When I started astrophotography hobby, I always had questions? Which is the best telescope for astrophotography? Which are different types of telescopes? What is the focal length and how to check the focal ratio?
If you are getting the same questions, I am here to help you with my experience so let’s start with all the necessary information so that you would be more accurate while getting your first telescope for astrophotography.
Basic Information of the telescope
The focal length is the measurement between the optical center of the lens to the camera. Means range of light travel inside a telescope from optical entry center of the lens to eyepiece or camera.
Size of the sky object is directly proportioned to the focal length of the telescope. It means focal length light travel length from entry.
Long the focal length sharper the image.
You may hear about telescope with 500 mm or 1250 mm or camera lens with 200 mm. All these values are the focal length.
The diameter of the telescope or optical tube is called Aperture. most common aperture for refractor is 90 mm or 102 mm.
A larger aperture allows extra more light. Large aperture helps to show dimmer objects which increase the resolution of the image.
Focal Ratio of the telescope
A focal ratio is a formula which gives how much light is going to hit to the camera. This denotes with value as “F.”
Focal ratio = focal length / telescope aperture
e.g., Suppose you have refractor with focal length as 500 and aperture of that refractor is 90 then the focal ratio of that refractor will be 500/90 = F 5.5
Small the focal ratio more light comes inside the telescope and more field of view. For deep sky astrophotography, it recommended using a small focal ratio telescope.
Large the focal ratio less light comes inside the telescope and less field of view. For Planetary imaging, it recommended using a large focal ratio telescope.
Magnification of telescope
Magnification is the process to magnify an object, to get closer, view bigger. In telescope terminology magnification is depends on the aperture size.
We can get magnification till 2X of aperture value means more the aperture size we can go for more magnification.
e.g. If you have 127 mm MAK telescope, then you can use 127 * 2 = 254 times magnification.
Field of View aka FOV
Field of view is the term which denotes the size of the sky we can see based on focal length and aperture. FOV is directly proportioned to the Focal ratio.
If focal ratio value is high, it gives less FOV. This means it provides you will get very small or magnified area of the sky.
In other hand small focal ratio gives large FOV .
Below are the examples of M8 Lagoon nebula ‘s FOV with a different focal length; this will give you a better understanding of focal length and FOV.
Example # 1
Object – M8 – Lagoon Nebula
Focal length – 300 mm
Aperture – 90 mm
Camera ASI224 MC
You can photography whole M16 Nebula for this given focal length, aperture and using the mentioned camera.
Example # 2
Object – M8 – Lagoon Nebula.
Focal length – 500 mm
Aperture – 90 mm
Camera ASI224 MC
You can see photography partial M16 Nebula for this given focal length, aperture and using a mentioned camera.
Example # 3
Object – M8 – Lagoon Nebula
Focal length – 1250 mm
Aperture – 90 mm
Camera ASI224 MC
You can photography hugely magnified M16 Nebula for this given focal length, aperture and using a mentioned camera.
You must have to check your FOV details before going to an astrophotography session which helps you to understand better how big or small you can get the object in your telescope.
Are you planning for astrophotography session? , then please follow this checklist.
Types of telescopes for astrophotography
There are three basic types of telescopes, all of them can use for astrophotography, all types are having their advantages or disadvantages.
This type of telescope mostly uses for deep sky astrophotography, this type of telescope has very less F factor which gives a perfect field of view to capture most of the deep sky objects.
Refracting telescope also recommended to beginners as a first telescope.
Light passes through refractor telescope and refracts to the eyepiece. Refractor telescope needs a diagonal mirror to view objects.
Refractor telescope has straightforward design and portable if aperture less than 110 mm. Easy to aim objects even in the dark sky and very convenient see for eyepieces.
The major problem with refactoring is a chromatic aberration, which means light is not gathered in the same direction. This problem is rectified with APO type refractor but APO type refactors are 5 X times more expensive. Long refractor gets impacted due to high wind and needs more proper balance.
Reflecting telescope or a Newtonian telescope.
Reflecting telescope is also known as Newtonian telescope as it was designed by Newton.
As the name says this telescope reflects light, this telescope has the primary mirror usually placed at the bottom of the tube which reflects rays to the secondary mirror and the secondary mirror reflects it to eyepiece or camera.
No Chromatic aberration in this type of telescope. Less Price amount or inexpensive in order to make large aperture telescope. The reflector is most of the time Faster which means small F factor and large FOV.
Significant problems with Reflecting telescopes are having Coma in a very fast telescope, which can be corrected. Light lost due to 2 mirrors. Contrast lose due to secondary mirror light reflecting. Large in size and very heavy in size, which means needs proper telescope mount which can carry heavy weight.
Compound telescope or Catadioptric telescope
This type of telescopes are involved both reflecting and refracting of light in order to reduce aberration.
This telescope has corrected lens on top of the telescope from which light passed to the primary mirror which is strongly curved and refracts light to a secondary mirror which then again refracts light to eyepiece or camera.
In this telescope, light takes a very long focal length path which is folded in the shorter tube. The main types of compound telescopes are Schmidt – Cassegrain and Maksutov-Cassegrain (MAK ).
The compound telescope has very low aberration. It is very portable in size with a large focal length large light gathering power.
Significant problems are, compound type of telescope are very expensive. It has a large amount of light loss due to multiple mirrors. It takes a long cooling time.
Barlow lens is a lens which uses to get to extend the focal length of the telescope. It helps to magnify objects.
Basic Barlow lens comes 2 X , 3X and 5 X power which magnify object based on X factor.
Reducersare also a Barlow lens which has power less than 1 . which helps to reduced focal length.
Basic reducers comes 0.63 , 0.7 , 0.5 etc.
Barlow and reducers are basically used to change FOV.
I use SVBONY Barlow lens and Astromania reducer, I brought it from Amazon.
Focusing is a very important factor in photography. Its very easy to focus on the bright object and object we can see with naked eyes.
But deep sky objects are very dim, and without focus, we cannot get got high image quantity.
In night sky to get focus we would need to use Bahtinov Mask ,Bahtinov mask is use to focus astronomical telescope.
Bahtinov Mask comes in a different size. It made up of the aluminum or hardened plastic.
If you have more than one telescopes with different aperture, then you can get single Bahtinov Mask which fits in the range of your optical tubes.
How to connect the telescope to DSLR camera
You can enjoy the direct viewing experience with the eyepiece, and Only bright night sky objects can be viewable directly with the eyepiece.
If you are thinking for astrophotography then definitely you are thinking to get deep sky object photography. So in order to take photographs, you would need to attach a DSLR camera or CCD camera to Telescope optical tube.
you need T ring or M42 adapter to connect DSLR camera to telescope. this adapter helps to connect DSLR and 1.25 inch or 2-inch optical tube end.
when I say astrophotography I mean to say deep sky photography and planetary imagining. I recommend have a separate telescope for both astrophotography style.
Best telescope for deep sky astrophotography
Deep Sky astrophotography needs, Large FOV, low F ratio, Fast telescope.
I would recommend refractor for deep sky astrophotography. When I started my first deep sky object session I had my refractor SV48, SVBony 500/90 mm refractor with dual speed focuser.
Refractor is very portable and easy to use and focus which is very much suitable for beginners. I agree with the fact that it has very high chromaticaberration, but we can eliminate it during image processing.
I would recommend Compound telescope MAK telescope for Planetary imaging. I have 1250 mm / 90 mm which is F13, very powerful MAK. MAK is a compact size telescope with large focal length. It is very portable.
I would recommend Compound telescope or MAK telescope for Planetary imaging. I have 1250 mm / 90 mm which is F13, very powerful MAK. MAK is a compact size telescope with large focal length. It is very portable.
With proper focal reducer, you can use this MAK as a subordinate of refractor for deep sky object astrophotography. So If you are planning to have only telescope then get MAK with additional accessories of a focal reducer, in this way you can achieve deep sky astrophotography and planetary imaging with a single telescope which will be your best telescope for astrophotography.
I am happy to hear about your first telescope if you already brought or planning to buy and why ?. Kindly comment below with your first telescope Story.