Camera Sensor size in relation to telescope focal length

[Cols Astro Journey]

Hi all, just wanted someone to confirm something for me.

I am a pro photographer and just getting into this hobby, so I have plenty of photographic equipment and am about to pull the trigger on a S/W Heq5 pro mount and the S/W 150pds telescope (750mm focal length) and I assume my camera sensor will dictate the focal length?

So can I assume with my full frame sensor (nikon d750) the focal length to be 750mm, with my crop sensor (nikon d5300) 1125mm and with my M43 olympus to be 1500mm.

I hope that makes sense but I cant find this information any where else?

 

[Freddie]

Putting a camera on a telescope does not change the scopes focal length. All putting different cams on the scope will do is allow more or less of the image to fit onto the cams chip.

[Cols Astro Journey]

Thanks for the response but assuming that all of the sensor is covered then that would surely magnify the image? or am I missing something.

Is it possible to cover different size sensors? as that would explain your response and if so what sensor size is suited to the S/W 150pds

Edited May 10, 2020 by Cols Astro Journey
extra q's

[Astrosharkey]

http://astronomy.tools/calculators/field_of_view/

 

Go to the above on the imaging tab and play with your scope and camera options, will give you a better idea, remember the scope I’d just like a lens the only way to change the magnification is with a reducer or barlow 

[miguel87]

The telescope is creating a real image at the focal plane (where you place the camera chip). The image being sampled does not change, at all. Different chips will just pick up more or less of it in greater or lesser detail.

Magnification and focal length are fixed by the scope.

Imagine an image of the full moon, different sized chips will show more or less of the space around the moon. Or even not the entire moon if the chip is too small. So it might look bigger but only because it is taking up more of the sensor. It is not actually more magnified.

This is how I understand it anyway.

Edited May 10, 2020 by miguel87

[Cols Astro Journey]

Hi thanks to the responses but just checked on astronomy tools (as very kindly suggested) and came up with this (see attached) which is what I was expecting but it different from your responses so I am now even more confused.

 

 

[miguel87]

12 minutes ago, Cols Astro Journey said:

 

Is it possible to cover different size sensors? as that would explain your response and if so what sensor size is suited to the S/W 150pds

I think... (somebody may correct me here)

That a full frame chip is better in any telescope. I think the image created by the telescope is bigger than normal camera chips anyway so I dont think people consider size matching sensors.

I am NOT an expert.

[miguel87]

8 minutes ago, Cols Astro Journey said:

Hi thanks to the responses but just checked on astronomy tools (as very kindly suggested) and came up with this (see attached) which is what I was expecting but it different from your responses so I am now even more confused.

 

 

You're just grabbing more of an already 'taken' image by the telescope.

So take your computer screen from your picture. The image of the nebula is at a certain magnification, regardless of what boxes your draw on the screen. All those images would be taken at the same magnification. Just a larger or smaller field of view.

I.e. I could not take a photo of the moon at 100x mag. Blow it up to twice the size on my laptop and post it online saying it was taken at 200x magnification.

Edited May 10, 2020 by miguel87

[Cols Astro Journey]

Thanks Miguel, I appreciate that its the same magnification from the telescope (750mm) but the sensor size in normal photography can effectively increase/decrease focal length by using the same lens on different cameras that have different sized sensors and I just wanted to confirm its the same using a telescope.

 

I guess I just need to know if its possible to use/focus this telescope (150pds) with all three sensors.

Anybody know if thats posible?

[Skipper Billy]

Not wishing to 'muddy the waters' but not all scopes will cover a full frame sensor.

[miguel87]

Just now, Cols Astro Journey said:

Thanks Miguel, I appreciate that its the same magnification from the telescope (750mm) but the sensor size in normal photography can effectively increase/decrease focal length by using the same lens on different cameras that have different sized sensors and I just wanted to confirm its the same using a telescope.

 

I guess I just need to know if its possible to use/focus this telescope (150pds) with all three sensors.

Anybody know if thats posible?

Yep definitely. The image created by the telescope I.e. the focal plane, never moves. It just hovers there in the focusser tube. So all camera chips have to reach the exact same position and boom, you are in focus. The only variable is how deep the chip is within the body of your camera.

Your focal length will always be 750mm.

[Cornelius Varley]

1 minute ago, Cols Astro Journey said:

I appreciate that its the same magnification from the telescope (750mm) but the sensor size in normal photography can effectively increase/decrease focal length by using the same lens on different cameras that have different sized sensors and I just wanted to confirm its the same using a telescope.

This is a myth. The focal length doesn't change, just the field of view changes which is dictated by the size of the sensor.

[miguel87]

2 minutes ago, Skipper Billy said:

Not wishing to 'muddy the waters' but not all scopes will cover a full frame sensor.

I stand corrected. Not my area of expertise there. For sure somebody on this forum knows about your particular setup and I am 90% confident that your purpose built astrophotography scope could cover a full sensor.

✌

[Cols Astro Journey]

Thanks Billy, much appreciated, is there anywhere I can find out which telescopes would cover a full frame sensor, or a table showing what each telescope is able to cover in terms of sensor size?

[Cols Astro Journey]

1 minute ago, Cornelius Varley said:

This is a myth. The focal length doesn't change, just the field of view changes which is dictated by the size of the sensor.

Your right focal length does not change but the field of view does but its the same difference

[Cornelius Varley]

8 minutes ago, Cols Astro Journey said:

Your right focal length does not change but the field of view does but its the same difference

Not really. The telescope/lens will stay the same. Only one thing changes and that's the FOV.

 

[Gfamily]

This thread may be useful.

Olly Penrice is very experienced - and in the thread he says that you need to look for an image size of 45-46mm to be able to use full frame. 

 

[Skipper Billy]

10 minutes ago, Cols Astro Journey said:

Thanks Billy, much appreciated, is there anywhere I can find out which telescopes would cover a full frame sensor, or a table showing what each telescope is able to cover in terms of sensor size?

Not that I am aware of - its a question of finding out what size the 'imagine circle' is for a scope ypu may have in mind - for the SW 150 you mentioned it wont cover the full frame sensor.

BUT

There are very few scopes that will and the benefit is questionable - it's not enough to just cover the sensor - you need to cover the sensor with a flat field that gives sharp round stars right into the corners with minimal vignetting.

The simpler and less of a shock to the wallet is to use a 'cropped frame' sensor which is what most scopes are made to suit.

 

[Cols Astro Journey]

Thanks Billy, I also have a crop sensor DSLR which should allow the telescope to cover the whole sensor. I was just hoping that If I was able to use all three of my DSLR's then I could effectively have three telescopes for the price of one.

Maybe its something the manufacturers should consider.😆

[Skipper Billy]

1 minute ago, Cols Astro Journey said:

Thanks Billy, I also have a crop sensor DSLR which should allow the telescope to cover the whole sensor. I was just hoping that If I was able to use all three of my DSLR's then I could effectively have three telescopes for the price of one.

The other thing to consider is the weight of a full frame camera - a 'crop frame' DSLR is a lot for the focuser to hold square and steady - for many scopes the extra weight of something like a 5D swinging on the focuser could create a whole new set of problems!