what is the magnification of a telescope without any eyepiece

Started 7 months ago | Questions
Metastro
Metastro Forum Member • Posts: 56
what is the magnification of a telescope without any eyepiece

Hi

I'm a bit confused and trying to work out how people use barlow lens' with telescopes

if magnification = Telescope fl / eyepiece fl

then a 2000mm fl telescope with no eyepiece would be:

2000 / 0 = undefined

Would you implicitally say 1 instead of 0, as the formula requires an eyepiece? then it resolves as 2000 / 1 = 2000x magnification, 2000mm / 20mm = 100x magnification etc

My actual question if you please consider this scenario:

You have a telescope, a x2 barlow lens and a camera, with no eyepiece (as you are imaging and not observing)

Telescope FL 2000mm  ->  x2 barlow lens  ->  Camera

If I'm not mistaken this would resolve as 4000x magnification

The max useful magnification is apparently 2.5x aperature in mm.

So a 200mm aperature, 2000mm fl scope, with x2 barlow, would be approximately 4 times the maximum useful magnification.

I am adament I have made several mistakes and incorrect assumptions, but given the above, I cannot see how, where or why anyone would use a barlow lens. Eyepieces are for visual observation, hense me removing them from the scenario when referring to imaging.

Thanks

ANSWER:
swimswithtrout Veteran Member • Posts: 3,783
Re: what is the magnification of a telescope without any eyepiece

Metastro wrote:

Hi

I'm a bit confused and trying to work out how people use barlow lens' with telescopes

if magnification = Telescope fl / eyepiece fl

then a 2000mm fl telescope with no eyepiece would be:

2000 / 0 = undefined

Would you implicitally say 1 instead of 0, as the formula requires an eyepiece? then it resolves as 2000 / 1 = 2000x magnification, 2000mm / 20mm = 100x magnification etc

My actual question if you please consider this scenario:

You have a telescope, a x2 barlow lens and a camera, with no eyepiece (as you are imaging and not observing)

Telescope FL 2000mm -> x2 barlow lens -> Camera

If I'm not mistaken this would resolve as 4000x magnification

N0, it's 4000mm "FOCAL LENGTH". Using  the generic 50mm =1X estimate,  4000/50=80X

The max useful magnification is apparently 2.5x aperature in mm.

So a 200mm aperature, 2000mm fl scope, with x2 barlow, would be approximately 4 times the maximum useful magnification.

I am adament I have made several mistakes and incorrect assumptions, but given the above, I cannot see how, where or why anyone would use a barlow lens. Eyepieces are for visual observation, hense me removing them from the scenario when referring to imaging.

Thanks

selected answer This post was selected as the answer by the original poster.
Metastro
OP Metastro Forum Member • Posts: 56
Re: what is the magnification of a telescope without any eyepiece

Thank you very much for helping me understand

I see I should be applying the barlow strength to the focal length of the telescope to get the new fl

2000mm * 2x barlow lens = 4000mm fl

Then if I had a 20mm eyepiece, the magnification = 4000 / 20mm, for example.

I have been googling myself regarding finding out magnification without eyepiece but for camera, and I can see now that this is dependant on the camera, and that I need to do some research on how arcsec / pixel related to magnification. So that's fine.

Thanks!

82Virago Regular Member • Posts: 289
Re: what is the magnification of a telescope without any eyepiece

50mm = x1 is pretty reasonably estimate when comparing to a 35mm equivalent.

For other cameras you need to factor in the sensor size. I use a Canon APS-C, so 1.6x to come with my estimate.

It is a thing that your initial idea of 4000x is not correct. Ask anyone...try imaging at 4000x!! LOL!   I think you would need a base on the moon, skies on earth would probably never have good enough seeing.

There is website that allows you to put in data and see an approximate 'view' with most of the popular cameras on the market, astro, dslr, mirrorless etc. I can't recall it but someone may post.

Cheers

Jon555 Veteran Member • Posts: 7,238
That's slightly complicated...

If you just attach a camera then it's the same as a lens, so a 600mm focal length telescope is the same as a 600mm camera lens for what you see. As to resolution that depends on the camera sensor size, MP count and Telescope quality.

With Barlows it depends where you put them. My 2x Barlow becomes 3.5x if I move it about in the chain of stuff between the Telescope and the Camera. So for my colour Moon photo I went:

Scope-Barlow-Diagonal-2" Extension Tube-T2 wide adaptor-T2 to EOS adaptor-Camera

(I needed the diagonal and extension tube to get enough back-focus.)

Which gave about 2200mm equivalent, and the light was spread out so much (f/6.6 Telescope) I needed ISO1600 and stacked the images to get a good result. Placed after the diagonal the Barlow would be about 2x.

For a sanity test the Moon is about 0.516 of a degree across (varies somewhat as its orbit isn't circular). So for example in my image it was about 53.5% of the image width, so giving a horizontal FoV of about 0.96 degrees and so a equivalent focal length (as was an FF camera) of around 2200mm.

Oh and remember you'll need to flip stuff shot in a Telescope to give a right-way-up look (or if you forget people will just assume you're in Australia/similar).

You can just use the focal length/50 for approximate magnification, but as you are unlikely to use the image uncropped it's a bit meaningless.

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Tristimulus Veteran Member • Posts: 7,557
Re: what is the magnification of a telescope without any eyepiece

Just a twist to the sound answers given above.

Pixel size also matter.

When using my 24MP FF image sensor the pixels are 6 microns squares. With this camera and 530mm focal lenght each pixel covers about 2.3 arc seconds x 2.3 arc seconds at prime focus. This is the plate scale.

When using my 24MP APS-C camera with 4 micron pixels the plate scale is 1.6 arc seconds.

The image sensor with smaller pixels will resolve finer detail when using the same focal lenght. So magnification becomes a pretty useless measure in this context.

It is still meaningful to say that a 50mm lens magnifies 1x when using a full frame camera, but that is only part of the story. When using this measure my 530mm telescope magnifies 10.6x compared to a 50mm lens. But resolution also depend upon pixel size.

When I want high resolution (sun, moon and planets) my tiny industrial video and stills camera with 2.4 micron pixels comes in handy - just extend the focal lenght to f/15 (in my case a 3x barlow lens is all that is needed) and the telescope and camera combination is diffraction limited. Longer focal lenghts will only give a larger and dimmer image without any added detail.

Metastro
OP Metastro Forum Member • Posts: 56
Re: what is the magnification of a telescope without any eyepiece

82Virago wrote:

50mm = x1 is pretty reasonably estimate when comparing to a 35mm equivalent.

For other cameras you need to factor in the sensor size. I use a Canon APS-C, so 1.6x to come with my estimate.

It is a thing that your initial idea of 4000x is not correct. Ask anyone...try imaging at 4000x!! LOL! I think you would need a base on the moon, skies on earth would probably never have good enough seeing.

There is website that allows you to put in data and see an approximate 'view' with most of the popular cameras on the market, astro, dslr, mirrorless etc. I can't recall it but someone may post.

Cheers

Haha yes definitely, I learnt about max useful magnification on the day I posted the original question, and I can see that 4000x magnification would be absolutely crazy.

you would need an aperature of 1600mm to make that useful! can you imagine?

Thanks!

Metastro
OP Metastro Forum Member • Posts: 56
Re: That's slightly complicated...

Jon555 wrote:

If you just attach a camera then it's the same as a lens, so a 600mm focal length telescope is the same as a 600mm camera lens for what you see. As to resolution that depends on the camera sensor size, MP count and Telescope quality.

With Barlows it depends where you put them. My 2x Barlow becomes 3.5x if I move it about in the chain of stuff between the Telescope and the Camera. So for my colour Moon photo I went:

Scope-Barlow-Diagonal-2" Extension Tube-T2 wide adaptor-T2 to EOS adaptor-Camera

(I needed the diagonal and extension tube to get enough back-focus.)

Which gave about 2200mm equivalent, and the light was spread out so much (f/6.6 Telescope) I needed ISO1600 and stacked the images to get a good result. Placed after the diagonal the Barlow would be about 2x.

For a sanity test the Moon is about 0.516 of a degree across (varies somewhat as its orbit isn't circular). So for example in my image it was about 53.5% of the image width, so giving a horizontal FoV of about 0.96 degrees and so a equivalent focal length (as was an FF camera) of around 2200mm.

Oh and remember you'll need to flip stuff shot in a Telescope to give a right-way-up look (or if you forget people will just assume you're in Australia/similar).

You can just use the focal length/50 for approximate magnification, but as you are unlikely to use the image uncropped it's a bit meaningless.

Thanks Jon555,

I was getting too caught up in the theoretical mathematics behind it I think. In my beginners case I am thinking purely about the scope, a barlow and straight to camera. So I can see now that, as you say, it's just the fl scope length, multiplied by the barlow strength.

As I do more research and have read everyones answers, I can also see that magnification is only a small aspect of this. It's difficult to calculate what is 'optimal' when using a camera as the camera sensor itself introduces many variables. So from what I can see there is no 1 size fits all. Its about:

Seeing conditions

the scope and any attachnments

the camera sensor

the subject

(and more I have yet to learn)

I understand now that you simply have to do the maths on a per subject basis - which is fine, my original question has definitely been answered!

Jon555 Veteran Member • Posts: 7,238
Re: what is the magnification of a telescope without any eyepiece

Metastro wrote:

82Virago wrote:

50mm = x1 is pretty reasonably estimate when comparing to a 35mm equivalent.

For other cameras you need to factor in the sensor size. I use a Canon APS-C, so 1.6x to come with my estimate.

Not so much, as you can get FF cameras with the same pixel density. It does apply to FoV, but it's rare to use all of it with very long lenses for Astro.

It is a thing that your initial idea of 4000x is not correct. Ask anyone...try imaging at 4000x!! LOL! I think you would need a base on the moon, skies on earth would probably never have good enough seeing.

There is website that allows you to put in data and see an approximate 'view' with most of the popular cameras on the market, astro, dslr, mirrorless etc. I can't recall it but someone may post.

Cheers

Haha yes definitely, I learnt about max useful magnification on the day I posted the original question, and I can see that 4000x magnification would be absolutely crazy.

you would need an aperature of 1600mm to make that useful! can you imagine?

Thanks!

 Jon555's gear list:Jon555's gear list
Nikon Coolpix 950 Fujifilm FinePix Real 3D W3 Sony RX100 V Canon EOS 5DS R Panasonic GH5 +31 more
Jon555 Veteran Member • Posts: 7,238
Re: That's slightly complicated...

Metastro wrote:

Jon555 wrote:

If you just attach a camera then it's the same as a lens, so a 600mm focal length telescope is the same as a 600mm camera lens for what you see. As to resolution that depends on the camera sensor size, MP count and Telescope quality.

With Barlows it depends where you put them. My 2x Barlow becomes 3.5x if I move it about in the chain of stuff between the Telescope and the Camera. So for my colour Moon photo I went:

Scope-Barlow-Diagonal-2" Extension Tube-T2 wide adaptor-T2 to EOS adaptor-Camera

(I needed the diagonal and extension tube to get enough back-focus.)

Which gave about 2200mm equivalent, and the light was spread out so much (f/6.6 Telescope) I needed ISO1600 and stacked the images to get a good result. Placed after the diagonal the Barlow would be about 2x.

For a sanity test the Moon is about 0.516 of a degree across (varies somewhat as its orbit isn't circular). So for example in my image it was about 53.5% of the image width, so giving a horizontal FoV of about 0.96 degrees and so a equivalent focal length (as was an FF camera) of around 2200mm.

Oh and remember you'll need to flip stuff shot in a Telescope to give a right-way-up look (or if you forget people will just assume you're in Australia/similar).

You can just use the focal length/50 for approximate magnification, but as you are unlikely to use the image uncropped it's a bit meaningless.

Thanks Jon555,

I was getting too caught up in the theoretical mathematics behind it I think. In my beginners case I am thinking purely about the scope, a barlow and straight to camera. So I can see now that, as you say, it's just the fl scope length, multiplied by the barlow strength.

As I do more research and have read everyones answers, I can also see that magnification is only a small aspect of this. It's difficult to calculate what is 'optimal' when using a camera as the camera sensor itself introduces many variables. So from what I can see there is no 1 size fits all. Its about:

Seeing conditions

the scope and any attachnments

the camera sensor

the subject

(and more I have yet to learn)

I understand now that you simply have to do the maths on a per subject basis - which is fine, my original question has definitely been answered!

Also stacking helps a lot on noise and detail. I wrote a step-by-step on stacking the Moon BTW, in case of interest:
https://www.cloudynights.com/topic/701078-the-moon-with-its-colours-enhanced/?p=10556559

(The image at the top of the thread is a down-sized version of the 2200mm one, the partial Moon one is actual pixels if you look at it full-res..)

 Jon555's gear list:Jon555's gear list
Nikon Coolpix 950 Fujifilm FinePix Real 3D W3 Sony RX100 V Canon EOS 5DS R Panasonic GH5 +31 more
Metastro
OP Metastro Forum Member • Posts: 56
Re: That's slightly complicated...

Incredible!

I will have to address stacking eventually too. The furthest I've got with any session was using PHD and a Bahtinov Mask to gain focus on Polaris, but the seeing conditions were so bad that I couldnt get a consistant reading close enough to 0.00 focus. I was advised that I should try shooting over grass instead of the concrete I was on. I'll attempt again soon.

Thanks

Metastro
OP Metastro Forum Member • Posts: 56
Re: what is the magnification of a telescope without any eyepiece

Hi Trollmannx

I recall your user name and believe you have assisted me before, so thank you.

My question originally arose as a consequence of discovering just how surprisingly poor my results of photographing Mars was. Bare in mind I didnt have amazing expectations, and was not tracking, so it's perhaps not all that surprising

But it did get me to question how people possible get decent photographs of Mars, to the extent you can see the ice and details on the surface. In my photo attached, you can tell it's mars, but thats about it!

I understand that using a 5x powermate with my edge HD 8 was possibly way too much for it's max magnification. Though I will have to learn just by how much I overstretched it by working it out via details of my Sony A7S. I think I knew deep down it wasn't going to work, but I sadly still make these mistakes just to learn from.

Astrozoid
Astrozoid Contributing Member • Posts: 852
Re: what is the magnification of a telescope without any eyepiece

Metastro wrote:

Hi

I'm a bit confused and trying to work out how people use barlow lens' with telescopes

Hi Metastro,

This might help, it's an intro to planetary imaging:

https://www.astropix.com/html/i_astrop/Planetary_Imaging.html

Jerry

Tristimulus Veteran Member • Posts: 7,557
Re: what is the magnification of a telescope without any eyepiece

Metastro wrote:

Hi Trollmannx

I recall your user name and believe you have assisted me before, so thank you.

My question originally arose as a consequence of discovering just how surprisingly poor my results of photographing Mars was. Bare in mind I didnt have amazing expectations, and was not tracking, so it's perhaps not all that surprising

But it did get me to question how people possible get decent photographs of Mars, to the extent you can see the ice and details on the surface. In my photo attached, you can tell it's mars, but thats about it!

I understand that using a 5x powermate with my edge HD 8 was possibly way too much for it's max magnification. Though I will have to learn just by how much I overstretched it by working it out via details of my Sony A7S. I think I knew deep down it wasn't going to work, but I sadly still make these mistakes just to learn from.

This photo would be pretty ok at my location (lots of lively turbulence) with my gear (do only have a small astrograph, pretty good, can be used with a good barlow lens to get pretty decent results - given the usual atmospheric conditions here).

So not much help from here on this one...

Happily I see the poster above has provided a good link.

Metastro
OP Metastro Forum Member • Posts: 56
Re: what is the magnification of a telescope without any eyepiece

Thank you, Jerry, that's really cool! can't wait to read.

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