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What is the telescope's prime magnification with NO eyepiece lens?


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  • 2 years later...
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So as you see, telescope is not magnifying at all! It is minifying - making Moon that is 3474 km in diameter, appear to be only 6.1mm!  

I love simple answers.    

That's possibly not the best response to get continued answered from people.They are giving you the answers and information you need to learn in order to understand. My answer gave exactly the in

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Simple answer (I hope) is divide focal length by the image diameter circle (the diagonal) of the camera sensor. Full frame is 43.3mm and this is the proper length of a prime lens to give 1:1 image although we normally use 50mm or 45mm. For micro four thirds the image circle diameter is  21.6mm so its 'prime' lens is 20-25mm thereabouts, and APS is somewhat variable, nominal 35mm. A typical guide camera might be a 1.2/3 sensor of 7.66mm diagonal. These give the equivalent eyepiece focal lengths when used on prime focus. Magnification is OTA focal length divided by eyepiece focal length.

A common misconception is that a larger sensor will see more light than a small one. Both see all the light gathered through the lens/OTA iris (or aperture if closed down). The sensor is at the image focal point and sees all the light. A 130mm reflector is 130mm iris. If 650mm focal length then 650/130 = f/5.

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I haven't waded through tbe rest of the answers, but my telecope has a focal length og 1200mm.  I always assumed that if I put my dslr into the focusser with no other Eye piece in place that it was like stcking a 1200mm lens onto the DSLR

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9 minutes ago, JOC said:

I haven't waded through tbe rest of the answers, but my telecope has a focal length og 1200mm.  I always assumed that if I put my dslr into the focusser with no other Eye piece in place that it was like stcking a 1200mm lens onto the DSLR

You’re exacty right

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4 hours ago, jefrs said:

A common misconception is that a larger sensor will see more light than a small one. Both see all the light gathered through the lens/OTA iris (or aperture if closed down). The sensor is at the image focal point and sees all the light. A 130mm reflector is 130mm iris. If 650mm focal length then 650/130 = f/5.

That is not misconception - large sensor will cover more of the sky and hence gather more light. Not more light from the same target - but more total light (small sensor won't gather any light from targets that fall outside of it's FOV - but large sensor will pick up on those).

If you want to do wide field shot with smaller sensor - you need to do mosaic - and spend much more time doing it to match output of large sensor.

Alternatively, you can use large sensor on large scope and get faster system. Large sensor is going to be faster if matched with proper scope compared to small sensor.

 

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9 hours ago, vlaiv said:

Alternatively, you can use large sensor on large scope and get faster system. Large sensor is going to be faster if matched with proper scope compared to small sensor.

 

Using the same scope but with two different sensors size, why would the combination with bigger sensor be faster when they have the same focal ratio? Please correct me if I misunderstood this.

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3 hours ago, ZiHao said:

bigger sensor be faster

Hi

I believe that on this forum, we're not allowed to use the terms fast and slow for anything other than motor vehicles!

If your target runs out of the field of view of the smaller sensor, you're going to have to move the camera and take more frames. Then combine them in software. That takes a long time. The large sensor get it all at once. No need to move the telescope and take more frames.

If your target fits into both the small and large sensors' field of view and the sensors are of equal sensitivity, then it takes the same time to build the image.

It doesn't matter what focal ratio you use. The only way you can get the image in less time in the latter case is to use a larger aperture telescope.

Sometimes theory and explanation just don't help.

When you've seen it first hand, you get it immediately. I too needed convincing. Try a side by side on the same target with a large and a small telescope. e.g. 80mm and 130mm. Markarian's Chain is a good target . Do 5 minute exposures in each. You'll not need telling which image is taken with which.

Same exercise with a large and a small sensor, but same telescope. The whole of Markarian's Chain fits into one frame with a large sensor but you'd need say 2 frames to cover the same area of sky with a small sensor. Try with a 183 and a dslr.

HTH

Edited by alacant
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2 hours ago, ZiHao said:

Using the same scope but with two different sensors size, why would the combination with bigger sensor be faster when they have the same focal ratio? Please correct me if I misunderstood this.

It is faster in one particular example - trying to get larger FOV than small sensor is capable of.

In this case you are limited (we exclude focal reducers for the time being or we assume that it is already fitted in form of CC / FF) to mosaic technique. Imagine that you have to take 4 panels to cover target with small sensor.

Large sensor will cover all of that in one go - let's say one hour. To get hour worth of signal per panel - you need to spend 4h total to get the same image with smaller sensor.

 

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The problum with this issue is when people try and understand the issues when two variable are changed (e.g. scope aperture and sensor size).

The easy way to understand each factor is to work through the effects of changing them one at a time.

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This table might help. 'Image size at fixed dpi' is, for example, how large it will appear if displays at 100% on a computer monitor:

 

FACTOR INCREASED Field of view Exposure  Time Required Image Size at fixed dpi
       
Aperture No change Reduced No change
Focal length Reduced Increased Increased
Focal Ratio Indeterminate* Reduced

Indeterminate*

Pixel size No change Reduced Reduced
Sensor size Increased No change Increased
Sensor quantum effciency No change Decreased No change
       
       
*Change in field of view or image size will depend on whether or not focal length is changed, this is why focal ratio alone is unhelpful without further information    
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19 minutes ago, Stub Mandrel said:

This table might help. 'Image size at fixed dpi' is, for example, how large it will appear if displays at 100% on a computer monitor:

There seems to be contradiction in the table provided. It might be me and my understanding.

What is considered as Image Size at fixed DPI?

I see two possibilities - Size of object and size of actual image - width x height - or size of FOV of the image. But we already have FOV - I'm inclined to believe it is Object size in the image?

If we say that it is object size, then:

Pixel size increased -> object size reduced is true statement

Sensor size increased -> object size increased is false statement

If we assume it is opposite - that image size is actual pixel count - width x height or FOV then:

Pixel size increase -> Image size decrease is true

Sensor size increase -> Image size increase is true

but

Focal length increase -> Image size increase is false

 

 

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3 hours ago, alacant said:

Hi

I believe that on this forum, we're not allowed to use the terms fast and slow for anything other than motor vehicles!

If your target runs out of the field of view of the smaller sensor, you're going to have to move the camera and take more frames. Then combine them in software. That takes a long time. The large sensor get it all at once. No need to move the telescope and take more frames.

If your target fits into both the small and large sensors' field of view and the sensors are of equal sensitivity, then it takes the same time to build the image.

It doesn't matter what focal ratio you use. The only way you can get the image in less time in the latter case is to use a larger aperture telescope.

Sometimes theory and explanation just don't help.

When you've seen it first hand, you get it immediately. I too needed convincing. Try a side by side on the same target with a large and a small telescope. e.g. 80mm and 130mm. Markarian's Chain is a good target . Do 5 minute exposures in each. You'll not need telling which image is taken with which.

Same exercise with a large and a small sensor, but same telescope. The whole of Markarian's Chain fits into one frame with a large sensor but you'd need say 2 frames to cover the same area of sky with a small sensor. Try with a 183 and a dslr.

 

56 minutes ago, vlaiv said:

It is faster in one particular example - trying to get larger FOV than small sensor is capable of.

In this case you are limited (we exclude focal reducers for the time being or we assume that it is already fitted in form of CC / FF) to mosaic technique. Imagine that you have to take 4 panels to cover target with small sensor.

Large sensor will cover all of that in one go - let's say one hour. To get hour worth of signal per panel - you need to spend 4h total to get the same image with smaller sensor.

Ah, I think I misunderstood the term "faster" here, I related it to the light gathering ability of the camera, light gathering ability as in like, for example, we have one deep sky object that both the sensor can capture completely, now we crop it to the same size so both images show the same features of the DSO, so the SNR for both of them will be the same, therefore for different sensor size using the same focal ratio scope, how fast they collect light is the same. I agree the larger sensor covers wider FOV whereas smaller sensor covers smaller FOV and larger sensor is faster, in another way, that they will cover bigger area of the sky and the total light collected will be greater. Thanks for clarifying, I hope that I didn't confuse myself again.

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4 minutes ago, ZiHao said:

 

Ah, I think I misunderstood the term "faster" here, I related it to the light gathering ability of the camera, light gathering ability as in like, for example, we have one deep sky object that both the sensor can capture completely, now we crop it to the same size so both images show the same features of the DSO, so the SNR for both of them will be the same, therefore for different sensor size using the same focal ratio scope, how fast they collect light is the same. I agree the larger sensor covers wider FOV whereas smaller sensor covers smaller FOV and larger sensor is faster, in another way, that they will cover bigger area of the sky and the total light collected will be greater. Thanks for clarifying, I hope that I didn't confuse myself again.

I understand term faster/speed like this:

- for a given target, setup and expected SNR - how long does on need to expose to reach expected SNR.

Where more time is slower than less time and other way around - less time is faster than more time (same as in real speed - how much time it takes to travel from A to B).

When we start thinking like that, then we tend to start thinking about components of the setup in similar terms. How fast is that scope, or how fast is particular sensor?

In it self - it does not make much sense as camera by itself can't perform the task of imaging - neither can telescope by itself. What we really mean is - this part of equipment has "the potential" to be fast if properly paired with other part.

In that sense F/ratio of the scope is not necessarily measure of potential to be fast, while sensor size is measure of potential to be fast.

We are used to speed of scope - being ratio of focal length and aperture, but it is aperture in reality that represents potential of telescope to be fast. Here is an example:

Take 100mm F/5 telescope and pair it with 10mm diagonal camera. F/5 telescope is fast, right?

Can we match that speed with F/10 telescope by any chance? We actually can. Take sensor that has 20mm diagonal and same pixel size as first sensor and put it with 100mm F/10 scope. It will result in same FOV but more pixels. Bin those pixels and you will get exactly the same setup as first one. Pixel size will be the same, FOV will be the same, aperture will be the same, sampling rate will be the same.

SNR (except for read speed - which you can mitigate by using different sub length, depending on your LP levels) will be the same in given time and hence speed will be the same.

How is that possible if one telescope is "slow" and other is "fast". Because F/ratio is not very suitable measure of potential to be fast. We have other two measures. Aperture and sensor size. In this case aperture plays a part same aperture - potential to have same speed - just pair it with suitable camera.

You can also look at it the other way around - larger sensor has potential to "go faster" - it can either offset "slow" scope and make it equally fast as "fast" scope, or it can make "same speed" scope much faster. If we take 200mm F/5 scope with larger sensor - it will again give same FOV, same sampling rate after pixel binning and so on, but aperture is now x4 in surface over 100mm F/10 example.

Instead of being of the same speed - 200mm F/5 scope with larger sensor will be much faster than 100mm F/5 with smaller sensor. Both have "same speed" scopes.

In the end, let me recap: we can talk about speed of the system and not components, but if we want to talk about speed of components - it's probably better to talk about potential to be fast rather than actual speed as single component can't get the job done without being paired with other parts of the system.

 

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3 hours ago, vlaiv said:

What is considered as Image Size at fixed DPI?

I see two possibilities - Size of object and size of actual image - width x height - or size of FOV of the image. But we already have FOV - I'm inclined to believe it is Object size in the image?

 

I should have been clearer!

Yes it's object size, and I should have said 'no change' with sensor size.

I've edited the table to correct this.

 

 

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  • 4 weeks later...

The effective "zoom" of a camera depends on pixel pitch and how useful that is depends on the number of pixels.  When I use a Celestron NexImage 5 with my Mead 10" telescope, the zoom is too high to see the whole moon. If I use my Sony Nex 5N, I can get the whole moon in.   Both images are interesting, but I want to try stiching NexImage 5 images to get a really high res picture. The second images has a square that shows the section from the NexImage 5 but the actually original covered a bigger area of the moon.

101457491_10223170659958195_1564952187056947200_o.jpg

 

102286912_10223170660398206_589213266119491584_o.jpg

101253445_10223170661398231_1678824607731154944_o.jpg

Edited by Astrokye
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  • 6 months later...

To the OP (if you're still tracking this thread), I believe the answer is your 700mm scope is essentially a 14x power lens when shooting prime.  Now from there you could use a Barlow, or perhaps an adapter capable of containing eye pieces, but doing this on a scope with a manual mount may be self defeating honestly.  I've never done this myself but once my setup arrives I'll be giving it a try on my 8" DOB.  I think it will work okay for the moon, all bets are off on anything else ;) 

The FOV (field-of-view) on a 35mm camera with a 50mm lens is debatably close to the FOV for the normal eye, or 1x power magnification.  As you know determining magnification is the scope focal length divided by the lens focal length.  When shooting "prime" and using the camera body as the eyepiece lens, simply substitute 50mm as you are essentially at 1x power or no power.  Thus 700mm / 50mm = 14x power magnification

Edited by dcobb
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