When you are shopping for your first refractor telescope, don’t forget to check whether you need a field flattener or not and how much it costs.
Budget refractors, in fact, may have an attractive price tag, but the cost of the final setup can, sometimes, double once you add the field flattener.
But what is a field flattener, and why you may need one?
What Is Field Curvature
A lens is a curved glass, and light rays passing through it are also curved. The resulting effect is that a flat object behind the lens looks curved to the observed on the opposite side of the lens.
We can simply illustrate this effect by looking at a squared sheet of paper with a magnifying glass.
One type of field curvature common in photography is barrel distortion.
This is typical of wide-angle lenses and fisheye. With this type of distortion, the image is stretched near the center of the frame.
Another type of distortion is when the lines are curved outwards: this is called Pincushion distortion. With this type of distortion, the image is stretched more and more going towards the corners of the frame.
Barrel and Pincushion distortions are examples of a simple field curvature, but one could have more complex distortions, such as mustache/wavy field curvatures.
In photographic lenses, extra optical elements are added in the lens design to reduce or correct these distortions, thus rectifying the image.
Field Curvature And Astrophotography
In astrophotography, we don’t have lines that could be curved by lens and telescopes suffering from barrel or pincushion distortions, and you may think field curvature is not an issue and does not matter. But it does matter.
For any practical consideration, the night sky is flat in the field of view of your instrument, with all the stars at infinite.
Because of this, all stars are on the same focal plane… at infinity. But field curvature will curve and bend the flat sky, increasing deformation towards the edges of the frame.
Because of field curvature, the closer the stars are to the edges of the frame, the more stretched, blurred and affected by chromatic aberrations they look.
As shown in the image above, when using refractors, field curvature is often a Pincushion one and it results in elongated stars creating a runaway effect. The stars are not sharp and display color separation.
Because planets are often small in the frame, field curvature is less an issue in planetary astrophotography, but when you fill the frame, like in closeups of the lunar surface or deep-sky images, the problem is real and readily visible.
What Is A Field Flattener?
When light passes through a series of lenses, you can have all sorts of optical aberrations, from chromatic aberrations to coma, and field curvature.
A field flattener is an optical element that corrects the field curvature in some types of refractor telescopes.
When Should I Use A Field Flattener And Will My Images Look Better If I Use One?
We often say less glass is better, but sometimes more glass is necessary.
If you are photographing the starry sky with photographic lenses, there is not much you can do to correct distortions. You can eventually stop the lens down a bit to improve the edges sharpness and reduce chromatic and coma aberrations, but that’s about it.
Petzval telescopes aside, where the field curvature is corrected in the telescope, if you are using a refractor for deep sky astrophotography, then you need a field flattener to correct for the field curvature of your instrument.
A field flattener flattens the field of view and ensures round and sharp stars all around the frame.
How To Use a Field Flattener
Using a field flattener is a no-brainer: just screw it at the back of your telescope or insert it in the telescope using a nosepiece. That’s it.
But there are a few technical things to sort out when using a field flattener:
- Available backfocus
- Distance between the camera sensor and the flattener
- Image Circle
Before buying a telescope and a flattener, you need to verify how much backfocus is available for that combination.
The back focus is the distance between the back of the telescope and the focal plane, i.e., how far back the telescope+flattener will focus the image. This is a critical parameter in astrophotography.
In order to be able to focus with your camera, the camera sensor must be placed at the proper distance from the back of the instrument. If the back focus is too long, it is not a problem: just buy some extension tube to have the camera sensor at the required distance.
But, if the back focus is very short, you may not be able to have the camera sensor that close to the instrument and you will not be able to focus.
I recently reviewed the combination of the Sky-Watcher Evoguide 50ED and field flattener, and while without a flattener a DSLR can be used, but with the dedicated Sky-Watcher Evoguide 50ED Field Flattener attached, the back focus is so short (17.5mm) that you can only use astro cameras.
With the Starizona EVO-FF field flattener, the available back focus is 34mm, enough for a mirrorless camera but still too short for a DSLR camera.
Set The Proper Distance
The back focus is a nominal value, but you may need to tweak it a little bit in practice, and you can find on the market spacers of 1mm or less.
For the EVO-FF field flattener, for example, Starizona declares a tolerance on the back focus of +/- 2mm.
The image below guides you in setting the proper distance between the flattener and the camera sensor by looking at the ways stars are elongated.
The field of view with a given telescope depends on the sensor size of the camera you use: the smaller the sensor, the narrower the field of view. Particularly with a DSLR, it is important to look at the image circle the flattener can correct.
The EVO-FF, a general purpose flattener for short focal refractors, has an image circle of 27mm, meaning the field is flat with sensors up to the APS-C format. With a full-frame camera, this flattener will still show some field curvature at the edges of the frame.
Field Flatteners And Reducers
Some flatteners do just that: they flatten the field removing the field curvature created by the telescope.
Some flatteners are also reducers: this means that while they correct the field curvature, they also reduce the focal length of the telescope.
This is often the case with long focal telescopes: they are great for the planets and the Moon, but difficult to tame for deep sky astrophotography, as they have very small fields of view and may be quite dark.
A focal reducer is the opposite of a Barlow lens and will make such instruments better suited to deep sky astrophotography by enlarging the field of view and reducing the f-ratio of the instrument, making them brighter.
Which Flattener Should I Buy?
Usually, field flatteners are available as specific accessories for a particular telescope, like the Sky-Watcher Evoguide 50ED Field flattener, made for the Evoguide 50ED. The same is true for other telescopes.
The Sky-Watcher Evostar 72ED and most of the William Optics refractors have their own field flatteners.
Therefore, I suggest you check among the available accessories for the scope you are interested in: the field flattener is not the most affordable accessory, but it is an absolute must have.
There are, though, some flatteners targeting a type of telescope rather than a specific model. The Starizona EVO-FF, for example, is suitable for refractors with a focal length ranging from 250mm to 450mm.
If you shop for a generic field flattener, mind that the available back focus may change with the focal length of the telescope you plan to use it with.
A Field Flattener is an expensive piece of equipment, sometimes reaching 50% or more the telescope’s cost. But for deep sky astrophotography, it is not optional: you need a flat field.
But shop wisely, and do your homework to avoid bad surprises: when considering to buy a flattener, backfocus and image circle are the main specs you need to look at.