is at infinity. 

So I was right after all.

Regards Andrew 

PS equations from "Telescope Optics" Rutten & van Venrooij

Thanks Andrew. When the day cools down and my brain starts working I will see if I understand all this.

I just got confused by wondering why this thread was in the Sponsor Announcement section... only on SGL could a thread introducing a new refractor end in physics equations about airy disk size! Every day is a school day 👍👍

[andrew s]

Note I made a mistake before now corrected.

Regards Andrew 

Edited May 31, 2020 by andrew s

[F15Rules]

4 hours ago, johninderby said:

How could anyone get by with just one scope? 😁

Looking forward to finding out just how good the 80mm f/15 is. Will it find a permanemt home or will it be good try but not quite up to it? 🤔🤔🤔

I'd be interested to hear your opinion on the new 80/F15 up against the Carton 100mm F13 John..that Carton spec sounds to me like almost a perfect recipe for achromat heaven, and as a user of several vintage 1980s Carton eyepieces, I know just how good their optics were/are..

Obviously the Carton would be a good deal brighter but the comparison in the other key areas, sharpness, contrast, colour rendition etc would be fascinating.

Dave