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Use of a 18mm lens for astrophotography

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Hi All,

I was out observing last night and tried taking a picture of the Orion Constellation using an 18mm lens with my DSLR. I have a Celestron 90LCM and once aligned, I removed the scope and replaced it with my DSLR which has a dovetail attachment on the bottom. I stacked 7 x 4 minute exposures where the camera was set at ISO1600 set at f4.5 (attached).

The telescope was centred on the Horsehead Nebula.

Is it normal when using an 18mm lens that the central stars will be fixed and the outer ones will start to trail even on a driven mount?

Would I be correct that to countermand this effect, I would need to use at least a 50mm lens or greater level of 'zoom'?

I was also wondering how I can effectively focus my camera on the stars. I have tried switching the camera to 'live view' mode on a bright star, then zooming in and fine tuning it so the star appears to the smallest point as possible. I still find this technique difficult and feel I can never achieve optimal focus. Is there another easy method that I could try for better focusing?

Hope someone can help.

All the best.


Orion complete 18mm lens (6 x 4 mins) 11-2-21.TIF

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Your telescope's mount is an altaz rather than equatorial I think, so it moves in tiny left right up down movements and the imaging exposure length is limited to how long before field rotation shows, as the mount doesn't track the Earth's rotation. Field rotation shows as star trails and nebula smearing.

When using your camera, a red dot finder mounted in the flash hot shoe can be used to align with) aim to keep your exposure length to within what you doing acceptable star shapes.

If you are using an equatorial mount then could be lens aberrations and to minimise this try not to focus on a star in the middle rather use a star where a third intersects and try stopping the lens down.


Edited by happy-kat
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Thanks for your comment Bright Giant. 

As previously mentioned, I have a Celestron 90LCM which tracks the stars using a drive and computer and was properly aligned prior to taking the long exposure images..

In the provided photo, the Orion Nebula region does not trail.

I'm not sure what you mean by the red dot finder mounted on the flash shoe (I have no trouble lining up what I want to image with my rig, just difficulties with the focusing)

I have attached another photo showing my setup.

Hopefully you can help further.

Thanks again,


Driven camera rig.JPG

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Field rotation is what happens without an equatorial mount. If you visualize, say, Orion on the eastern horizon, then track it overhead to the western horizon, you'll see that unless you rotate the camera, it will have flipped 180 degrees. Or, for a simpler example, if you visualize the Little Dipper going around the pole, keeping your camera pointed perfectly at its center doesn't mean that the constellation won't rotate in the FOV.

Equatorial mounts do more than track a target, they also compensate for this effect.

Field rotation is most notable at the edges of the field, which is why it shows up nicely with an ultrawide lens. The only way around it is (a) to rotate the camera in the same direction, which is the approach taken by some really big telescopes that physically can't be equatorially mounted, (b) use an EQ mount, or (c) use exposures short enough that the trailing isn't noticeable. There's an online calculator if you're interested.

For focusing, try searching online for "Bahtinov mask". That's the best technique I know of for razor-sharp focusing without computer assistance. It is difficult, and no mistake.

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Thanks very much for your reply, that makes sense (the previous person was very helpful too 🙂).

I've just ordered a Bahtinov mask from a local supplier in Scotland. to fit my 8" Newtonian.

I will also have a wee peek at the  online calculator.

I think I may need to bite the bullet and purchase an EQ5 EQ mount with a motor.

All the best,



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You might find this post encouraging.


As a rough guide exposure length might be anything between 15-45 seconds depending where you point and elevation. Best directions for a altaz tracking of static mount are East or West upto about 60 degrees latitude. After stacking crop the edges of the stack as the frames will have very slightly rotated when processed.

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