When checking the collimation of a Celestron 14" SCT I took this image:

My question is: how much will the particles causing the local aberrations affect the image quality? (i.e. do I need to clean the optics?).

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    $\begingroup$ A third question might be how to figure out on which surface the dust particles reside; corrector, primary or secondary. $\endgroup$ – uhoh Aug 11 at 0:23
  • $\begingroup$ Is the picture taken looking through an eyepiece? if it is, I'd check the eyepiece first for dirt and hair. $\endgroup$ – Dr Chuck Aug 11 at 7:01
  • $\begingroup$ @a_donda I've read the manual. It's.... not the most useful manual I've encountered. It explains how to collimate but not how to tell when collimating is needed. It doesn't say anything useful to answering my first question of how much this amount of dirt will affect the performance. $\endgroup$ – William Miller Aug 11 at 9:50
  • $\begingroup$ @a_donda When you say "my bet is the sensor"... what exactly do you mean? Are you saying you think the dirt is on the sensor? $\endgroup$ – William Miller Aug 11 at 9:52
  • $\begingroup$ @a_donda It seems I am not being clear. I understand that the manual explains how to check the collimation and then gives a procedure for adjusting it. My question isn't 'how do I adjust the collimation?' or 'how do I check the collimation?' it is 'how do I know when the collimation is good enough for the telescope to perform at its peak?' (I.e. how symmetric does the interference pattern need to be before collimation will not noticeably affect the quality or clarity of the image). When I say "when collimating is needed" I mean it in the sense of the latter. I apologize for being unclear $\endgroup$ – William Miller Aug 11 at 10:00

Will this affect image quality? It depends on "where" the dust is located. If on the telescope, then not very much. If on the sensor, you'll get the shadows of the dust on your images (use of dark-frames can be used to "subtract" the dust shadows when calibrating the images via software).

Based on the sharpness of dust, I suspect your dust is probably very close to the image sensor.

When light travels through the telescope, you can think of it as taking all possible paths to reach the sensor. This means that while some photons are blocked by the dust, other photons are taking paths that bypass the dust. The dust wont show up on the image because the farther away it is much too far away from any focal plane -- so it's completely out of focus to the point of being invisible. But as some photons are technically blocked... it may fractionally reduce some contrast if the dust gets pretty bad.

The closer the dust is to the image sensor, the easier it is to spot. Since your dust shows up fairly clearly, my guess is that it's fairly close to your camera sensor. You mentioned the telescope is a C14, but you did not mention the camera and/or any filters being used.

Cleaning Telescope Optics

After about 4 years without any cleaning, I removed to the dew shield and found this:

C14 with dirty optics

I spent a few hours very careful cleaning the corrector plate (which is where I suspect you'll find the bulk of dust) using Dr. Clay's cleaning solution recipe (you can find that here: http://arksky.org/asoclean.htm ).

The technique has you gently daubing a lint-free cleaning pad with cleaning solution applied to it. This pulls off the dust but leaves the optics rather messy. Avoid rubbing action while the optics are still caked in dust as this will just drag the dust around on the surface. Again, the corrector plate is much more durable than mirror surfaces and I generally never need to clean the mirror surfaces since Schmidt Cassegrain Telescopes aren't "open" to the air.

Once the dust is mostly eliminated, I'll give it a second cleaning pass and this time I use gentle cleaning action (almost no pressure).

Lastly, I'll use a mist of distilled water as a rinse and daub dry with a clean microfiber cleaning cloth.

The result was this:

C14 after cleaning

While this "looks" clean, my advice is ... never get too close to your telescope optics while holding a bright light. You'll see all kinds of imperfections. My advice is to leave them be -- getting too aggressive can result in damage.

With that aside, a corrector plate is not the same as a first-surface mirror. Mirrors have a soft coating and are easily damaged. Corrector plates have anti-reflective coatings, but they are hard and not so easily scratched as long as you only use very mild cleaning solutions and are gentle.

Cleaning Camera Sensors

I use a solution called "Eclipse" (the manufacturer is "Photographic Solutions, Inc." and it is available from most stores that sell supplies to professional photographers. Eclipse is nearly pure methanol. This means it will act as a solvent to remove not only dust, but oil splatter (some DSLRs will occasionally splatter oil if the reflex mirror was generously lubricated). Since the substance is nearly pure methanol it will evaporate almost completely and leave virtually no trace.

I tend to clean camera sensors in phases of aggressiveness.

  1. Some cameras have a self-cleaning cycle ... a piezoelectric effect which vibrates a filter in an effort to shake the dust loose (helps to point the camera down while doing this.)

  2. Use a soft blower with ordinary air. Avoid canned compressed air because those usually aren't "air" and the propellant in the can can leave a haze on the camera ... and that has to be wet-cleaned.

  3. Use a very soft and pristinely clean paint brush (not the type you use to paint a house... the type an artist uses to paint a picture.) There are versions of this that have a grounding wire which can help release static cling.

  4. Wet cleaning. This is where I use the Eclipse solution. The same vendor makes lint-free cleaning pads (PEC Pads). It only requires a few drops and often just one or two wipes to clean the sensor.

After each of these phases, I inspect the sensor with a magnifying loupe and/or take a sample image. Cover the front of the telescope with a piece of plain white fabric (e.g. white t-shirt is a popular choice) and stretch it so that there are no wrinkles. Set the camera to a middle exposure so that the image comes out as a middle gray. The telescope need not be focused. Inspect the images carefully to for signs of dust shadows. Keep in mind that the images on the camera is rotated 180°. E.g. a dust speck appearing in the upper-right corner is really in the lower-left corner on the camera sensor.

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  • $\begingroup$ +1 just for taking the time. Though I tend to not encouraging beginners to clean the (unshielded) camera sensor themselves. They are likely to damage it. $\endgroup$ – user34599 Aug 12 at 18:06
  • $\begingroup$ Thanks for the answer, one question though: aren't flat frames usually used to remove shadows from dust? I've never heard of dark frames being used for that purpose $\endgroup$ – William Miller Aug 13 at 0:30
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    $\begingroup$ Apologies ... yes, I stand corrected. "Flat" frames can subtract dust shadows (as well as vignetting). Good catch! $\endgroup$ – Tim Campbell Aug 13 at 1:49

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