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Focal Reducer and mount tracking


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It would make a more forgiving situation for a mount with poor tracking by, simply, making the times of shooting an image faster - which is why a scope operating at F/3.3 requires less time for a good image than one at f/10. I'll leave a link to a good paper on how to put such together. Though written around the venerable MallinCam-line of cams, the principles are applicable regardless of the brand used:

http://www.mallincam.net/uploads/2/6/9/1/26913006/focal_reduction_for_dummies.pdf

I hope this helps you - or someone,

Dave

 

<edit> Ouch! You stepped on my foot! :D

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2 hours ago, SilverAstro said:

At the cost of making the object of interest smaller as well ? !

Yep, the object does get smaller by the same ratio.  however, If the size of the object is the most critical thing, you should really decrease the focal ratio by increasing the aperture of the scope without increasing it's focal length.   Doing that is a whole load more expensive than reducing the focal length with a focal reducer ;-) 

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2 hours ago, cuivenion said:

Thanks for the replies. It would make the object smaller, but it's useful for fitting large objects in the field of view.

Yes !  If the field of view is the object of interest, but your original question was about making tracking easier which to me implied making the tracking error smaller so that you couldnt see it ! Anyway, you seem to have a handle on it so you dont need me any more :)

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2 hours ago, SilverAstro said:

Yes !  If the field of view is the object of interest, but your original question was about making tracking easier which to me implied making the tracking error smaller so that you couldnt see it ! Anyway, you seem to have a handle on it so you dont need me any more :)

Cool, always appreciative of replies.

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You can also compensate somewhat for the smaller image scale of the telescope by choosing a camera with smaller pixels. This increases the available resolution of the system - although it is often limited by atmospheric seeing. In effect, smaller pixels allow you the "blow up" the image to a larger size without losing detail, much like any photographic enlargement. As a rule of thumb, large DSOs can look pretty good at about 2"/pixel (or even larger), and small ones (or planets) often benefit from 0.25-0.5" / pixel. You may have heard of "under-sampling" and "over-sampling" - this refers to how much you deviate from an "ideal" resolution for the object. As with everything, it's all a juggling act - there is no point in having more resolution than your scope or the seeing will support, but if you have less, you are losing some information. For 2-dimensional images it's often stated that an image scale about one-third of the limit of seeing or your scope's resolution is ideal. So if you have a 4" scope (c. 1.1" resolution) and 3" seeing, then an image scale of 1"/pixel would be the lowest worth considering. A bigger scope may allow you to have a finer image scale, particuarly for planetary imaging where "lucking imaging" can help counter the limits of seeing.

To answer you original question, yes, a shorter focal length (irrespective of aperture) will reduce the effect of angular tracking errors (or periodic error) of your mount. You will still need to use an auto-guider unless you have (a) very short focal length or (b) short exposures (< 30-60s) or (c) a very expensive (c. $10,000) mount. 

The added advantage is that by reducing for focal length you also reduce your focal ratio, making the whole system faster so you can choose to reduce the duration of your subs whilst getting the same signal.

 

John

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