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Will a 0.5x focal length reducer help me to reduce the exposure time if I am imaging deep sky objects? I'm using a 1200mm focal length optical tube (f/6), a 32mm super view telephoto extension eyepiece, and a Nikon D5500 (sensor size of 23.5x 15.6mm). I do not have a computerised mount.

If the focal length reducer can help, then how will it help me? Or is there any other way I can reduce my exposure time to 15 or 20 seconds?

Addendum: For now, I connect a 32mm superview projection eyepiece directly to the DSLR's T2 thread and place it into the focuser. This should give me a wide view and maybe 10 to 15 sec exposures can be taken...would this be possible?

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  • \$\begingroup\$ I edited the answer. \$\endgroup\$
    – Euri Pinhollow
    Jun 27, 2016 at 7:56
  • \$\begingroup\$ 15-20s is a very short exposure time for most DSOs. Exposure times for these are usually measures in minutes or even hours. What exposure times are you currently using that you wish to get down to such a short time? Are you stacking images and without a computerised/motorised mount, how are you compensating for movement (from the Earth's rotations)? \$\endgroup\$
    – Steve Ives
    Jun 27, 2016 at 15:03

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If the object is at fixed distance, has fixed size (and is moving at constant angular speed) and fits frame well: no, you cannot get better photograph of it with focal length reducer.

Focal length reducer has three effects:

  1. it makes image brighter
  2. it reduces the number of pixels which object occupies
  3. if it contains glass it looses some small amount of light

1 is totally negated by 2 (and 3). Motion blur remains the same in relation to object size, you do get visibly less motion blur but it becomes just the same if you crop the photograph to frame obtained without reducer.

You can only get a better photograph of fixed size & distance object if you increase the entrance pupil, or prolong the exposure (with motorized mount).

If the object is larger than the frame with current setup, you will benefit from reducer for sure, but not if the object fits the frame well already.

Chris suggested that you could try stacking of multiple images to either compensate for noise in underexposed images to maintain the maximum usable exposure time or to improve good exposures even more - if the object is considerably smaller than the sensor. Removing noise improves percepted resolution as well. What you could do is: take a series of exposures withing tolerable exposure time and then merge them with some shifting using visible anchor points.

However, I haven't practiced it and I cannot give good software recommendation. It sure can be done manually in GIMP though.

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  • \$\begingroup\$ What would be the best entrant pupil size i can get..??? Although id love to own a motorised mount but cant afford one at the moment. :( @Euripinhollow \$\endgroup\$
    – dwayne dias
    May 6, 2016 at 20:20
  • \$\begingroup\$ @dwayne-dias: entrant pupil size is focal length divided by F-number. In your case it is 1200/6=200mm. You can only get bigger entrant pupil with another telescope. \$\endgroup\$
    – Euri Pinhollow
    May 6, 2016 at 20:35
  • \$\begingroup\$ So should this b good rnough for deep space imaging..?? For now i connect a 32mm superview projection eyepiece directly to to the t2 thread of my dslr and place it into the focuser. This should give me a wide view. And as calculated i may b able to tske 10 to 15 sec exposures. Would this b possible...??? \$\endgroup\$
    – dwayne dias
    May 6, 2016 at 20:45
  • \$\begingroup\$ I guess its best learned and understood once experimented :) @euripinhollow. \$\endgroup\$
    – dwayne dias
    May 6, 2016 at 21:05
  • \$\begingroup\$ @dwayne-dias: yes, you'd better try it. If the object is larger than the frame with current setup, you will benefit from reducer for sure, but not if the object fits the frame well already. \$\endgroup\$
    – Euri Pinhollow
    May 7, 2016 at 7:55

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