What happens when you reach max magnification

[Solo05]

Hi All

I have just got a skywatcher 200pds and I am using the Celestron zoom eyepiece and all works fine.

I also have a 5mm celestron which also works fine.

I have put a cheap 2 x balow in the chain and the zoom still works but the 5mm is black or should I say you can see nothing.

This should be at the far end of what is possible i.e x400 as the 200pds is f1000.

Anyone out there can explain the phenomenon.

Thanks.

[dav1d]

It could just be that putting your Barlow in the chain has pushed you waaaay out of focus.

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[Stephen]

400x may be the max rated magnification, but in practice you will struggle to go above 200x on an average night in the UK due to the the skies.

[John]

As you exceed the maximum that is useful on a particular night (this varies night to night and can be a lot less than the theoretical max of the scope) the image of the object you are looking at gets larger but also looses sharpness and contrast so you can't see any more detail, in fact you usually start to see less. Backing off the magnification can then enable you to discern those details again, albeit with a smaller image scale.

None of the above is subject to fixed thresholds though - you need to judge as you use the scope how it's responding to magnification under the conditions that pertain. This is why astronomers often have a number of higher power eyepieces in their eyepiece case - so they can find the best balance to suit the conditions.

[Solo05]

Thanks for the replys guys but I don't think you quite understand what happens.

When I say it is black it's like the main lens cap is on. I have saturn in the centre of the 5mm focussed fine. Put the barlow in and tried focus from one end to the other. No out of focus blob or anything just absolute black.

At the very least I would expect to see an out of focus Saturn.

[coulthamst]

Have you tried using the eyepiece on the moon? This should make it easier to be certain that you are pointing at an unmistakeably bright object, and you can take it from there.

[John]

Barlows do change the focus point of an eyepiece / scope combination, usually moving it inwards, sometimes by quite a bit. It's possible, with the barlow in place, that you just can't bring the 5mm eyepiece to focus. I would have thought you would at least see an out of focus image of some sort though

[Stu]

It does sound like Saturn is just moving outside the fov when you add the barlow. I would try gently panning around to find it again as you should at least see it out of focus as the others have said. The moon idea is also a good one to try

Stu

[dav1d]

I have had this problem from both sides when using my webcam mod. Too far out of focus (easy to do with non-standard kit) and there isn't even a disc - Also at high mag, planets you think you have centred are in fact, not.

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[Solo05]

Hi all,

I went out again last night and tried again and found the problem.

The FOV in the 5mm is fairly small anyway but add in the barlow and I assume this gets even smaller. This means the exit pupil size must also change, becoming even smaller than normal. Basically I was just not looking directly down and centred in the eyepiece and just totally missing any light coming from the target image.

I could not achieve a crisp focus but it was not far off. Maybe the seeing was not up to par + this was done in a light polluted back garden.

I wonder if a better than the real cheap barlow I have would improve things as well.

I know I want a better widefield low mag eyepiece for DSO`s. Any sugestions that don`t cost more than a scope.

Thanks again to all who replied.

[Andymarrison]

I think that the exit pupil is the focal length of the eyepiece divided by the f-number of the scope (f5 for your 200mm aperture scope with a 1000mm focal length). This means that the exit pupil would have gone from 1mm with your 5mm eyepiece to 0.5mm with a 2x Barlow (which effectively halves the focal length of the eyepiece)

As far as I understand it the maximum usable magnification usually quoted is related to the smallest exit pupil reckoned to be practical - the usual rule of thumb for magnification is based on an exit pupil of 0.5mm, or put the more usual way 2xmagnification per mm of scope aperture (since exit pupil is also equal to scope aperture divided by magnification). Sounds like your practical experience bears out the rule of thumb that 0.5mm of exit pupil is about as low as you can go.

[KimG]

I have much the same scope and have only very rarely found going to 400X times to be of much use, if I am trying to split a close double I usually only get a fizz of light rather than a point source, and planetary details actually look better at 2-300X max. That doesn't mean that you should not try it, or get a decent setup for achieving it, but if you are going to fork out a lot of money (like I just have) I would go for something similar to my own choice which is a Baadar Hyperion 13mm with a GSO typ Apochromatic Barlow 2.5X

This will give me an effective 5.2 when Barlowed (190X) 76X without plus the optional fine tuning or spacer rings can make the lens an 8mm and 4mm Barlowed which would then give 125X and 250X

All this in an excellent eyepiece which should certainly get me just about the best images possible. You can of course pick out a different focal length to go for but I already have a 5mm Vixen Lanthanum, and three Plossl's, 10, 20, and 32, from my experience so far (15 years) I reckon the 13mm will do the best all round job for both planetary and DSO observations.

Of course, have only just got the thing I have actually yet to see the evidence, but with quality like Baader I am pretty confident.

Incidentally, something I learned recently, the cheaper Barlows which have the removable lens at the end can be used rather differently to good effect, unscrew the lens assembly from the Barlow and screw it into an eyepiece (where the filter would go), this give a lower magnification (I think about 70% but not certain) I have found that a very effective way of getting an in between magnification with some surprisingly good results.