90mm f6 or 102mm f7

[ollypenrice]

On 05/12/2022 at 15:58, wongataa said:

There isn't anything different between them in the numbers that specify them.  A telescope is a type of lens.  Aperture and focal length are properties of a lens/telescope that mean exactly the same thing for both of them.  Your photographic knowledge applies to telescopes.

Things like seeing also apply to daytime photography but is far less important so no one really cares about it there.

It depends on how deep your photographic knowledge is. If it ends with the idea that F ratio defines the speed of the system (which is usually enough knowledge for normal photography) it is certainly not adequate for understanding the speed of astrophotographic instruments. In the normal photo world, both focal length and pixel size are constants when different F ratios are compared. When these constants disappear without being acknowledged, you end up with nonsense like this, just copied from the Starizona website; The HyperStar 8 lens converts a standard Celestron 8" SCT from f/10 to f/1.9, making the system 25 times faster.

It doesn't do any such thing. To make the system 25 times faster you would have to increase the area of the telescope objective by 25 times. 

I might just as well say,  'I can make your car twice as fast by reducing the distance you travel by 50%.'

lly

[Louis D]

54 minutes ago, ollypenrice said:

nonsense like this, just copied from the Starizona website; The HyperStar 8 lens converts a standard Celestron 8" SCT from f/10 to f/1.9, making the system 25 times faster.

Are you also saying focal reducers are nonsense and don't actually reduce exposure time while simultaneously reducing focal length?  I thought if aperture remains constant but focal length decreases, f-ratio must go down as well.  When the f-ratio goes down, the exposure time required for the same exposure density also goes down.  The HyperStar gets rid of the 5x magnifying effect of the SCT secondary mirror, reducing the focal length and f-ratio by 5x in the process.

57 minutes ago, ollypenrice said:

It doesn't do any such thing. To make the system 25 times faster you would have to increase the area of the telescope objective by 25 times.

You're assuming the focal length remains constant, which it does not; and they make no claims that it does.  Further down, they show the decrease in both in a table:

The 25x comes from the squaring effect of f-ratios on light gathering.  I think it is really 2**5 or 32x, actually.  That much they did screw up.

[vlaiv]

31 minutes ago, Louis D said:

  I thought if aperture remains constant but focal length decreases, f-ratio must go down as well.  When the f-ratio goes down, the exposure time required for the same exposure density also goes down.

Exposure time goes down - but not because of F/ratio, but because you also change sampling rate, or effective pixel size.

If you reduce focal length but at the same time reduce pixel size to keep effective pixel size (in arc seconds, or in another words sampling rate) - you won't speed up anything. You'll need same amount of time to target SNR as with original focal length or F/ratio.

This shows that it is not F/ratio that is important - but two other quantities - aperture surface and effective pixel surface (or how much sky is covered by single pixel).

[ollypenrice]

41 minutes ago, Louis D said:

 

The 25x comes from the squaring effect of f-ratios on light gathering.  I think it is really 2**5 or 32x, actually.  That much they did screw up.

The effect of focal reducers on light gathering can be stated with absolute certainty. It is zero. How can a lens at the back of the lightpath increase the amount of light going in at the front?

What a focal reducer does is change the way in which the captured light is distributed onto the chip. That is all it does.

What Starizona should do is stop comparing a Hyperstar with the scope it was originally, since it is now a totally different kind of scope suitable for totally different targets. They should, instead, compare it with other scopes of comparable focal length. This is what I did recently in a magazine article on the RASA 8. Its 400mm focal length is comparable with that of a fast 85mm refractor, say. The area of clear aperture is 4.5x larger for the RASA so it is 4.5 times faster at taking the same picture.  This comparison is meaningful.  (Note that a simple comparison of F ratios, 4.7 versus 2.0, would have made the RASA 5.5 x faster so I looked only at clear aperture.)

Hyperstar stick with the meaningless '25x' faster claim up front because it is an attention grabber and I maintain that it is also a lie.

Olly

Edited December 14, 2022 by ollypenrice
False click.

[Louis D]

2 minutes ago, ollypenrice said:

The effect of focal reducers on light gathering can be stated with absolute certainty. It is zero. How can a lens at the back of the lightpath increase the amount of light going in at the front?

It can't, but it can increase the amount of light per unit area on a sensor by compressing the image circle as with a refractor.  I get the whole pixel business, but if you take the imaging device out of the equation and simply look at the optics, the light flux per unit area at the imaging plane has increased.

5 minutes ago, ollypenrice said:

What Starizona should do is stop comparing a Hyperstar with the scope it was originally, since it is now a totally different kind of scope suitable for totally different targets.

Hopefully their customers who are willing to pay their prices are savvy enough to understand this trade-off.  If it was a $100 device, then I would be concerned about noobs being misled.

[900SL]

Going back to my original question, sort of, how do you calculate the relative light intensity difference between two scopes of different apertures and focal lengths, assuming the same camera pixel size and sensor size? And does the size of the image circle come into play? 

[vlaiv]

6 minutes ago, 900SL said:

Going back to my original question, sort of, how do you calculate the relative light intensity difference between two scopes of different apertures and focal lengths, assuming the same camera pixel size and sensor size? And does the size of the image circle come into play? 

Crude speed comparison is diameter^2 * pixel_scale^2

More accurate comparison is to replace diameter^2 with "clear aperture surface" for both scope - this one is harder to calculate as we have unknowns, but we can approximate things.

Clear aperture surface is equivalent light gathering surface after we factor in all the losses in optical train.

Say that we have triplet refractor scope. It will have say ~99.5% effective coatings on each air/glass surface. Total loss will then be ~ 0.995^6 = ~0.97 or

97% of light will pass onto sensor.

Maybe we have 102mm aperture - so final clear aperture will be 51^2 * 0.97 = ~7926mm2 of light gathering surface.

Say that we want to compare that to 6" newtonian with 32% central obstruction. It has standard mirror coatings that have 94% reflectivity.

We have x2 mirrors so 0.94 * 0.94 = 0.8836 is reflectivity factor and we have to calculate in central obstruction as well so it will be

152mm diameter -> 76mm radius, 76 x 0.32 = 24.4mm radius of secondary obstruction.

(76mm^2  - 24.4mm^2) * pi * 0.8836 = ~14381mm2

Now we just need pixel size and focal lengths to get arc second per pixel - and square those as well and multiply with clear aperture equivalent - and you'll get two numbers that represent relative speed of systems.

By the way - standard losses are between 0.5% and 1% for glass/air interface with refractors and 94% or 97% for standard and enhanced mirror coatings and 99% for dielectric mirror coatings.

 

 

[900SL]

Many thanks Vlaiv. So it's basically aperture and pixel scale, which makes sense, with adjustment for losses in the system.

[vlaiv]

1 minute ago, 900SL said:

Many thanks Vlaiv. So it's basically aperture and pixel scale, which makes sense, with adjustment for losses in the system.

Yep.

Forgot to answer this one:

52 minutes ago, 900SL said:

And does the size of the image circle come into play? 

It sort of does in some cases.

Say you have 4" scope and ASI1600 and you want to speed up things, but you don't want to loose FOV - you really like your FOV.

In turns out that ASI6200 has x4 the surface (roughly) of ASI1600, so if you get scope with twice the focal length - you'll have the same FOV.

You get 6" scope, that is "slower" as far as F/ratio goes - but has twice the focal length. You bin pixels of ASI6200 to match imaging scale you had with ASI1600 and you have the same FOV - now you have faster system as 6" is larger than 4" and everything else is the same (pixel scale) - and you managed to keep FOV the way you like.

To be able to do above scenario - you need to have 6" scope that has large enough imaging circle for ASI6200. This is when imaging circle comes into play.

Another example is if you want fastest possible scope for small galaxies - again, sensor size plays a part there as you can match it with very large aperture and still have enough FOV to capture galaxies (you'll fix pixel size to match your needs with binning).

All good and dandy - but you also need big enough imaging circle to have corrected image over such large sensor.

Bottom line - imaging circle comes into "speed" equation as it lets you use large sensor and with large sensor you can use longer focal length for same FOV - which means you can use larger aperture. In some sense - size of sensor is part of "speed equation" as it allows you to use bigger scope (larger aperture) and with pixel binning still be on target sampling rate.

[ollypenrice]

6 hours ago, Louis D said:

It can't, but it can increase the amount of light per unit area on a sensor by compressing the image circle as with a refractor. 

I don't understand the second part of this (ie 'compressing the image circle.')

What we haven't talked about is the important issue of 'object photons.' If your object of interest will fit on the chip with and without the reducer then you get the same number of photons from the object with and without the reducer. The reducer compresses them onto fewer pixels so you get a smaller image more quickly than you would get a larger one. This is hardly a surprise! It takes my car longer to do 100 miles than to do 50 miles.  I would like to point out to Starizona that its speed is not greater when it makes the shorter journey in half the time. 

If your 'object of interest' is everything on the wider field provided by the reducer then you will reach an acceptable S/N ratio in a time reduced in accordance with the classic F ratio rule. 

Olly

 

[gorann]

Obviously aperture rules - the more aperture you can afford (and your mount can handle) the more photons you will gather. But you also want to collect those photons on your sensor, and that is where I can see that a focal reducer can be an advantage. If you have a telescope with an imaging circle of, lets say 40 mm, and you have a sensor with a diagonal of, lets say 30 mm, then it would make sense to use a focal reducer to bring down the imaging circle towards 30 mm as more of the captured photons would end up on your sensor.

[vlaiv]

34 minutes ago, gorann said:

If you have a telescope with an imaging circle of, lets say 40 mm, and you have a sensor with a diagonal of, lets say 30 mm, then it would make sense to use a focal reducer to bring down the imaging circle towards 30 mm as more of the captured photons would end up on your sensor.

That makes sense, but only if target is larger than 30mm or more nicely framed on 30mm sensor when using reducer.

[Louis D]

1 hour ago, ollypenrice said:

I don't understand the second part of this (ie 'compressing the image circle.')

I'm just pointing out how focal reducers work from a layman's perspective.  Also known as telecompressors, they compress the image circle into a smaller area, increasing the photon flux per unit area, simultaneously reducing image scale.  As you say though, what good is a faster acquired image if it is tiny on the sensor?  However, if someone is after large, expansive, low resolution sky survey images acquired in the least amount of time possible, then perhaps the Starizona solution is for them.  It's similar to a Schmidt camera which is used for such purposes.

[ollypenrice]

8 hours ago, Louis D said:

I'm just pointing out how focal reducers work from a layman's perspective.  Also known as telecompressors, they compress the image circle into a smaller area, increasing the photon flux per unit area, simultaneously reducing image scale.  As you say though, what good is a faster acquired image if it is tiny on the sensor?  However, if someone is after large, expansive, low resolution sky survey images acquired in the least amount of time possible, then perhaps the Starizona solution is for them.  It's similar to a Schmidt camera which is used for such purposes.

Oh, I agree entirely about the virtues of the fast, widefield system and am using one at the moment, a RASA 8.

Olly

[900SL]

Blimey. That was quick. Ordered a couple of days ago, held for QC check, dispatched yesterday, arrived in Finland tonight.

There's a test certificate included. Does this tell me anything meaningful? 

 

[900SL]

I think I'm going to be happy with the new scope

 

A quick 60 minutes total exposure from the balcony, bortle 7, IDAS NBZ dual narrowband, no calibration frames.

Vixen SXD2, ASI 533MC Pro, 178MM guide cam, Askar FMA 180 guide scope

 

 

 

Edited December 28, 2022 by 900SL

[Mr Spock]

0.985 Strehl is pretty good - not really needed for widefield imaging, but, as you can see from your image, you have a good one 👍

[900SL]

Thanks Mr Spock, with a Vulcan salute.

I'm relieved to see that it all seems OK. It was shipped Germany to Finland and picked up a couple of dents in the outer packaging  but seems to have emerged unscathed, probably due to the gazillion foam peanuts the scope box was nested in. 

I'm delighted with it. Resolution and detail seem pretty good for the first light. Looking forward to clear skies longer than the odd hour at present Ive not really checked backfocus yet, but it seems to be working OK with the William Optics 6Aiii reducer I repurposed from my GT71. Small sensor helps, I will do some test frames with a APS C Nikon at some point

 

Edited December 28, 2022 by 900SL