90x Per Inch!!...How?

[Sunshine]

Recently i have been doing a whole load of planetary observing, there were a couple of nights where seeing very good, very little shimmering as i cranked up the power. On a few occasions, i have been experimenting with exactly how high i could ramp up the power while still maintaining a decent image. On one night in particular, i was floored by my ability to push power up to 90x per inch with my 5mmXW plus a 2.5 powermate while maintaining a dimmer but still sharp image of saturn, cassini division still well defined. Never would i have thought this would be possible without making a mess of the image, i have had many scopes (never a refractor before) and such high per inch powers would turn planets into blobs. it is no Tak but, Is this a trait that refractors can display? handling such high powers per inch compared to other designs? or, do i have a particularly one with better optics then lets say the same scope made a week later? one of a particularly good batch?. 

[Alkaid]

Nice! What scope?  Refractors, particularly good ones, can be pushed hard.

Edit a few days later - just seen your signature, sorry...it was the Orion triplet...

 

Edited August 11, 2020 by Alkaid

[Peter Drew]

9ox per inch is quite doable with small, high quality refractors under excellent seeing conditions.  It doesn't scale up well though, the seeing limits the ultimate magnification useable..  It is also important to consider the object to be viewed, M31 or M42 would not be vey impressive at 90x per inch but a close double star might.     🙂

[vlaiv]

4 minutes ago, Peter Drew said:

9ox per inch is quite doable with small, high quality refractors under excellent seeing conditions.  It doesn't scale up well though, the seeing limits the ultimate magnification useable..  It is also important to consider the object to be viewed, M31 or M42 would not be vey impressive at 90x per inch but a close double star might.     🙂

I have to ask - what defines usable magnification?

On a both good and bad scope in good and poor seeing conditions - after certain threshold image just stays the same only magnified. Yes, it will get dimmer, but regarding the detail - it will remain more or less the same (at some point image will be too dim and some features will start disappearing - but I don't believe we are reaching that threshold in this case).

At what point in this magnifying process do we say - ok, now image is no longer "usable"?

Let's consider these:

First one can be considered I don't know x100 with 8" scope when you are suitable distance from computer screen (depends on pixel size of your computer), and others are respectively x200, x300 and x400.

Which one of these is no longer usable?

[Peter Drew]

All of those images I wold find usable, I would love to have an image as good as that 400x version, unfortunately my local seeing doesn't support it.      ☹️

[Stu]

As long as adding image scale is helping you to see detail in the image then there is some merit in high mags. Small, high quality apo refractors (60 to 120mm roughly) very much break the normal rules in terms of mag/mm.

I do get a little frustrated when I see posts suggesting that an exit pupil of 1mm is the minimum or saying that using a mag the same as the aperture in mm should be the maximum Eg x200 in a 200mm scope). These ‘rules’ might apply to larger newts and compound scopes, but if I only ever used x100 with my 4” apos I would miss a huge part of their capability. Of course they are excellent at low power views too, but when conditions permit are wonderful at high power on the Moon, planets, doubles and the sun.

[vlaiv]

6 minutes ago, Stu said:

As long as adding image scale is helping you to see detail in the image then there is some merit in high mags. Small, high quality apo refractors (60 to 120mm roughly) very much break the normal rules in terms of mag/mm.

I do get a little frustrated when I see posts suggesting that an exit pupil of 1mm is the minimum or saying that using a mag the same as the aperture in mm should be the maximum Eg x200 in a 200mm scope). These ‘rules’ might apply to larger newts and compound scopes, but if I only ever used x100 with my 4” apos I would miss a huge part of their capability. Of course they are excellent at low power views too, but when conditions permit are wonderful at high power on the Moon, planets, doubles and the sun.

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.

[Kevin Kretsch]

I feel like magnifying the blurryness makes my brain unhappy. It "wants" to see the details at a certain scale. Getting too far away from that doesn't help. For me, I'd vote image no 2 or 3. 1 is nice but too small to see the details, and 4 is too big, just looks blurry even though it has the same amount of detail. Throw in the loss of brightness and I'll vote for number 2.

FWIW, I've never successfully used more than ~40x per inch with Jupiter in my ETX-90. And I don't recall a tight double within Rayleigh criterion that I couldn't split at 25x per inch.

[vlaiv]

Actually, I just realized there is one factor that can mess up image at higher magnifications - and it's not the scope nor the eyepiece.

Scope produces same image at focal plane regardless of what eyepiece is used to magnify it. For this reason, it can't be scopes fault if we see magnified image as worse than it should be when magnified.

Second suspect is eyepiece - and indeed, not all eyepieces are equally sharp, but I think that we can rule out this one as well - there are plenty observers that report on this that have expensive glass and very sharp eyepieces in short focal lengths.

Third reason that I can think of is human eyeball.

When we say someone has 20/20 vision (and we base our calculations of what can be seen on that) - that figure is obtained by measurement performed in such way that wave front hits quite a bit of eye lens. In normal daytime / artificial light of exam room - it should be something like 3-4mm, right?

Bring that down to 1mm and many observers that suffer astigmatism in above conditions start observing without problems - at 1mm aperture - eye is not out of shape locally enough to cause issues associated with astigmatism.

But smaller exit pupil than this - above 0.5mm could be suffering from another problem - local wavefront aberrations caused by physical defect being now larger relative to pupil size. Similarly to that how smaller apertures suffer less from seeing and dominant aberrations being tilt - here it would be somewhat different due to eye placement - with such small exit pupil - rays at different angles will hit eye lens in different spots and possibly experience different level of tilt - which compounds to produce blur at this level of scale?

Not sure if above makes sense, but could be something to it? Exit pupil too small - not only image gets dimmer but also blurrier due to eye?

[Sunshine]

Understanding everything mentioned above, i am not one to enjoy magnifying an image and, sacrificing detail or image quality. Sometimes, i find i can increase image scale far beyond what i thought possible based on other scopes I've owned and still maintain well defined edges and detail. Yes, the image does dim, i expect that.

[Stu]

48 minutes 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.

Science can say what it likes, I get benefit from using up to x200 or even x300 with my 100mm scopes so I’ll continue to do that!

I know you are not seeing any additional detail, but the detail there is easier to view because of the extra image scale.

[vlaiv]

1 minute ago, Stu said:

Science can say what it likes, I get benefit from using up to x200 or even x300 with my 100mm scopes so I’ll continue to do that!

I know you are not seeing any additional detail, but the detail there is easier to view because of the extra image scale.

Don't get me wrong - I'm not implying that you should stop using magnification that suits you just because science says so.

I'm just trying to figure out why people report things that they report. I have quite good understanding why would someone use higher magnification - it is easier to see things and eye/brain does not need to work as hard and not everyone has 20/20 vision.

What I don't understand is - why people report that too much magnification "breaks down" the image. From telescope perspective - it projects the same image of focal plane and does not really care what sort of eyepiece is used to magnify the image, so there is no change in image itself with magnification (one that telescope produces) - hence it can't "fall apart".

But it looks like there could be both real physical reasons (exit pupil size vs eye shape) and psychological reasons for image break down (brain can't tolerate certain level of blurriness and dim image).

[mikeDnight]

36 minutes ago, Sunshine said:

Understanding everything mentioned above, i am not one to enjoy magnifying an image and, sacrificing detail or image quality. Sometimes, i find i can increase image scale far beyond what i thought possible based on other scopes I've owned and still maintain well defined edges and detail. Yes, the image does dim, i expect that.

You no doubt have an excellent refractor Sunshine, and there are many out there, not all are Tak's. Personally, I don't worry about the math or science, I know what an acceptable image looks like, and I instantly know when I've tipped the scale. I'd say continue pushing the mag and increasing the image scale. Things will vary according to seeing conditions and the chosen target of course, but more often than not, being adventurous in this was will pay off on many occasions. A few nights ago i observed Mars while it was still relatively low in the east and in less that first class seeing, yet I was able to use 400X on my 100mm frac and still retain an acceptably well defined image showing some really nice detail. Crazy isn't it!

[Sunshine]

35 minutes ago, mikeDnight said:

You no doubt have an excellent refractor Sunshine, and there are many out there, not all are Tak's. Personally, I don't worry about the math or science, I know what an acceptable image looks like, and I instantly know when I've tipped the scale. I'd say continue pushing the mag and increasing the image scale. Things will vary according to seeing conditions and the chosen target of course, but more often than not, being adventurous in this was will pay off on many occasions. A few nights ago i observed Mars while it was still relatively low in the east and in less that first class seeing, yet I was able to use 400X on my 100mm frac and still retain an acceptably well defined image showing some really nice detail. Crazy isn't it!

Agreed! My point exactly, I was always convinced that after about 50x per inch, things went downhill very fast, hadn't even attempted pushing that  envelope. Now i realize i can go well beyond it as long as both seeing permits and, i am happy with the image. That being said, i would be curious to see how different designs perform side by side at a given power per inch. Would a given target 90x per inch look as sharp and defined in an 8" SCT and a 5" frac side by side? keeping in mind the difference in magnification but, this should be offset by light gathering capability, all relevant of course.

[andrew s]

The human visual system is a lot more than the optics and receptors in the eye. We learn to see. As a learnt skill we will all develop our prefered strategy. 

As an example I tend to pull away from blured or pixellated images to try to sharpen them up. I am currently reintroducing myself to visual astronomy and it will be interesting to see how this develops.

Regards Andrew 

Edited August 10, 2020 by andrew s

[Sunshine]

8 minutes ago, andrew s said:

The human visual system is a lot more than the optics and receptors in the eye. We learn to see. As a learnt skill we will all develop our prefered strategy. 

As an example I tend to pull away from blured or pixellated images to try to sharpen them up. I am currently reintroducing myself to visual astronomy and it will be interesting to see how this develops.

Regards Andrew 

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?.

Edited August 10, 2020 by Sunshine

[andrew s]

22 minutes 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?.

 

Whatever floats your boat I take between 1000 an 2000 images/spectra a night of the same star  to try to catch it flaring. Regards Andrew 

[Sunshine]

3 minutes ago, andrew s said:

Whatever floats your boat I take between 1000 an 2000 images/spectra a night of the same star  to try to catch it flaring. Regards Andrew 

That is an observation I have never heard about, interested in knowing more about that if you care to elaborate.

[mikeDnight]

48 minutes 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?.

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

[Geoff Barnes]

1 hour ago, mikeDnight said:

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

Are they lemons? I see them as eggs! 🤔

[Sunshine]

15 minutes ago, Geoff Barnes said:

Are they lemons? I see them as eggs! 🤔

Those would be some big eggs! lol, those are fake lemons, a decorative thing my wife used to fill up counter space a bit.

[Marvin Jenkins]

I tried a few different filters and eye pieces and I am not convinced. Perhaps on a night of better seeing I could be persuaded they are eggs.

[andrew s]

9 hours ago, Sunshine said:

That is an observation I have never heard about, interested in knowing more about that if you care to elaborate.

@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

[andrew s]

As part of my reintroduction to visual astronomy I have been reading "Foundations of Vision" by BW Wandell. It's fascinating stuff at least to me. One, amongst many, things I just learnt was that vision goes from trichromatic to dichromatic to monochromatic with both increasing spatial and temporal frequency. Put along side the loss of colour perception with lowering light intensity it's no wonder we all have different  experiences with magnification. 

Regards Andrew 

PS They look like lemons to me, at a distance, with the lights off.

Edited August 11, 2020 by andrew s

[JeremyS]

33 minutes ago, andrew s said:

As part of my reintroduction to visual astronomy I have been reading "Foundations of Vision" by BW Wandell. It's fascinating stuff at least to me. One, amongst many, things I just learnt was that vision goes from trichromatic to dichromatic to monochromatic with both increasing spatial and temporal frequency. Put along side the loss of colour perception with lowering light intensity it's no wonder we all have different  experiences with magnification. 

Regards Andrew 

PS They look like lemons to me, at a distance, with the lights off.

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 🙂