Astrographs: an Introduction

In astrophotography, most of us begin with a DSLR and a tripod. The next best upgrade you can get is a tracking mount. But, if you want to shoot some serious deep sky images, then you must get an Astrograph.

An astrograph is a telescope designed for the sole purpose of astrophotography. They are used by astrophotographers for shooting deep-sky objects. A refractor is what comes to mind when we think of an astrograph. But, all kind of telescopes can be designed for astrophotography –

  1. Reflector
  2. Refractor
  3. Schmidt-Cassegrain

Improvements in photography in the middle 19th century led to designs dedicated to astrophotography. Scientists used astrographs to survey the night sky, and one of the famous projects was Carte du Ciel. Pluto was discovered by using an astrograph. Rather than looking through the telescope, it was discovered by using images taken by an astrograph.

Here, in this article, we will talk about different designs of refractors and other technical details you have to consider before buying one. So, let’s get into it –

Specifications of an Astrograph –

  • Type of Design
    1. Simple refractor: No color correction is done in the design. You will see this design in binoculars and cheap telescopes. Just one objective lens element is used.
    2. Achromat: Brings red and blue wavelengths at the focus. You will not see any blue or red halos around bright objects.
    3. Apochromat: Brings red, green, and blue wavelengths at the focus. Practically no false color will be visible. Optically perfect images.
  • Number of lenses
    1. Doublet: Two lenses are used, they are usually used in an achromat.
    2. Triplet: Three lenses are used, usually in apochromats.
    3. Quadruplet: Four lenses are used. Very expensive, they are.
    4. 5-element: Five lenses. Typically, a doublet objective lens and triplet corrector group.
    5. 6-element: Six lenses.
  • Grade of Glass
    1. ED – extra-low dispersion 
    2. LD – low dispersion
    3. SLD – Special low dispersion
    4. ELD – Extraordinary low dispersion
    5. UL – Ultra-low dispersion
  • Specific Glasses, according to their Abbe number.
    1. FCD100: 95.10
    2. Calcium Fluoride: 94.99
    3. FPL-53: 94.96
    4. FPL-52: 90.32
    5. FPL-51: 81.61
    6. FCD1: 81.61. Purple and cyan fringing. The main issue is with stars. Only when you zoom in.

Abbe Number:

In optics and lens design, the Abbe number is a measure of the material’s dispersion, with high values of V indicating low dispersion.

Remarks:

  1. Achromats are those which use two lenses for correcting the chromatic aberration. And Apochromats use three lenses.
  2. There is no clear line where ED glass starts and where it ends. Always look for what’s the Abbe number of the glass.

After going through the article, you might say that triplet is better than a doublet. Well, that’s not the case all the time. There are other factors to consider, and then only one can say which telescope will give better results. For instance, here’s a comparison between various astrograph designs and their PSF:

MTF plots for selected 4 and 6-inch refracting objectives.

Point Spread Function describes the response of an imaging system to a point object. A refractor with a PSF value of 1 will generate a perfect image. You can see ’60’ and ’40’ are perfect refractors of 4″ and 6″.

On studying the above graph, we can conclude two points:

  1. A doublet FPL53 performs better than a triplet FPL51. Although under night-sky these refractors will perform differently. But this graph shows us that there is not much difference in a doublet and triplet in most cases. So, for your first astrograph, you can go with a doublet with great optics.
  2. Fast doublets with small apertures can compete with triplets. While there is no visible correlation with large aperture ones.

In Conclusion,

Getting your first astrograph can be tough with all the various specification and details to consider. There is too much things to look for like fov, portability, design, optics etc. We have just covered the optics part here in this article and will cover others in the next ones.

~AK

Published by Anand Krishna

Amateur astronomer and astrophotographer. Interested in astrophoto processing, astrostatistics, comet hunting, visual and radio astronomy.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s