Reduction in telescope magnification

[Colin]

1). Question:- Is it possible to reduce the magnification of a Maksutov-Cassegrain telescope for use with a CCD camera?

2). Background:- I have recently purchased a low cost Celestron Neximage 5 camera which is giving good results. This is slotted directly (or via a diagonal) into the 2" / 1.25" mount at the rear of the telescope in place of my normal 40mm or 25mm focal length eyepieces. My telescope is a Skywatcher 180mm (7") diameter, 2700 focal length unit.

3). The issue:- The Neximage camera is equivalent to an ~5mm focal length eyepiece. With my telescope this is giving an effective magnification of some 540x.

As a beginner I would like to greatly reduce such magnification so as to be able to "home in" on planets and nebulae more easily.

I am able to "bin" images to a 4x reduction using the Neximage software.

I have seen that a "reducer" tube (for example the Celestron 94178 2x field of view reducer) is a possible solution. Do readers have experience of such devices or can you offer other advice on getting magnification down to a reasonable target level of, say, ~50x to~100x with my telescope and Neximage camera?

Regards,
Colin Bayliss
Streatley,
West Berkshire.



Colin Bayliss

[Dan Davidson]

As you have discovered, it is possible to use a focal reducer.  A focal reducer makes your optical tube assembly look like it has a shorter focal length and typically gives you both a wider field of view and lower magnification.

I am not at all certain that there is a better solution for you than that Celestron part.  The only focal reducer I know of which will give greater focal reduction would be Meade's 0.3x focal reducer - but I am pretty sure it would not fit your imager.

One other difficulty is that it is not clear that anything other than that Celestron part will fit your imager.  I've seen reports indicating that your imager has non-standard threads and that could seriously limit your ability to use other (standard) focal reducers.

Sorry I cannot be more helpful.  But if it helps, there is a decent chance that if your Celestron Neximage reducer is of decent quality, you may be able to do useful imaging of the Moon, Jupiter, Saturn, etc.

FWIW

DTVBA

[Robin Scagell]

Hi Colin,

I have no practical experience of using a focal reducer. It will probably work, but may not produce a flat enough field for imaging (see below). However, most people with shorter focal length telescopes that yours usually find that they have the opposite problem, that of getting a sufficiently long focal length for decent planetary imaging. On my LX90, for example (200 mm aperture, 2000 mm focal length) I have to use a 2x Barlow and extension tube to get me up to 8000 mm focal length so as to get a good image scale. Smaller images just don't include all the detail that the scope is capable of yielding. It sounds as though your problem is not so much the image scale when actually taking the pictures as locating the object in the first place.

Though not a cheap solution, I use a guiding eyepiece (with crosswires) to locate the planet first visually, then I pop in the webcam. Usually the planet is then in the field of the webcam when using the Barlow, though if I am working at the full 8 m focal length it is a bit more problematic. I also have what's called a parfocalising ring around the eyepiece so that it focuses at the same point as the webcam. Sounds complicated, but it's nothing more than an adjustable ring round the eyepiece barrel. A bit of gaffer tape in the right place would work nearly as well, stopping the eyepiece from sliding so far into the barrel.

Rather than buy a guiding eyepiece, make sure that the planet really is dead centre in your 25 mm eyepiece, which you could adapt as above to the same focusing point as the webcam, then pop in the webcam. It should then be nearly in focus, which is important in cases where the image is so out of focus that you don't see anything. Alternatively, you might have to do it the other way round, putting the tape around the webcam barrel so as to match the focus position of your 25 mm eyepiece.

I know that the latest Neximage will give long exposures for nebulae, for which you really would prefer a wider field of view. But many of the nebulae where wide fields are needed are so large that you really need a much larger chip anyway, such as a DSLR. I think you should concentrate on the smaller objects using the setup you have got, rather than trying to expand your field of view. I don't know how flat the field of view will be that is provided by the reducer you mention. Flat fields are essential for imaging, whereas for visual work you can accept a bit of out of focusness around the edges.

Smaller objects include planetary nebulae and galaxies. Have a go at the Eskimo Nebula in Gemini, for example, which requires all your focal length if not more. Then proceed to galaxies, such as M81. Plenty to do with what you have got.

Let us know how you get on!

Robin Scagell

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[Colin]

Dear Dan,

Many thanks for your helpful and practical advice.  It is great to get such assistance over the net.  I'll let you know how I get on.  The sensitivity of the Neximage camera is probably only really good enough for solar objects anyway and I'll get the short tube reducer and let you know how that works out.

Best wishes and hoping for clear skies in 2013.

Colin Bayliss

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[Colin]

Dear Robin,

Once again many thanks for the very helpful practical advice - you are quite correct in that it is the problem of locating the object of interest in the first place if the magnification is too high!  I'll certainly let you know how I get on.

Best wishes and hoping for clear skies in 2013.

Colin Bayliss