APOD: Pillars and Jets in the Pelican Nebula (2015 Mar 04)

[Chris Peterson]

Actually, Chris, I think I just figured out why you keep mentioning a 14X magnification. It is the aperture divided by the dark-adapted pupil size (100/7 for a 4" scope). This is the minimum magnification theoretically achievable for a 4" scope, without wasting light on the eye. But it would require a very expensive eyepiece, methinks.

Correct, I was just defining the optimal magnification for sOnIc's scope. And for a low contrast object of this extent, 14X isn't an unreasonable magnification. It is trivially achieved in his case with an simple 3X focal reducer and a common 32 mm EP. Nothing exotic or expensive.

I wasn't suggesting that his scope was the ideal visual instrument for this target, just that it was more than adequate.

[Nitpicker]

Most of the cheap, or reasonably priced focal reducers I've seen online, ranging from nominal 0.33x to 0.63x reductions, claim they are only recommended for little webcams and the like, not visual observation, which I assume means they aren't great in the outer parts of the FOV. The better quality ones aren't cheap (probably more than five times the cost of an average 32 mm EP) and tend not to have nominal reductions as low as 0.33x. Also, at least with my SCT, I'd expect vignetting issues. But I must admit, I don't own and have never used a focal reducer. If anyone has good advice for me or sOnIc on the matter, please let us know.

[Chris Peterson]

Most of the cheap, or reasonably priced focal reducers I've seen online, ranging from nominal 0.33x to 0.63x reductions, claim they are only recommended for little webcams and the like, not visual observation, which I assume means they aren't great in the outer parts of the FOV. The better quality ones aren't cheap (probably more than five times the cost of an average 32 mm EP) and tend not to have nominal reductions as low as 0.33x. Also, at least with my SCT, I'd expect vignetting issues. But I must admit, I don't own and have never used a focal reducer. If anyone has good advice for me or sOnIc on the matter, please let us know.

I have an inexpensive Chinese 2x reducer that works quite well with eyepieces (it screws onto their tube), and was around $30. I have a somewhat more expensive 3x reducer that is intended mainly for imaging, although it works so-so with eyepieces as long as you only pay attention to the central part of the field.

FWIW, I'm not recommending this as the ideal of visual astronomy, just pointing out what is possible with a small scope. Really, a 100 mm scope with a 1400 mm focal length isn't a very good design for any sort of wide field viewing. It's more of a planetary instrument.

[Nitpicker]

I have an inexpensive Chinese 2x reducer that works quite well with eyepieces (it screws onto their tube), and was around $30. I have a somewhat more expensive 3x reducer that is intended mainly for imaging, although it works so-so with eyepieces as long as you only pay attention to the central part of the field.

Thanks. Maybe I'll buy one of the cheap ones (that screw into the nosepiece of an EP or camera). I haven't quite figured out the advantage of the much more expensive ones that screw into the base of the OTA (other than providing a seal on an SCT).

[Nitpicker]

I have an inexpensive Chinese 2x reducer that works quite well with eyepieces (it screws onto their tube), and was around $30. I have a somewhat more expensive 3x reducer that is intended mainly for imaging, although it works so-so with eyepieces as long as you only pay attention to the central part of the field.

Thanks. Maybe I'll buy one of the cheap ones (that screw into the nosepiece of an EP or camera). I haven't quite figured out the advantage of the much more expensive ones that screw into the base of the OTA (other than providing a seal on an SCT).

So, I bought a cheap "nose piece" focal reducer: ~95 mm FL, quoted as a 0.5x to 0.7x reducer, with which I can achieve 0.22x to 0.65x reductions with my DSLR, depending on spacing. But the image quality is poor and it doesn't appear to allow me to detect anything fainter than without the reducer (even in the centre of field). My scope already vignettes a little at the edge of my sensor, without a reducer, although it is not really noticeable in my images. But with the reducer, the vignetting becomes horribly obvious and my images are smaller, fuzzier, more distorted and show more aberrations. Woo hoo. (At least I learnt a bit more about my scope.) I won't be making wide-field images anytime soon.