Opinions - New Askar 103mm APO triplet

[Icesheet]

14 minutes ago, vlaiv said:

There are spot diagrams published for all combinations - with x1 flattener, x0.8 reducer and x0.6 reducer.

Here we have RMS radius of spot diagram. With focal length of 700mm and reduction of 0.6 - that turns out to be 420mm of effective focal length.

6um RMS spot size is equal to 3" at 420mm - so that is quite low resolution.

If we were to calculate effective aperture it would be

3" RMS radius spot size is equivalent to 3" * 2.355 = 7.065" FWHM which is in turn equal to 2.44 * 7.065" / 1.025 = 16.82" airy disk diameter

That is the same as 16.46mm aperture scope. Like I said - not much of a resolving potential.

 

I’m interested in learning more about spot sizes and how to translate that into imaging potential. Can you direct me to a good source? For example, I just ordered a FRA400, but now I’m looking closer at spot diagrams the FRA300 looks better. How is the aperture and FL affecting this and how can we make objective comparisons?

[vlaiv]

1 minute ago, Icesheet said:

I’m interested in learning more about spot sizes and how to translate that into imaging potential. Can you direct me to a good source? For example, I just ordered a FRA400, but now I’m looking closer at spot diagrams the FRA300 looks better. How is the aperture and FL affecting this and how can we make objective comparisons?

I just use simple formulae that are available for different things.

For example - airy disk radius in radians is 1.22 x lambda / aperture diameter, where lambda and aperture diameter are expressed in meters (or micrometers - it does not matter as long as they are the same). Lambda is usually taken to be 550nm (or 0.55um) as that sits in the middle of 400-700nm visible range.

Find more information here:

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

For FWHM - look here:

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

It is usually taken that FWHM = 2.355 * RMS as that holds for normal distribution (Gaussian bell shape) - when you have RMS spot radius.

In the end there is handy formula to relate angles and micrometers for a telescope (a bit of trigonometry really) which goes:

angle in arc seconds = size_in_um * 206.3 / focal_length_in_mm

It is handy to calculate arc seconds per pixel - if you for example put some pixel size

3.75 * 206.3 / 700mm = 1.105"/px

(700mm is focal length, 3.75um is pixel size and it solves for angle in arc seconds). Alternatively it can serve to convert micrometers to arc seconds for some focal length - just use pixel size of 1um so you get

1 * 206.3 / 700mm = ~0.295"

So there is 0.295" per 1um at 700mm or 1/0.295 = ~3.4um per arc second.

In the end - if you want to get ideal sampling rate for long exposure - you take FWHM size of star and divide that with 1.6 to get arc seconds per pixel for optimum sampling (explanation for this is rather complex and involves Fourier transform and convolution theorem and Nyquist sampling).

With that we can see that above telescope with x0.6 reducer will produce around 7" FWHM stars without even having influence of seeing or mount guide error. That is ~4.375"/px or at 420mm ideal pixel size would be 8.9um based on that alone.

It is important to remember - shorter the focal length - "tighter" the spot diagram needs to be in micrometers to provide sharper image - because with less focal length - there is more sky covered with every micrometer.

 

[Vash]

13 hours ago, licho52 said:

The reviews will come from either people who have bought it already (they will love it and gush) or YouTuber Reviewers who get this stuff from Sharpstar to review (they will love it and gush).  But yeah, it will be fun to watch them do flips trying to hide any glaring problems..

They will undoubtedly gush but they will show pictures taken with the scope and discribe how they took and pictures and more importantly how they processed them and you can't dress that up, especially as some reviewers will provide access to unprocessed images. 

 

The sample pictures sharpstar have published look good to me, the problem is that you have no way of knowing if they passed them through blurxterminator or any other star reduction process so they are very limited in the information they can provide. 

[Vash]

12 hours ago, vlaiv said:

.

With that we can see that above telescope with x0.6 reducer will produce around 7" FWHM stars without even having influence of seeing or mount guide error. That is ~4.375"/px or at 420mm ideal pixel size would be 8.9um based on that alone.

A pixel size of 3.75um with the 0.6x reducer gives about 2" per pixel so while you are undersampling it is not drastic and if you compare RMS radius to that of a very good scope like the FRA400 there is less that 1um difference - but as mentioned before there is a 10nm difference in the wavelengths used for the ray trace! 

[vlaiv]

1 hour ago, Vash said:

A pixel size of 3.75um with the 0.6x reducer gives about 2" per pixel so while you are undersampling

In that combination you are actually over sampling rather than under sampling. From above, telescope is able to deliver ~4.3"/px and you want it to record 2"/px.

 

1 hour ago, Vash said:

f you compare RMS radius to that of a very good scope like the FRA400 there is less that 1um difference - but as mentioned before there is a 10nm difference in the wavelengths used for the ray trace! 

How about if we compare it to perfect 100mm aperture to see the difference.

We have RMS radius of about 3" with this scope.

FWHM of airy disk is 1.025 * lambda / diameter_of lens (in radians) so this gives us 1.025 * 0.55um / 100000um in radians. That is (180 / pi) * 1.025 * 0.55 / 100000 in degrees or

58.728174 / 100000 degrees. Now we need to convert that to arc seconds so we multiply it with 3600 (which is 60*60) and we end up with 58.728174 * 3600 / 100000 = 211421.42 / 100000 = 2.11"

We have that FWHM of perfect telescope is 2.11" - we need to calculate equivalent RMS so we divide with 2.355 and we get ~0.9" RMS

That is x3.33 times less than above telescope.

[Icesheet]

14 hours ago, vlaiv said:

I just use simple formulae that are available for different things.

For example - airy disk radius in radians is 1.22 x lambda / aperture diameter, where lambda and aperture diameter are expressed in meters (or micrometers - it does not matter as long as they are the same). Lambda is usually taken to be 550nm (or 0.55um) as that sits in the middle of 400-700nm visible range.

Find more information here:

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

For FWHM - look here:

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

It is usually taken that FWHM = 2.355 * RMS as that holds for normal distribution (Gaussian bell shape) - when you have RMS spot radius.

In the end there is handy formula to relate angles and micrometers for a telescope (a bit of trigonometry really) which goes:

angle in arc seconds = size_in_um * 206.3 / focal_length_in_mm

It is handy to calculate arc seconds per pixel - if you for example put some pixel size

3.75 * 206.3 / 700mm = 1.105"/px

(700mm is focal length, 3.75um is pixel size and it solves for angle in arc seconds). Alternatively it can serve to convert micrometers to arc seconds for some focal length - just use pixel size of 1um so you get

1 * 206.3 / 700mm = ~0.295"

So there is 0.295" per 1um at 700mm or 1/0.295 = ~3.4um per arc second.

In the end - if you want to get ideal sampling rate for long exposure - you take FWHM size of star and divide that with 1.6 to get arc seconds per pixel for optimum sampling (explanation for this is rather complex and involves Fourier transform and convolution theorem and Nyquist sampling).

With that we can see that above telescope with x0.6 reducer will produce around 7" FWHM stars without even having influence of seeing or mount guide error. That is ~4.375"/px or at 420mm ideal pixel size would be 8.9um based on that alone.

It is important to remember - shorter the focal length - "tighter" the spot diagram needs to be in micrometers to provide sharper image - because with less focal length - there is more sky covered with every micrometer.

 

Thanks for the explanation. I think I may still have some gaps that I need to fill in but from what I can see the FRA400 is probably not diffraction limited at any wavelength, while the FRA300 seems to be at least in the centre. Now I'm questioning my purchase. I have a TSA120 and Tak states that spots are 10microns across a 35mm sensor with the flattener which is about diffraction limited across the field. So maybe I'm expecting too much from cheaper optics. This is just theoretical of course and maybe seeing/ guiding etc negates much of this but to me it's still surprising.

Edit: Also with processing techniques it’s now easier to reduce stars and recover detail so maybe this also negates it. Plus it’s easier to fix bloated stars, rather than misshapen stars and at least they seem fairly uniform across the files on the Askars. 

Edited August 8, 2023 by Icesheet

[vlaiv]

3 minutes ago, Icesheet said:

Thanks for the explanation. I think I may still have some gaps that I need to fill in but from what I can see the FRA400 is probably not diffraction limited at any wavelength, while the FRA300 seems to be at least in the centre. Now I'm questioning my purchase. I have a TSA120 and Tak states that spots are 10microns across a 35mm sensor which is about diffraction limited across the field. So maybe I'm expecting too much from cheaper optics. This is just theoretical of course and maybe seeing/ guiding etc negates much of this but to me it's still surprising.

I don't think you should be that harsh on spot diagrams of astrographs.

As a rule, they have optical elements that are aimed at providing good correction over larger flat field. That comes at a price - you simply can't maintain diffraction limit in that scenario.

For the most part - that is fine as final resolution of the image depends not only on optics but on mount performance and of course seeing. If you put a good 4" astrograph next to regular 4" diffraction limited telescope and compare their center field in average imaging conditions - you won't see much difference - both will be limited to about 2"/px sampling (astrograph will be just a tad less sharp in center of the field - but it might be barely noticeable).

In any case, they are sharp if you treat them the right way. Problem with majority of imaging today is that pixels are getting too small. Just a decade or two, in CCD era, pixel size of about 5-9um and even more - was standard. Nowadays it is at least half that size if not less.

Above telescope will produce sharp images at 9um pixel size - no problem (we calculated that).

Nice and simple way to check what your optics is capable of is to get artificial star, do some shots of it across the field and then measure FWHM of those star profiles (just make sure artificial star is far enough when doing this - like 50m or more). Divide FWHM with 1.6 and that is your optimum sampling rate for optics alone. For larger diffraction limited scopes - you might actually run into issue of pixels being too large. Planetary imagers often need barlows to match plate scale to pixel size. For this reason, it is sensible to perform this test on astrographs rather than diffraction limited scopes

[The Admiral]

1 hour ago, Vash said:

if you compare RMS radius to that of a very good scope like the FRA400 there is less that 1um difference

Well is the FRA400 a very good scope? I have one and I'm not so sure, though obviously there must be sample to sample variation. You could argue it's as bad as the FRA400! I much preferred the images from my Altair 102 triplet to be honest, but that became too heavy for my back and I passed it on, unfortunately.

Ian

[Vash]

1 hour ago, vlaiv said:

In that combination you are actually over sampling rather than under sampling. From above, telescope is able to deliver ~4.3"/px and you want it to record 2"/px. 

Thanks for the clarification, I always get those 2 mixed up. 

 

1 hour ago, vlaiv said:

How about if we compare it to perfect 100mm aperture to see the difference.

Let's no talk about diffraction limited optics or I will end up with another newtonian!

But you are right. Optics is all about compromises. It is very hard to make well corrected, fast (f/4 and lower) refractors. Hence all the f/5.6 refractors on the market. 

Speaking personally, I have a 200mm f/5 newt that I image with as well as shorter camera lenses. I want a scope in the 400-600mm focal length range but I also want it to be fast and not break the bank. I would also rather not get another newt. I hope this refractor fits the bill or I will get a quattro 150p. 

[scitmon]

On 03/08/2023 at 21:12, ollypenrice said:

I know that lots of words are devoted to glass type but I would rather see them devoted to collimation, field curvature and... the focuser.

This!  In a world of narrowband imaging and post processing, glass is less important than build quality imo.  The reason I upgraded my doublet was 90% to get a sturdier focuser.

[vlaiv]

26 minutes ago, Vash said:

. I want a scope in the 400-600mm focal length range but I also want it to be fast and not break the bank. I would also rather not get another newt. I hope this refractor fits the bill or I will get a quattro 150p. 

When you say scope in 400-600mm range - what actually do you mean?

Why do you need that FL range? Is it the FOV? Is it the sampling rate? What sort of camera do you have and what sort of resolution of the image do you want to do?

[Vash]

6 minutes ago, vlaiv said:

When you say scope in 400-600mm range - what actually do you mean?

Why do you need that FL range? Is it the FOV? Is it the sampling rate? What sort of camera do you have and what sort of resolution of the image do you want to do?

I am after a larger fov than I get with my newt ( https://imgur.com/gallery/p4FooD8 ) for medium sized targets like m31, rosette nebula etc. 

I image with a modified dslr at the moment, Canon 800d, aps-c, 3.72 um pixel size. 

[vlaiv]

9 minutes ago, Vash said:

I am after a larger fov than I get with my newt ( https://imgur.com/gallery/p4FooD8 ) for medium sized targets like m31, rosette nebula etc. 

I image with a modified dslr at the moment, Canon 800d, aps-c, 3.72 um pixel size. 

Well, in that case, if it is FOV that you are interested - just see what will give you fov that you want and is within your budget and that you can easily mount. As long as you don't try to get high resolution images and just want to do wider field, you don't have to worry.

Do be aware that you'll probably over sample with that camera in most cases and maybe use super pixel mode to debayer your images, or bin x2 data after stacking.

If you want to have both high resolution and wide FOV - then consider creating mosaics

[Vash]

12 minutes ago, vlaiv said:

Well, in that case, if it is FOV that you are interested - just see what will give you fov that you want and is within your budget and that you can easily mount. As long as you don't try to get high resolution images and just want to do wider field, you don't have to worry.

Do be aware that you'll probably over sample with that camera in most cases and maybe use super pixel mode to debayer your images, or bin x2 data after stacking.

If you want to have both high resolution and wide FOV - then consider creating mosaics

Thank you. I do usually bin at x2 with the newt. 

I would love to do mosaics but with the UK weather it is sometimes (usually!) difficult to get a nice run of clear nights. Maybe once a become a bit less impatient I will! 

[Icesheet]

Sorry to keep banging the spot diagram drum here but looking at these is confusing me further! To keep it consistent (if that’s possible) here’s the diagram for the 0.6x and 0.8x reducers. First of all, the wavelengths are the opposite way round. I’ll assume that’s a printing error somehow. I’ll also assume the colour key does not relate to the wavelength since the green spot isn’t actually in the green wavelength. Then, actually looking at the spot diagram itself. The centre spot for the 0.6x has a size of ~20um, according to the scale. However the RMS radius for the centre shows 6um which would be 12um spot size presumably. Or am I interpreting this wrong? Is it the spot from the GEO radius they are representing on the diagram? Then finally, the RMS and GEO radius. Are they representative of all wavelengths or just one?

Btw, the error in my initial post was I thought the box size was 20um so miscalculated spot size.  

 

 

Edited August 8, 2023 by Icesheet

[vlaiv]

2 hours ago, Icesheet said:

Then, actually looking at the spot diagram itself. The centre spot for the 0.6x has a size of ~20um, according to the scale. However the RMS radius for the centre shows 6um which would be 12um spot size presumably. Or am I interpreting this wrong?

Yes, that is right, but in order to relate RMS to something meaningful - it is best to think in terms of Gaussian distribution:

So sigma is just a measure of this bell shaped curve. Another way to think about it in terms of spot diagram is - about 68% of all rays land within circle with RMS radius.

We have similar thing with guiding - when you have RMS value and this:

RMS is just root square mean of where all rays land with respect to dead center (their distance from center - where they would ideally land). Roughly that translates into "blur" of sort of Gaussian shape - but actual shape of blur is much more complicated to calculate and involves wave superposition.

2 hours ago, Icesheet said:

Is it the spot from the GEO radius they are representing on the diagram?

Diagram is simply plot of all points and does not show "density" of points properly. There are other ways to do spot diagram - for example this one:

here we are zoomed in enough so you can see individual ray hits. How dense spot diagram is - depends on two things, first how many rays are cast and second - how "zoomed" in spot diagram is and how large individual spots are.

These "sparse" spot diagrams show just a bit better density of spots than "compact" diagrams like that one above. For example - in middle image we can see that red is concentrated in center but does have little tail. This is actually some coma (and possibly something else) and actual star image would look like this:

(I managed to find wave simulation as well as corresponding spot diagram - they look similar except that wave simulation shows "ripples" and interference stuff from light waves).

Here you can see that "core" is bright and that coma tail is fainter - which corresponds to spot density.

In any case - GEO radius is simply radius of a circle that contains all ray hits (again that does not mean it is full extent of the blur as you need to account for wave properties of light - but like in above example they do correspond well enough).

2 hours ago, Icesheet said:

Then finally, the RMS and GEO radius. Are they representative of all wavelengths or just one?

I think that they are given for all wavelengths unless otherwise stated.

[Icesheet]

Thanks again @vlaiv. Thinking of it as standard deviation helps me visualise it and understand it better (even if it’s not exactly the case).  

4 hours ago, vlaiv said:

I think that they are given for all wavelengths unless otherwise stated.

This is also useful as the scope I’m looking at looks particularly bloated in blue. Someone before mentioned it, but using one of these fringe killer filters might really help tighten things up. 
 

Interesting stuff!
 

 

[Elp]

Looking at YT and the 2600 duo reviews and what pops up, Nico Carver's got one but hasn't uploaded a review yet. The F4 images didn't look too bad from the outset.

[Concordia000]

On 03/08/2023 at 18:38, Stuart1971 said:

Looks a very nice scope, and a good spec for the cost…but the only real way to know is for some real world reviews…

I bought a Takahashi FSQ85 and thought it would be superb, and ended up selling to buy an Esprit 100, due to the Tak being really poor with astigmatism due to me using a modern small pixel camera, QHY268m, it was superb with my old CCD cameras with massive pixels….it’s an issue with them for sure, and to try and cure it they bought out another flattener for it, which is not included in the package, so with this Petzval design 4 element scope, to image, you have to use 2, yes 2 flatteners together, the one built into the scope and another on the back, it’s ridiculous TBH…

But this one being a new design should be fine on that front…it does seem a really good buy on paper for sure…with good spot diagrams, .If you do get it, keep us informed how you get on…

Just bought the same scope myself, but I think with modern small pixel cameras you are supposed to use the dedicated 1.01x flattener. I think with the flattener the stars should look excellent (haven’t tested it yet myself).

[Stuart1971]

5 hours ago, Concordia000 said:

Just bought the same scope myself, but I think with modern small pixel cameras you are supposed to use the dedicated 1.01x flattener. I think with the flattener the stars should look excellent (haven’t tested it yet myself).

I assume you are referring to the Tak I owned, if so the I’m afraid you are wrong,  I used that new flattener with mine, and it makes a very small difference…and why should you have to use 2 flatteners with such a premium scope, it’s a joke…I agree they SHOULD look excellent but they don’t at all….☹️

Edited August 13, 2023 by Stuart1971

[Elp]

Be interested to know your thoughts. As they're not out yet via the usual outlets did you buy them direct from Askar?

[Stuart1971]

10 minutes ago, Elp said:

Be interested to know your thoughts. As they're not out yet via the usual outlets did you buy them direct from Askar?

I think he was referring to the tak I owned, if your question was aimed at me…sorry if not…

[Elp]

Both to be honest, wires crossed. The Esprit is widely considered to be one of the best for imaging.

[Stuart1971]

8 minutes ago, Elp said:

Both to be honest, wires crossed. The Esprit is widely considered to be one of the best for imaging.

Yes, that what I have now, and it blows the Tak FSQ85, out of the water…

 

[Elp]

Nico's (p)review is in, seems decent with the lower power reducers, should be okay for visual too I'm guessing: