Refractor for planets, mostly visual, 4" or 5"

[Paz]

Regarding diagonals, when I was reading up on binoviewers I recall reading that the greater the distance between the diagonal and the eyepiece the more of a challenge is presented to the diagonal and the quality of the diagonal matters more.

If that is the case then if testing/comparing diagonals I wonder if maximising the diagonal to eyepiece distance would help to discriminate between them, e.g. put extensions between the eyepiece and diagonal and rack the focuser in to compensate?

[Peter Drew]

The further away from the eyepiece is from the diagonal then the further into the focal point the diagonal will need to be to achieve focus.  Therefore more of the diagonal optical surface will be in use and better the diagonal quality needs to be.    🙂

[mikeDnight]

On 02/02/2021 at 18:22, Deadlake said:

This link was of interest to me:

https://www.damianpeach.com/simulation.htm

The effect of seeing cannot be underestimated. However who are we kidding, if we could we would end up with both scopes a 4-5" Refractor and a large SCT/Reflector for those exceptional nights...

Short answer scope of 10-15 cm in size deal well with poor seeing...

But then, for the refractor nut, on those exceptional nights the 10 - 15cm refractor would still be the instrument of choice, as it will be even better than it is on a mediocre night. 🔭

[jetstream]

45 minutes ago, mikeDnight said:

But then, for the refractor nut, on those exceptional nights the 10 - 15cm refractor would still be the instrument of choice, as it will be even better than it is on a mediocre night. 🔭

Hey Mike, would you still talk to me if I bought one of those new Takyhashi reflectors? A Mewlon 250CRS?😃

[Voyager 3]

1 hour ago, jetstream said:

Hey Mike, would you still talk to me if I bought one of those new Takyhashi reflectors? A Mewlon 250CRS?😃

No I assume 😎 .

[Deadlake]

7 hours ago, jetstream said:

Hey Mike, would you still talk to me if I bought one of those new Takyhashi reflectors? A Mewlon 250CRS?😃

If I could permanently mount it a reflector would be on my list, given the cost of the mount as well for a 180mm APO to equal a large SCT on goog nights.

https://www.cloudynights.com/articles/cat/user-reviews/telescopes/schmidt-cassegrains-scts/a-c14-confronts-an-ap-180-r881
 

7 hours ago, mikeDnight said:

But then, for the refractor nut, on those exceptional nights the 10 - 15cm refractor would still be the instrument of choice, as it will be even better than it is on a mediocre night. 🔭

Seeing determines the maximum aperture for visual as you well know:

https://www.cloudynights.com/articles/cat/articles/how-to/what-is-the-best-planetary-telescope-r402

The amount error added to an image is culmative, so poor glass will make average seeing poor.

We still don’t know how good a lense needs to be to suuport quality of seeing presentat a location.

Aperture and quality we could be over buying???

 

Edited February 8, 2021 by Deadlake

[vlaiv]

2 hours ago, Deadlake said:

The amount error added to an image is culmative

I would not quite put it like that. Sometimes two errors cancel each other out and you get perfect optics. If you have spherically under corrected optics - you can insert corrector that over corrects and if two are matched - you'll get perfect correction (think catadioptric telescopes - spherical mirror + corrector)

There is no guarantee that two match - if phase shift due to seeing is matching optics - you'll get better image, but if its opposite - you'll get worse image (in cumulative way).

Since seeing is random and ever changing - I would say that errors, at least in wavefront - add like any sort of noise - "in quadrature". Not sure how that translates to perceived image quality.

[mikeDnight]

11 hours ago, jetstream said:

Hey Mike, would you still talk to me if I bought one of those new Takyhashi reflectors? A Mewlon 250CRS?😃

I'd move in next door! 😆

[Deadlake]

1 hour ago, vlaiv said:

Since seeing is random and ever changing - I would say that errors, at least in wavefront - add like any sort of noise - "in quadrature". Not sure how that translates to perceived image quality.

If seeing is random then I'd imagine there would be a distribution of errors associated with the waves interaction with the atmosphere before the lights touches the telescopes optics.

The error must be predictive in nature otherwise active optics would not work.

Which brings be to correctors you've mentions, they have to be active in nature, not something I could modfiy a C11 with.

[Alan White]

5 minutes ago, mikeDnight said:

I'd move in next door! 😆

So you can have a peak through the Reflector? or
So you can allow a peak through your Refractor when the seeing is too poor for a larger scope?
 

[JeremyS]

28 minutes ago, Deadlake said:

The error must be predictive in nature otherwise active optics would not work.

 

Do you mean adaptive optics? Adaptive optics help respond to atmospheric distortion. Telescopes with such systems measure the atmospheric distortion and modify the optical train in real time, I.e they are not predictive. They operate at 100 to 1000 Hz.

Active optics optics on the other had change the optical system to prevent distortions of the optics (usually the mirror) being deformed by e.g. gravity as the telescope load shifts when it is moved across the sky. Response is slower, typically 1 Hz. This can be predictable 

[vlaiv]

1 minute ago, Deadlake said:

If seeing is random then I'd imagine there would be a distribution of errors associated with the waves interaction with the atmosphere before the lights touches the telescopes optics.

Yes, I've seen some tables - but I think that it depends on particular night and location

1 minute ago, Deadlake said:

The error must be predictive in nature otherwise active optics would not work.

I think that prediction is only in very narrow window. Active / Adaptive optic works similar to guiding.

It tracks wavefront deformation on either a star with primary or secondary optics - Active (telescope or guide scope) or projected laser beam - Adaptive.

Issue with both of them is that they are useful over very narrow field of view - much less than planet size for example.

Take a look at this video:

https://www.youtube.com/watch?v=cazY2gKqQrw

You will see that planet is "boiling" - or wobbling

See those deformed edges? That is because tilt component of the seeing wavefront error - is different (tilt PSF moves star or point position away from its true location).

This shows that every single point in the image is affected by different wave front aberration. This is the same for optical aberrations of the system. Some of them depend on distance from optical axis / angle - like coma for example in Newtonians with parabolic mirror.

Optics might be perfect - strehl 1.0  - but move away from the optical axis and you'll get increasing coma.

In the end, it is forth noting that there is temporal relationship between seeing aberrations between points in the image. "Seeing moves" across the image - in multiple layers - because it is generated by multiple layers in atmosphere and in general air moves in atmosphere.

If we want to simulate effects of the seeing - we need to account of all of those things:

1. fact that it is random

2. fact that every point in the image of the planet is affected by different wavefront aberration

3. fact that "seeing moves" in multiple layers.

It is rather simple to generate some sort of seeing aberration. For example, it is enough to do this:

I just create random wavefront and it results in this:

Question is, how likely is that above seeing aberration is going to be encountered on particular night at any given moment?

 

[andrew s]

I am working on a Shack Hartman wave front sensor coupled to a deformable contact lens to allow visual observers to benefit form adaptive optics. This will finally put the refractor v reflector issue to bed. Aperture will win.

Regards Andrew

PS crowdfunding post to follow.

👹

[Deadlake]

13 minutes ago, JeremyS said:

Do you mean adaptive optics? Adaptive optics help respond to atmospheric distortion. Telescopes with such systems measure the atmospheric distortion and modify the optical train in real time, I.e they are not predictive. They operate at 100 to 1000 Hz.

Active optics optics on the other had change the optical system to prevent distortions of the optics (usually the mirror) being deformed by e.g. gravity as the telescope load shifts when it is moved across the sky. Response is slower, typically 1 Hz. This can be predictable 

Depends on definition of predictive. Adaptive optics in this case is using a wavefront-sensor to calculate the mean wavefront perturbation in each pixel in the sensor to correct for the atmosphere. Predictive in this sense is a non-random algorithm determining the perturbation to be applied. The atmospheric perturbation being driven by measurement in this case.

Edited February 8, 2021 by Deadlake

[vlaiv]

1 minute ago, Deadlake said:

Depends on definition of predictive. Adaptive optics in this case is using a wavefront-sensor to calculate the mean wavefront perturbation in each pixel in the sensor to correct for the atmosphere. Predictive in this sense is a not-random algorithm determining the perturbation to be applied. The atmospheric perturbation being driven by measurement in this case.

It does not work like that.

You are correct that Adaptive optics compensates for wavefront error - but it does not happen on pixel level - that is impossible. It happens on level of the optics - hence the name adaptive optics - optics adapts to wavefront and counters it so that combined error is 0 - or optics cancels wavefront error created by atmosphere.

This can only happen up to a point because we don't have infinite control over optical surfaces - there is finite number of actuators that bend mirror into wanted shape - and you are right - that happens deterministically, but it happens only after we have made a measurement of atmospheric wavefront deformation - so it is not predictive  - it is reactive - much like guiding - we react after we have detected error.

 

[andrew s]

Somewhat more seriously,  I have been looking at the recent literature on the effect of the atmosphere on astronomy.

They split the issue into two, seeing and scintillation.  Seeing is caused by a tilt in the wavefront shifting the object in the focal plane.  While scintillation is caused by curvature in the wavefront focusing and refocusing the light. 

Correcting tilt is simpler than correcting curvature.

The latter might sting a bit on the eyeballs (see previous post).

Regards Andrew 

[Deadlake]

1 minute ago, vlaiv said:

It does not work like that.

You are correct that Adaptive optics compensates for wavefront error - but it does not happen on pixel level - that is impossible. It happens on level of the optics - hence the name adaptive optics - optics adapts to wavefront and counters it so that combined error is 0 - or optics cancels wavefront error created by atmosphere.

This can only happen up to a point because we don't have infinite control over optical surfaces - there is finite number of actuators that bend mirror into wanted shape - and you are right - that happens deterministically, but it happens only after we have made a measurement of atmospheric wavefront deformation - so it is not predictive  - it is reactive - much like guiding - we react after we have detected error.

 

Wiki entry:

in order to perform adaptive optics correction, the shape of the incoming wavefronts must be measured as a function of position in the telescope aperture plane. Typically the circular telescope aperture is split up into an array of pixels in a wavefront sensor, either using an array of small lenslets (a Shack–Hartmann wavefront sensor), or using a curvature or pyramid sensor which operates on images of the telescope aperture. The mean wavefront perturbation in each pixel is calculated. This pixelated map of the wavefronts is fed into the deformable mirror and used to correct the wavefront errors introduced by the atmosphere. It is not necessary for the shape or size of the astronomical object to be known – even Solar System objects which are not point-like can be used in a Shack–Hartmann wavefront sensor, and time-varying structure on the surface of the Sun is commonly used for adaptive optics at solar telescopes. The deformable mirror corrects incoming light so that the images appear sharp.

Pixel in this case relates to an array of lenselet sensors...

[andrew s]

5 minutes ago, vlaiv said:

It does not work like that.

You are correct that Adaptive optics compensates for wavefront error - but it does not happen on pixel level - that is impossible. It happens on level of the optics - hence the name adaptive optics - optics adapts to wavefront and counters it so that combined error is 0 - or optics cancels wavefront error created by atmosphere.

This can only happen up to a point because we don't have infinite control over optical surfaces - there is finite number of actuators that bend mirror into wanted shape - and you are right - that happens deterministically, but it happens only after we have made a measurement of atmospheric wavefront deformation - so it is not predictive  - it is reactive - much like guiding - we react after we have detected error.

 

I think you misunderstood.  It does sense the error on the pixels of the Shack  Hartmann sensor. It then corrects via deforming the optics.

Regards Andrew 

Edited February 8, 2021 by andrew s

[vlaiv]

11 minutes ago, andrew s said:

I am working on a Shack Hartman wave front sensor coupled to a deformable contact lens to allow visual observers to benefit form adaptive optics. This will finally put the refractor v reflector issue to bed. Aperture will win.

Regards Andrew

PS crowdfunding post to follow.

👹

I'm interpreting this to be sarcasm - you can't correct for multiple wavefront deformations at the same time - adaptive optics works only on a single point in FOV perfectly. Planet size objects will have very different wavefront aberrations from one edge to another.

[andrew s]

1 minute ago, vlaiv said:

I'm interpreting this to be sarcasm - you can't correct for multiple wavefront deformations at the same time - adaptive optics works only on a single point in FOV perfectly. Planet size objects will have very different wavefront aberrations from one edge to another.

Sorry it was intended as humour, ligh relief. 

No offense was intended to anyone. The emoji was ment to signal humour. 

Regards Andrew 

[vlaiv]

1 minute ago, andrew s said:

I think you misunderstood.  It does sense the error on the pixels of the Shack  Hartmann sensor. It the corrects via deforming the optics.

Regards Andrew 

Could be. I just responded to what was written - don't know how Shack Hartmann sensor work - will need to look it up.

Ah, ok - fair enough - it is easy to understand and it is simple in its operation.

6 minutes ago, Deadlake said:

Wiki entry:

in order to perform adaptive optics correction, the shape of the incoming wavefronts must be measured as a function of position in the telescope aperture plane. Typically the circular telescope aperture is split up into an array of pixels in a wavefront sensor, either using an array of small lenslets (a Shack–Hartmann wavefront sensor), or using a curvature or pyramid sensor which operates on images of the telescope aperture. The mean wavefront perturbation in each pixel is calculated. This pixelated map of the wavefronts is fed into the deformable mirror and used to correct the wavefront errors introduced by the atmosphere. It is not necessary for the shape or size of the astronomical object to be known – even Solar System objects which are not point-like can be used in a Shack–Hartmann wavefront sensor, and time-varying structure on the surface of the Sun is commonly used for adaptive optics at solar telescopes. The deformable mirror corrects incoming light so that the images appear sharp.

Pixel in this case relates to an array of lenselet sensors...

Sure - just checked it. My bad - I thought that you were talking about pixels in focal plane of telescope - but here Pixel refers to Lenslet sensor as a whole - or subdivision of aperture.

 

[vlaiv]

Just now, andrew s said:

Sorry it was intended as humour, ligh relief. 

No offense was intended to anyone. The emoji was ment to signal humour. 

Regards Andrew 

No offense taken - I was just confirming if I got it right

[Deadlake]

9 minutes ago, vlaiv said:

I'm interpreting this to be sarcasm - you can't correct for multiple wavefront deformations at the same time - adaptive optics works only on a single point in FOV perfectly. Planet size objects will have very different wavefront aberrations from one edge to another.

If you mean the field of view is limited due to the adaptive mirror being used, then yes. You can use multiconjugate adaptive optics to increase the area where the correction is being applied, but it is still small.

I think the main issue is that you need a guide start, region mag 12-15 or to use an artificial guide star from a laser being pointed into the sky.

Maybe not very practical..  

Edited February 8, 2021 by Deadlake

[vlaiv]

2 minutes ago, andrew s said:

Sorry it was intended as humour, ligh relief. 

No offense was intended to anyone. The emoji was ment to signal humour. 

Regards Andrew 

If anything, my question should be understood as a compliment - given your level of knowledge, for a moment it seemed very plausible that you were working on something like that - and maybe it was me who was not fully understanding the topic

[andrew s]

The attached describes AO for ESO covering ~2 arc minutes (albeit in the IR). It includes an image of Jupiter with 90 milli -arcsecond resolution.  

AO and Jupiter

Regards Andrew

Edited February 8, 2021 by andrew s