Focusing problems with barlow

[Georgio]

Howdi stargazers, I have a simple 70mm x 700 and was looking at orion nebula last night through my 10mm eye. When I put the Barlow x3 into the mix I could see nothing at all. I tried readjusting focus and double checked the finder with stays at dead center. Can someone tell me what I'm doing wrong please. The Barlow will be a cheapie as it came with the Scope. 

Many thanks.

 

 

[Moonshane]

In general the maximum usable magnification for a scope is 2x aperture in mm and this is in decent sky conditions. So your scope has an aperture of 70mm and therefore maximum usable magnification of 140x. Your 10mm eyepiece provides 700 / 10 = 70x and therefore with the barlow x 3 = 210x which is far too much. At 70x you should get a decent image of the Orion Nebula.

For 140x you'd need a 15mm eyepiece with the 3x barlow but in truth a 5mm eyepiece or 2x barlow with your 10mm might be a better option. That said, I'd just enjoy what you have before committing to more stuff.

 

[michael.h.f.wilkinson]

One important issue is that less magnification is often more when it comes to nebulae. A 20mm (35x) or 25mm (28x) EP will probably give better views.

[Georgio]

Thanks Moonshane makes much sense. I enjoyed the view without it yes. Perhaps I'll look again with the 4mm. I'm considering trimming the eye lashes too ✂️

 

[Georgio]

Thanks Michael what's the best average to get?  The purse is not so deep. 

[rockystar]

I suspect your 4mm won't be very good either. If it's anything like my first scope, they chuck a 4mm & 3x barlow in, just so that they can advertise "over 500x magnification", i soon learned (as I'm sure you will) that magnification isn't everything in astronomy.

having said that, you won't know until you try it

 

[rockystar]

1 minute ago, Georgio said:

Thanks Michael what's the best average to get?  The purse is not so deep. 

I assume you mean eyepiece - don't ask what's best at this stage, the price will make your eyes water

 

[Pieter]

I had a very nice result with a x162 (2x barlow and a 12.5mm) 2 nights ago.

[Georgio]

Thanks. I suspected as much. The finder Scope is the same rubbish but I've modified it and now it's fine. Think some decent EP's would be the best route. The thanks for your help

[cjdawson]

Let me come at this from my own perspective.  It might give you some ideas and maybe even show that it's not all about magnification.

My scope is an 8" Meade LX-90 EMC.  It's the original from 2001.  Frankly it's a fantastic scope and I've been using it happily.   It will be great until the electrics die - which shouldn't be for a very long time.

The main apteture is 200m, the focal length is 2000mm.   Using the double rule, that means that my scope is capable of 400x mag.    BUT, I've never managed to get that out of my scope in almost 20 years.

Why?  Sky conditions are a limiting factor.  Actually the highest that I go to is about 250x, even thought the optics are supposed to be capable or more.

Using my 400x/250x = 1.6 * the apteure is a more realist viewing limit (well, for me anyway)

 

Let's test this out with my SkyWatcher st-80

80mm * 1.6 = 128x mag maxium.   With my eyepieces, the most that I can get out of my scope is about 80x.  This is using a 10mm eyepiece, and a 2x barlow.  The image looked good to me at that.  I did try using my 4x powermate, that put it up to 160x, but the image wasn't clear enough to for me to say it was a success.

 

So, let's put the same thing onto your telescope. 

 

70mm apeture * 1.6 = 112x Mag.   I reckon that in the UK, unless under really good sky conditions, you'll be struggling to get much above 120x mag with a 70mm scope.

 

note: be aware that this is all back of envelope maths, your mileage may vary.

[Geoff_L]

AIUI, most barlows increase the eye relief of the eyepiece, so you might get somewhere just by moving your eye away from the eyepiece a little. That said, I suspect the scope might be the same Bresser 70/700 Skylux that I bought from a well known German supermarket for about fifty quid last year. If so, the problem might be a combination of the mount and the poor optics of the supplied barlow and eyepieces. FWIW, although the box claims a rediculously high maximum magnification, the maximum usable is probably around 140x (your 10mm eyepiece + 3x barlow = 210x) and at that magnification the shake due to the wobbly mount prevents decent observation. To be honest, my Bresser is OK with the supplied 20mm eyepiece and no barlow (35x). It's also OK with a 12.5mm Plossl eyepiece and no barlow (56x), but I wouldn't want to go much higher than that with the mount that came with the 'scope. HTH, Geoff