Optical quality vs seeing

[jetstream]

Just now, John said:

That is a good point. Many features are more about contrast differences rather than angular resolution. I think this is where Modulation Transfer Function MTF comes into play but it is also where my knowledge peters out so I can't "shed any light" on MTF really 😄

Yes, I believe it to be about MTF as well possibly. I hope through conversation here, ideas about it and how seeing can potentially affect these certain features as described by spacial frequency sheds some light on it. I think there might be something to it as the MTF graph describes optical quality in a different but related way to the idea of "diffraction limited or above" etc.

[globular]

39 minutes ago, jetstream said:

Excellent observation Michael. Most people attribute this to cooling, thermal issues which is a factor. Another factor, possibly overlooked is the average optical quality of SCT's. I know some who have tested them and most are just below diffraction limited and a few at or just above. Combine this with the cooling factor and the seeing issues being discussed it does not surprise me at all that your dob shows better and more consistently I presume.

I'm not trying to disrespect SCTs or their owners by my comments, but many have reported the same thing as you.

I suspect another overlooked, and perhaps with the biggest impact too, thing when using SCT is focussing.  People seems to think that because the focuser will let you move the mirror a long way to bring things to focus, that it's fine if they do this - and so they don't think about the optimal position of their mirror and just focus as much as needed.

In my SCT an addition of 10mm to the focal length introduces 0.25 wave aberration.  And 10mm to the focal length is achieved when the focal plane is only 3.5mm away from optimal. Not many turns of the focus knob at all.

I have parfocalised all my eyepieces and have a number of diagonals and adapters pre-configured so that all my viewing, whether cyclops or in a binoviewer, with or without and ADC, with or without filters, all take place with my mirror in it's optimal position.

I am still impacted by seeing, like everyone else, but I do not consider that my SCT is significantly more susceptible to bad seeing than other telescope designs.

YMMV

[jetstream]

2 minutes ago, globular said:

I suspect another overlooked, and perhaps with the biggest impact too, thing when using SCT is focussing.  People seems to think that because the focuser will let you move the mirror a long way to bring things to focus, that it's fine if they do this - and so they don't think about the optimal position of their mirror and just focus as much as needed.

In my SCT an addition of 10mm to the focal length introduces 0.25 wave aberration.  And 10mm to the focal length is achieved when the focal plane is only 3.5mm away from optimal.

This is an excellent point and my buddy YKSE was big on this years ago. Your method of focusing preserves the optical quality of your SCT and the Edge models are reputedly of higher optical spec. I didnt mean to disrespect your scope with my previous comment.

[globular]

39 minutes ago, jetstream said:

This is an excellent point and my buddy YKSE was big on this years ago. Your method of focusing preserves the optical quality of your SCT and the Edge models are reputedly of higher optical spec. I didnt mean to disrespect your scope with my previous comment.

No problem.  I quickly developed a thick skin while browsing the forum - there does seem to be a lot of people doing a lot of 'looking down noses' at SCTs. I accept that there does seem to be a large number of poor quality SCTs in existence (or a lot of people using them sub-optimally perhaps?) but I don't really understand why that means the SCT as a telescope design gets so devalued.  They have their pros and cons - like all scope do.  Normally I just read the comments, allow myself a knowing smile and move on.  I replied this time because I hoped it would add something to your thread - not because I felt disrespected.

[Deadlake]

2 hours ago, jetstream said:

Yes, I believe it to be about MTF as well possibly. I hope through conversation here, ideas about it and how seeing can potentially affect these certain features as described by spacial frequency sheds some light on it. I think there might be something to it as the MTF graph describes optical quality in a different but related way to the idea of "diffraction limited or above" etc.

For planetary observations some of the best planetary views I had are by attaching a Barlow on the scope ahead of the diagonal (now running at F20) and using a 10 mm UFF EP or some BV's.

This helps with bring up contrast from observation and also brings out the reds on Jupiter.

[jetstream]

41 minutes ago, Deadlake said:

For planetary observations some of the best planetary views I had are by attaching a Barlow on the scope ahead of the diagonal (now running at F20) and using a 10 mm UFF EP or some BV's.

This helps with bring up contrast from observation and also brings out the reds on Jupiter.

I am always very interested in reports of seeing colour in Jupiters bands or lack of. I see ruddy red in the bands with every scope I have and my reflectors do a great job and with the 15" really opening them up with very fine riffles in them. I really like observing Jupiter just at dusk and afterwards a bit. I tend to get the best views then but I only really obs it when well placed.

I'm always puzzled when some cant seem to get much colour out of Jupiter. Theres so much in there, white ovals, barges and the hooked purple spikes aka festoons.

[Deadlake]

4 minutes ago, jetstream said:

I am always very interested in reports of seeing colour in Jupiters bands or lack of. I see ruddy red in the bands with every scope I have and my reflectors do a great job and with the 15" really opening them up with very fine riffles in them. I really like observing Jupiter just at dusk and afterwards a bit. I tend to get the best views then but I only really obs it when well placed.

I'm always puzzled when some cant seem to get much colour out of Jupiter. Theres so much in there, white ovals, barges and the hooked purple spikes aka festoons.

I has to experiment with different EP’s and diagonals. However on nights of good seeing all is possible. 

Sometime best to have multiple scopes out to experiment with Barlow or BV’s side by side…

[jetstream]

Just now, Deadlake said:

I has to experiment with different EP’s and diagonals. However on nights of good seeing all is possible. 

Sometime best to have multiple scopes out to experiment with Barlow or BV’s side by side…

Yes I get puzzled when some have to experiment to get colour unless the seeing is a major factor. Out H130 gives a deep ruddy colour and I use a NZ 3-6 zoom mostly in it. I really dont have a clue why seeing colour in Jupter is an issue. When its high up and very bright- highly contrasted here because of the very dark sky and overpowering, I do a couple of things. One is to use a slower scope to use the smaller exit pupil to advantage. The second is to use binoviewers where the beamsplitter halves the volume of light to each eye making detail much easier to see. The best view of Jupiter I have had was just over 300x in the 15" with the Binotron 27's. It was like looking at a good image , a very good image.

The third thing is obs at sunset and just after using the lighter sky to tame down Jupiters brightness. This is just me however but it does work for me.

[Deadlake]

12 minutes ago, jetstream said:

Yes I get puzzled when some have to experiment to get colour unless the seeing is a major factor. Out H130 gives a deep ruddy colour and I use a NZ 3-6 zoom mostly in it. I really dont have a clue why seeing colour in Jupter is an issue. When its high up and very bright- highly contrasted here because of the very dark sky and overpowering, I do a couple of things. One is to use a slower scope to use the smaller exit pupil to advantage. The second is to use binoviewers where the beamsplitter halves the volume of light to each eye making detail much easier to see. The best view of Jupiter I have had was just over 300x in the 15" with the Binotron 27's. It was like looking at a good image , a very good image.

The third thing is obs at sunset and just after using the lighter sky to tame down Jupiters brightness. This is just me however but it does work for me.

In the UK really need the planets high up, last few years low down and large depth of atmosphere to peer thru..

[jetstream]

44 minutes ago, Deadlake said:

In the UK really need the planets high up, last few years low down and large depth of atmosphere to peer thru..

Here too we're at the same latitude -me- I observe other stuff until the planets are higher. Even the TSA120 had false colour obs Jupiter when very low, just AD.

[jetstream]

2 hours ago, Deadlake said:

For planetary observations some of the best planetary views I had are by attaching a Barlow on the scope ahead of the diagonal (now running at F20) and using a 10 mm UFF EP

I think the lower .55mm exit pupil that your barlow gives is the reason behind your better views and not the f20 part. This is a great way to get detail under challenging conditions!  Excellent!

[JeremyS]

1 hour ago, jetstream said:

I am always very interested in reports of seeing colour in Jupiters bands or lack of. I see ruddy red in the bands with every scope I have and my reflectors do a great job and with the 15" really opening them up with very fine riffles in them. I really like observing Jupiter just at dusk and afterwards a bit. I tend to get the best views then but I only really obs it when well placed.

I'm always puzzled when some cant seem to get much colour out of Jupiter. Theres so much in there, white ovals, barges and the hooked purple spikes aka festoons.

I was shocked by the colour in Jupiter’s bands when I first turned my M210 to him.

[jetstream]

1 minute ago, JeremyS said:

I was shocked by the colour in Jupiter’s bands when I first turned my M210 to him

A perfect scope for it, great optics and enough focal ratio to provide a low exit pupil should you need it and also a great image scale all round. Is the M210 a tak? Shhh, we need a code word.

[JeremyS]

2 minutes ago, jetstream said:

A perfect scope for it, great optics and enough focal ratio to provide a low exit pupil should you need it and also a great image scale all round. Is the M210 a tak? Shhh, we need a code word.

I couldn’t possibly say 🤣🤣🤣

[John]

6 minutes ago, JeremyS said:

I was shocked by the colour in Jupiter’s bands when I first turned my M210 to him.

When Jupiter was higher in the sky in past years I've had a similar shock observing with my 12 inch F/5.3 dobsonian. Especially when there was still plenty of light left in the sky - the colours seemed even more startling then.  

[Captain Scarlet]

31 minutes ago, John said:

When Jupiter was higher in the sky in past years I've had a similar shock observing with my 12 inch F/5.3 dobsonian. Especially when there was still plenty of light left in the sky - the colours seemed even more startling then.  

I only started observing when Jupiter was down at the start of its “low years”. Now it’s going to be higher and higher over the next 2-3 years I cannot wait, especially as I now have that much more experience and equipment. I am one of those who has still not had anything near a startling view of the planet. I have properly just about discerned the GRS and its colour only once.

Edited April 3, 2023 by Captain Scarlet

[John]

22 minutes ago, Captain Scarlet said:

I only started observing when Jupiter was down at the start of its “low years”. Now it’s going to be higher and higher over the next 2-3 years I cannot wait, especially as I now have that much more experience and equipment. I am one of those who has still not had anything near a startling view of the planet. I have properly just about discerned the GRS and its colour only once.

Your lovely new 140mm refractor will really be able to "strut it's stuff" on the planets soon Magnus 👍

[Geoff Barnes]

I guess I was incredibly fortunate when I first bought my 12 inch  Dobsonian that the likes of Jupiter Mars and Saturn were very high in the sky, over 70 degrees altitude down here in Oz. The resulting views were amazing, so much detail and colour. Now they are rapidly lowering in the sky and while still not bad, the views are visibly poorer most of the time and it is a struggle to tease out the details I saw a couple of years ago. If the seeing is poor it's not even worth bothering and I tend to stick to doubles and clusters now.

[jetstream]

1 hour ago, Captain Scarlet said:

I only started observing when Jupiter was down at the start of its “low years”. Now it’s going to be higher and higher over the next 2-3 years I cannot wait, especially as I now have that much more experience and equipment. I am one of those who has still not had anything near a startling view of the planet. I have properly just about discerned the GRS and its colour only once.

Wait until you get the SV 140 on Jupiter...and also your excellent dobs.  I cant wait for you to get at the planets when high up Magnus!

[jetstream]

On 03/04/2023 at 16:46, Captain Scarlet said:

I only started observing when Jupiter was down at the start of its “low years”. Now it’s going to be higher and higher over the next 2-3 years I cannot wait, especially as I now have that much more experience and equipment. I am one of those who has still not had anything near a startling view of the planet. I have properly just about discerned the GRS and its colour only once.

Hey!! You got a bigger scope banner than me!!😬

                                       

[dan_adi]

On 03/04/2023 at 15:50, jetstream said:

Heres a question that is not meant to be controversial.

Over the years I have heard others, respected observers having had many scopes and having high end glass report that the better the optical quality of the scope the more resistant it is to poor seeing. The reports Ive seen refer to refractors basically .

I have zero interest in "bias" and dont want it brought up here. I do have a great interest in this idea of optical quality vs seeing and would appreciate anyones thoughts or experiences with this. Myself, I dont know and am going to test it out coming up. Ive never really tested it out because as seeing deteriorates I go back in the house.

Gerry

Seeing affects all scopes. If you have 3" seeing and the best refractor in the world you will not get 3" fwhm stars. This is because there are other sources of error that can add up during an exposure, like tracking error, focus error, collimation (mostly for reflectors), focuser sagging, loose connections, wind, etc. 

I image with a 8 inch apo refractor, and if the reported seeing for the night is for example 1.5-1.8 ", my stars have a median fwhm between 2.2-2.8". The scope is paired with a good mount (Mesu) and a good focuser (Optec Leo). 

I don't know how this imaging experience translates to visual observing, but my guess is the lower quality scope will add more errors, so if you compare 2 scopes, in the same bad seeing situation, I would expect the higher quality glass to pull ahead, but by how much? 

So, if you plan to upgrade your scope to a better one, do it for the nights with good seeing, not for the nights with bad seeing. In good seeing the higher quality one will pull ahead for sure

 

Edited April 8, 2023 by dan_adi

[IB20]

What I do know is that my 3” scope is affected less than a 4 and my 8” scopes when seeing is challenging. When seeing is complete garbage then it really doesn’t matter what scope I pull out.

[niallk]

Can I ask a probably silly question please (& apologies for veering a bit off topic): when it's said that a smaller aperture suffers less than larger aperture in moderate to poor seeing, do people mean comparing the views at similar magnification?  I presume the answer is yes...

I tend to check the jetstream forecast before committing to cooling my 15" dob if I want to look at planets.  Usually i can then use >= 200x on Jupiter, but I've often been intrigued by all the advocates for a 4" frac!

Edited April 12, 2023 by niallk