Fibs, damn lies, telescopes, and astrophotography

Most people are surprised by the gap between the images on the box of their telescope and what they see at the eyepiece. Many 60 mm telescope have promised 600 X colorful views of Jupiter, and the real view is a small grey blob.

Damn lies

Visual views through a telescope are limited by the laws of nature and many telescopes are sold with promises that are impossible to keep.

The magnifying power of a telescope is calculated by dividing its focal length by the focal length of the eyepiece. This is only part of the story.

Images are blurred by diffraction, because light behaves as a wave. The amount of diffraction is determined by the aperture size (diameter of the primary mirror or lens) of the telescope. This limits the amount of magnification possible with a telescope, before the image becomes too dim and blurry to look good or see more detail.

A good rule of thumb for visual observing is 2X magnification for every millimeter (50X per inch) of objective diameter. For a 60 mm telescope this is about 120 X.

A bright, crisp, 120 power view of a planet is much smaller than anything you would notice in an advertisement or a big telescope box.

Fibs and cameras

The gap between what you can see visually, and the image that can be captured with a digital camera and post processing is another surprise.

These images were taken with an 89 mm telescope. The first is a close approximation to a typical image that I see visually using this telescope:

Three of Jupiter's moons are visible and the cloud bands. On a night of very good seeing, I can make out the Great Red Spot (if it is facing earth) and even glimpses of shadow transits. Picking out details by eye takes a lot of patience and practice. It gives you an appreciation for the discoveries of astronomers who didn’t have big telescopes or digital cameras to work with.

Let's take a look at what can be done with a digital camera and the same telescope. Making this image took many hours of work. I took 2000 images in a three minute span, found the best 128, statically stacked them to produce a single low noise image, deconvolved the image to make it sharper, and cropped and tweaked the final image.

Sweet Home Alabama Jupiter with Io transit & Europa

Wow, quite a difference! If I’m selling the telescope, I can honestly put the 2nd image on the box. If I’m lazy or more deceptive, I’ll put a Hubble Space Telescope or space probe image on the box.

Most telescope vendors go with the last option. A realistic visual observing image, like my first one above, is buried in the manual, if shown at all.

Capturing an image like this requires more than the telescope and a suitable camera. You need a very long equivalent focal length, 5000 mm or so. Capturing a long focal length image is very sensitive to camera shake. It requires a very solid tripod and mount. The added equipment easily costs several times that of a moderately priced telescope and mount. Astrophotography with a telescope, is difficult and costly, not for everyone. If you are interested, the best way to get started in astrophotography is without a telescope.

Visual observing with a telescope

Many people prefer visual observing. You can see an detailed image like the one above by eye in an amateur telescope. It takes about a 400mm aperture class instrument on a very good night. Detailed visual views of the planets require the cost, weight, and complexity that goes with a large amateur telescope.

Visual observing with a small telescope can be great fun. Don't be discouraged by unrealistic expectations. Bright close up views of the Moon's craters, watching the dance of Jupiter's moons and trying to spot features, jewel-like star clusters, and the contrasting colors of binary stars are all within reach. The smaller and easier a telescope is to set up, the more often you will use it.

Getting help

A great place to get advice on selecting a telescope best suited to your needs is Starizona's Telescope Basics. They also provide detailed information on how to use the scope and mount that you already have.

I keep a list of other print and on-line resources that I recommend..

This note was rewritten from an answer that I wrote to a question about why Jupiter was just a round white blob in a telescope on Quora.

Content created: 2017-08-09




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