Friday, June 30, 2017

A Beginner's Guide to Solar System Photography with the Celestron 127 SLT (and other Alt/Az Scopes) Part 2

Update: Based on feedback, I have broken the original large post into four smaller posts to make it easier to read.


The amount of magnification you can use and get good results is limited by aperture and atmospheric conditions. The larger your aperture, the more light your scope gathers, and the more you can magnify an image. If you try to use too high a magnification for your aperture, your image will get dim and fuzzy, and you'll have a really hard time focusing, even with a Bahtinov mask.

For a given aperture and focal length telescope paired with a given camera, you can estimate the magnification at which you will get optimal detail. It is generally best to shoot at this magnification, and then rescale your resulting image in an image editing program. 

The focal ratio of a telescope (or camera lens) is defined as the focal length over the aperture. The use of a Barlow lens to increase magnification multiplies the focal length - the top number of the ratio - so it linearly changes your focal ratio.

focal ratio = focal length / aperture

The general guideline for determining magnification is this:

When seeing is average, use a focal ratio of 5 times your camera's pixel size in microns. 
If the night is excellent seeing, you may be able to push to 7 times pixel size in microns

The ASI290MC has a pixel size of 2.9 microns. My optimal focal ratios are therefore:

Average seeing: 5 x 2.9 = f/14.5
Exceptional seeing: 7 x 2.9 = f/20.3

With no Barlow or extension tube, the 127 SLT has the following focal ratio:

1500 mm focal length/127 mm aperture = F/11.8.

A Barlow effectively multiplies your focal length, so:

With 1.5x Barlow: 1.5 x 11.8 = F/17.7
With 2x Barlow: 2 x 11.8 = F/23.6
With 2.5x Barlow: 2.5 x 11.8 F/29.5

Therefore, the guidelines indicate that for good to exceptional seeing, I should run a magnification of 1.5x. Even on the best nights, 2x is not optimal for detail.

I tested it. The results are shown below. The night I used 2x was significantly better seeing than the night I used the 1.5x, but the image shot at 1.5x is significantly sharper, and you can see a detail that you can't make out at 2x.

I learned from a post on a forum that the inexpensive Celestron 2x Barlow is super flexible. If you use it as-is, you get 2x magnification. If you unscrew the lens portion and attach it to your camera using the filter threads, you get a 1.5x magnification.

If you have a different telescope, run the math above - you'll likely get a different answer because your starting focal length is different.

Image Orientation

Be aware that the view you see through the viewfinder or on the camera may be different than the orientation of your target. Refractors, reflectors, and Mak-Cass telescopes all behave differently because of the way the optics work.

A great summary of the different configurations and their effect on image orientation is available this article on star diagonals

This is especially useful when looking at the moon, because an upside-down right-left flipped image is hard to navigate.

Dew Control

If you have a refractor or Mak (like the 127SLT) you'll likely run into the problem of dew forming on your objective. As the temperature approaches the dew point, it will become more and more likely. You might think that as long as the temperature of the air is above the dew point, you don't need to worry about it, but it's more complicated than that. When pointed at a dark sky, your telescope actually radiates heat off into space, chilling it below the ambient temperature and allowing moisture to condense out of the air just as it does on the outside of a cold beverage glass.

The dew will form so gradually you won't immediately notice it - you will just notice that you are losing sharpness at first. You can't safely wipe the dew off without risking damaging the coatings on your objective - if it forms, you are done for the evening. Just let it air out inside with the covers off.

You can reduce the odds of dew forming be adding a simple foam shield that extends out past the end of your scope. It should be at least 1.5 times as long as your aperture is wide. Many people report good results with using part of a foam camping mat about 1/2" thick - that was a little too rich for my blood, so I used a couple sheets of 1/8" foam from a craft store laminated together with hot glue. I added velcro strips to attach it to the scope. This has pretty much eliminated the problem for me for sessions of up to three hours.

If you live in a area where dew formation is more of a problem, or you intend to have very long observing sessions, it's possible to buy or make small low power electrical heaters to prevent dew from forming on the glass surfaces of your telescope, including eyepieces and finders.

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