SCT Performance

The easy guide to optimizing your SCT

The SCT Design

The Schmidt–Cassegrain (or SCT) is a very popular design among amateur astronomers. It is a catadioptric design i.e. it includes both lenses and mirrors. The optical path is folded which allows the packing of a long focal length in a very short tube making it very compact and light.

Despite many criticisms of the design they can be superb scopes. For example My EdgeHD 8 shows cloud shadings on Venus, breathtaking detail in Jupiter’s bands and moon transits, the icecaps and most features of Mars and Saturn’s rings in HD detail. It can even in average seeing quite easily take 400x-500x on the Moon. In good seeing it can easily take 600x+ with image remaining incredibly sharp. Deep-sky is superb and 8″ of aperture really starts to resolve globulars and objects start to look wonderful from dark skies.

Earlier models from both Meade and Celestron had quite a bit of optical quality variation but these days quality is a lot more consistent and quite good.

But like any other telescope design extracting performance requires an understanding of the design. We often forget these are precision optical and scientific instruments.

Three critical factors that impact the performance of an SCT

  1. Observing conditions: when I experience poor seeing the EdgeHD 8 struggles even at 200x and the stars have flares and look comatic.
  2. Thermal Equilibrium: Tube thermal currents will absolutely wreck havoc with the view. If the scope is not acclimated everything will appear fuzzy.
  3. Collimation: Poor collimation will make objects looks fuzzy. Even minor mis-collimation will soften the view and combined with either 1 or 2 above make things really fuzzy

(1) is usually out of the control of the observer but (2) and (3) can be optimized.

How to diagnose

  1. For 1 above, check your forecast. Also when you look up if stars are twinkling a lot or its windy seeing is likely to be poor
  2. For 2 above, defocus a bright star till you see a donut. If you see slow moving plumes of air it is likely tube currents.
  3. for 3 above, center and defocus a bright star. If the donut is not centered then your collimation is likely off quite a bit.

How to solve

  1. Observing conditions: Not much you can do here except change your observing site or wait for a better evening. When I experience poor seeing the EdgeHD 8 struggles even at 200x and the stars have flares and look comatic.
  2. Thermal Equilibrium: Leave the scope outside for an hour to acclimate. If that is not an option I have found that wrapping the OTA is reflectix works really well especially for quick observing sessions. Combine with a cooling fan before observing for even better results. Another beneficial side effect is that you will get less dew on the corrector.
  3. Collimation: There are 2 steps to SCT collimation. Step 1 is coarse collimation and step 2 is fine collimation. There are many detailed guides on the internet for SCT collimation but see a brief overview below.

SCT Collimation

My goal is to give a high level overview / framework here to orient you. You can then go research of these approaches further.

Step 1: Coarse Collimation

Defocus the star till you see the donut. Make sure the star is centered. Adjust the screws on the secondary till the inner ring is centered. When you adjust the screws the star will move in the fov as it changes the tilt of the secondary. Hence, make sure you center the star / donut each time after you make an adjustment to the screws.

After this step your collimation will be pretty good and you should see minimal flaring of stars and quite pinpoint stars. For deep-sky this is likely to be sufficient depending on how careful you have been and how picky you are. I have even done planetary imaging after this step with very good results but I do recommend the next step for planetary work.

Step 2: Fine Collimation

This step is required for precise collimation and necessary for the best crisp and detailed planetary and lunar views. There are three approaches:

  1. Using a star: This requires good seeing. Center the star in the fov and focus as precisely as you can. Use high enough magnification such that you can see the airy disk surrounded by concentric rings. Now tweak the secondary screws such that the spot in the center is perfectly centered. If you have a CMOS imaging camera then there is software available which allows collimation even in average seeing. I use a 850nm IR pass filter, a ASI290 mono camera and Sharpcap to collimate. It works even in below average seeing. The longer wavelengths are less affected by the Earth’s atmosphere and in these wavelengths the star’s image is be more stable. Another software is Metaguide which can help with collimation.
Collimation using a camera, 850 IR Pass filter and SharpCap
  1. Using an artificial star: If good seeing is not available then you can use an artificial star instead of a real star. The challenge with this approach is that you need 50-100 feet distance to get focus and collimate. Likely not an option for folks who live in apartments.
  2. Using the Possion Spot: This approach will not give as precise a collimation as 1 or 2 above but does allow you to refine the coarse collimation one step further. Can be done at the same time as Step 1. A good choice if 1 & 2 above are not an option. You need to center the center spot in the donut known as the Possion spot. Similar to steps above make sure the star is perfectly centered after each adjustment of the secondary screws.

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