90x Per Inch!!...How?

[JeremyS]

58 minutes ago, andrew s said:

@Sunshine I did a short note on it for the BAA. You can see it here https://www.pixelskiesastro.com/article (with permission of the BAA Journal Editor) Regards Andrew

This is an excellent article by Andrew that shows how innovative thinking can be brought to modern amateur astronomy. Inspiring stuff!

[andrew s]

4 minutes ago, JeremyS said:

I'm on the same journey, Andrew. Another text that is on my bookshelf from the 1990's is "Visual astronomy of the deep sky" by Roger N. Clark, which was, almost literally, an eye opener when it was published. Years since I've consulted it, what with those pesky CCDs getting in the way 🙂

 

Not one I have got but Anazon has one for £216.25 second hand so look after it.

Regards Andrew 

[JeremyS]

3 minutes ago, andrew s said:

Not one I have got but Anazon has one for £216.25 second hand so look after it.

Regards Andrew 

Crikey! That made me go to the bookshelf to check. It's there in excellent condition apart from a slightly faded spine.

I recall the main point was don't be afraid to use high mag on faint and fuzzies. There: I've saved you £216!

Seriously, Clarke explains how the visual system works and how high mag helps.

You are welcome to borrow it.

[Pixies]

Roger N Clarke  has his own (old) website:

https://clarkvision.com/visastro/

[Telescope40]

11 hours ago, Sunshine said:

Welcome to visual astronomy! sometimes I think some forget the joys of such an idea while lost in plate solving and such haha. Asside from the odd planetary image, my idea of astrophotography is setting up my scope, taking images of it as though it were a model, then I enjoy looking through it. Here is a favorite, it looks smashing in my kitchen, wouldn’t you say?.

Hello. May I quickly enquire about the RDF you have fitted to this lovely scope. 
Don’t think I’ve seen one before. 
regards.  John 

[Pixies]

1 minute ago, Telescope40 said:

Hello. May I quickly enquire about the RDF you have fitted to this lovely scope. 
Don’t think I’ve seen one before. 
regards.  John 

I beleive it's https://www.firstlightoptics.com/finders/celestron-starpointer-pro-finderscope.html

 

[Telescope40]

3 minutes ago, Pixies said:

I beleive it's https://www.firstlightoptics.com/finders/celestron-starpointer-pro-finderscope.html

 

Many thx.  John 

[vlaiv]

6 minutes ago, Pixies said:

I beleive it's https://www.firstlightoptics.com/finders/celestron-starpointer-pro-finderscope.html

 

I don't think I would put such thing on my scope - that design is just begging to be mistaken for a handle (Eggs! Never use word lemon next to a fine scope!).

[iPeace]

13 hours ago, vlaiv said:

Yes, but science says that for 100mm of aperture x113.2 (or less - depending on what you are using as a criteria, this is "worst" case scenario - using twice resolving power of human eye) will show you all you need in order to see it all.

That's very interesting. I have often struggled with high magnification, thinking there's more to be seen. It's a welcome thought that there just isn't (given certain variables).

I (think I) would like to understand this a bit better. How is this calculated for a given aperture? Which criteria can be used?

Provisionally assuming that the calculation renders 'max.' magnification proportional to the aperture, this would mean 67.92x for 60mm (if I'm not mistaken). While I do certainly enjoy the view at 60x-70x, it does take more magnification for me to see (notice?) certain things. Resolving (not: splitting) a double star like Izar with 60mm doesn't happen for me below 90x, it just takes that much power for me to see 'the blue dot' (Izar's secondary). Given that you can't magnify more than there is to be magnified in the first place (as presented by the scope) this would mean that my eye just cannot resolve the image to that extent below a certain magnification, even though that is more magnification than is / should be needed to see all there is to be seen...am I missing something?

To be clear, I'm not calling science into question, only trying to understand my own experiences within that which science tells us.

[PaulM]

1 hour ago, Telescope40 said:

Hello. May I quickly enquire about the RDF you have fitted to this lovely scope. 
Don’t think I’ve seen one before. 
regards.  John 

Its a Celestron Star Pointer - awful piece of kit, is very cheaply put together, unstable. I took mine off after 2 sessions with it and replaced with a Rigel Quickfinder

[vlaiv]

4 minutes ago, iPeace said:

That's very interesting. I have often struggled with high magnification, thinking there's more to be seen. It's a welcome thought that there just isn't (given certain variables).

I (think I) would like to understand this a bit better. How is this calculated for a given aperture? Which criteria can be used?

Provisionally assuming that the calculation renders 'max.' magnification proportional to the aperture, this would mean 67.92x for 60mm (if I'm not mistaken). While I do certainly enjoy the view at 60x-70x, it does take more magnification for me to see (notice?) certain things. Resolving (not: splitting) a double star like Izar with 60mm doesn't happen for me below 90x, it just takes that much power for me to see 'the blue dot' (Izar's secondary). Given that you can't magnify more than there is to be magnified in the first place (as presented by the scope) this would mean that my eye just cannot resolve the image to that extent below a certain magnification, even though that is more magnification than is / should be needed to see all there is to be seen...am I missing something?

To be clear, I'm not calling science into question, only trying to understand my own experiences within that which science tells us.

You can use several different criteria and they all give very low magnifications. This is the latest one that you can actually test "in-house".

First part of the equation is human vision and how well can we see. That is pretty much standardized and we say a person with 20/20 vision has good eyesight.

https://en.wikipedia.org/wiki/Visual_acuity

Is a good starting point, but here is important part:

In order to resolve something, we need to be able to spot a gap that is wide 1 arc minute (it talks about 6/6 above - that is same as 20/20 which is American standard - also explained in the text).

You have probably seen this (or similar chart) at some point in your life (eye exam):

In order to see where the symbol is pointing you need to be able to see the gap - which is 1 arc minute wide for 20/20.

Now that we know what constitutes a good vision, we need to match that to image that is formed in focal plane of the telescope.

Let's use simple case - Rayleigh criterion:

We say that two point sources are resolved if they look like top image. Condition being that two point sources are at Airy disk diameter away from each other. In order to see that with 20/20 vision, we need to map that to roughly three 1 arc minute segments. White / black / white pattern (star - gap - star).

Two airy disk diameters need to be 3 arc minutes wide when magnified. For 100mm of aperture, Airy disk diameter is 2.57" wide. Two of them will be 5.14" and those need to be magnified to be equal to 3x1' = 3 x 60" = 180".

Magnification is therefore 180" / 5.14" = ~ x35

This is very "strict" criteria, and I used much more relaxed criteria above - one that is comparable to images that I posted. That one relies on sampling resolution for imaging. Same math involved, but in this case, I did not use "star-gap-star" as basis for resolving - I used single pixel for resolving. With 100mm of aperture, critical sampling rate for green light at 510nm is 0.53"/px. So a single pixel is 0.53" wide and we want to be able to "see it" (with this sampling there will never be case of white pixel / dark pixel / white pixel because there will be need for multiple pixels per airy disk - about 5 or so). We want to magnify 0.53" to 60" (1 arc minute) and that is 60/0.53 = x113.2

What ever criteria you use - you actually get rather small magnifications that you need to be able to resolve things at lower bound. I gave you two examples - one very strict and one rather loose.

You can check this in house - generate star pairs or image of planet for display on screen using one of these two criteria and then stand at certain distance to the screen. Distance you need to stand away from the computer screen can be calculated based on your display DPI in order for single pixel to be 1 arc minute (or in first case if you want to actually have double star / Rayleigh criterion - you'll need to do a bit of math - again optimum sampling / pixel resolution - distance, and of course properly generated and sampled image).

 

 

[andrew s]

1 hour ago, iPeace said:

To be clear, I'm not calling science into question, only trying to understand my own experiences within that which science tells us.

I have set myself the personal goal of researching the science of visual astronomy with the aim of trying to satisfy myself  that if all the relevant science is taken into account does it explain the claims of skilled visual observers or not. 

I think a lot of the issue is that partial explanations from limited fields  (say just optics) is used and often out of context. An example might be that "a 1mm exit pupil is optimum" without context or qualifications. 

Regards Andrew 

Edited August 11, 2020 by andrew s

[andrew s]

2 hours ago, Pixies said:

Roger N Clarke  has his own (old) website:

https://clarkvision.com/visastro/

Thanks @Pixies that was a fantastic read. I still need to follow up the references but it translated a lot of what I had been reading into the astronomy context.

Thanks Andrew.

[Sunshine]

2 hours ago, Telescope40 said:

Hello. May I quickly enquire about the RDF you have fitted to this lovely scope. 
Don’t think I’ve seen one before. 
regards.  John 

It’s the Celestron star pointer, stay clear of it, it has since been replaced with a telrad, the star pointer is flimsy and shaky.

[iPeace]

@vlaiv, that is fascinating stuff, thanks!

[iPeace]

1 hour ago, andrew s said:

a 1mm exit pupil is optimum

Thanks, I'll try that.

[vlaiv]

4 minutes ago, iPeace said:

Thanks, I'll try that.

That one is in very good agreement with above "relaxed" criteria - sampling point per arc minute for critical sampling. It gives around x113 for 100mm of aperture and that equates to 100 / 113 = ~0.9mm exit pupil.

I believe that one can see detail with even less magnification, but we can say that this is "safe" lower bound of magnification, and indeed, it is no wonder that people report seeing all there is to see with very sharp image at 1mm exit pupil.

[mikeDnight]

2 hours ago, andrew s said:

I have set myself the personal goal of researching the science of visual astronomy with the aim of trying to satisfy myself  that if all the relevant science is taken into account does it explain the claims of skilled visual observers or not. 

I think a lot of the issue is that partial explanations from limited fields  (say just optics) is used and often out of context. An example might be that "a 1mm exit pupil is optimum" without context or qualifications. 

Regards Andrew 

Here's a little trick to add to your curiosities collection Andrew. I'm not sure of the reason why, but when observing linear features such as the belts of Jupiter,  if I turn my diagonal so that the belts are vertical, detail becomes just a little more certain. For whatever reason I see vertical lines better defined than when they're horizontal. It could be just my eyes, but I seem to remember a science documentary going back many years that end me to believe its more to do with how the brain works. Anyhow, its worth trying just to see if it enhances the view or not. 

[iPeace]

1 minute ago, mikeDnight said:

Anyhow, its worth trying just to see if it enhances the view or not.

Certainly is! That would be great. Will try.

[JeremyS]

1 hour ago, mikeDnight said:

if I turn my diagonal so that the belts are vertical, detail becomes just a little more certain.

Another curiosity. Visual variable star estimates can be affected by orientation of the field.

[andrew s]

2 hours ago, JeremyS said:

Another curiosity. Visual variable star estimates can be affected by orientation of the field.

Thanks @JeremyS it's clear there is a whole wealth of information on visual observing that lays forgotten in old journals. More to ferret out.

Regards Andrew 

[JeremyS]

5 minutes ago, andrew s said:

Thanks @JeremyS it's clear there is a whole wealth of information on visual observing that lays forgotten in old journals. More to ferret out.

Regards Andrew 

I hope you will write it all up at the end, Andrew 🙂

 

[F15Rules]

19 hours ago, mikeDnight said:

She's gorgeous, and the lemons add that fruity zest to the pic. 

Lovely scope, Sunshine..but is that a finder or a handle it's carrying? Or a combination of both..a fandle??😁

Dave

Edited August 11, 2020 by F15Rules

[John]

I don't know quite how this feeds into this interesting thread but one of the times when I find using very high magnifications useful is when I'm trying to see faint point sources such as the dimmer planetary moons.

When I've observed Neptune's moon Triton and Titania and Oberon at Uranus I've found that really high magnifications (400x plus) have been helpful in teasing these faint points of light out of the background sky and the glare of their parent planets.

Sometime over the next couple of months I'm hoping to use my 12 inch dobsonian in this way on Mars to try and spot Phobos and Deimos.

 

[andrew s]

18 minutes ago, John said:

I don't know quite how this feeds into this interesting thread but one of the times when I find using very high magnifications useful is when I'm trying to see faint point sources such as the dimmer planetary moons.

When I've observed Neptune's moon Triton and Titania and Oberon at Uranus I've found that really high magnifications (400x plus) have been helpful in teasing these faint points of light out of the background sky and the glare of their parent planets.

Sometime over the next couple of months I'm hoping to use my 12 inch dobsonian in this way on Mars to try and spot Phobos and Deimos.

 

It fits very well. When you increase magnification you dim the background sky as the light is spread but not for unresolved point sources thus it increases the contrast

Regards Andrew