Good, Cheap Barlow Lens

[Moonshane]

am using a skywatcher deluxe x2 barlow, does the job but makes my viewing very dark. is that normal?

generally the higher the magnification, the darker the image. sometimes this can help apparently with making things stand out a little more especially if you suffer from light pollution. this is one reason I have bought a decent zoom eyepiece and it may help with framing etc. typically been cloudy ever since I bought it!

[alfingido]

2x and 3x seems to be the mag that is the most common - however I was thinking of getting a 5x (Revelation Astro 5x Barlow Lens) the price seems very reasonable - but is that a bad thing?

Looks like you have a pretty comprehensive range of eyepieces already, do you really need a barlow, especially a 5x barlow? The only ep that you could really use it with would be the 32 mm which would make your images very dim and probably very fuzzy.

Allan

[MishMich]

From what I can remember, I found the 102 struggled with the 2x barlow and 6mm or 9mm EPs. I stopped using the stock 25mm & 9mm EPs, and now use a 32mm, 15mm and 6mm TMB EP. I have managed to use a x2 barlow with the 6mm EP on the Meade 5" refractor, when seeing was very good, but not the ED80 (which I find I can push more than the 102). I found the 102 quite dark anyway - darker than the ED80 or the 5" Mease. I used the x2 barlow with the 15mm EP mainly, because my first 6mm EP was like a pinhole, and I found the 15mm with barlow easier on the eye than the 6mm. Now I have a TMB I don't need to do that, and the nights I can push that with a barlow are few.

I don't know about the 114, but I doubt the situation is much different from the 102. I'm not sure how much advantage you will get with even a x2 barlow, unless you want to try it with the 10 or 15mm EPs. I have one of the GSO/Revelation type x5 barlows, and I have never found it to be of any use visually. I bought it for imaging planets. It dims down the planet considerably, which with Mars recently is a benefit, which can be a benefit imaging (but on Saturn is less so, because that is dimmer to start with). It works best with my 8" Newtonian, which is f6, so it pushes the focal length up considerably. On the Meade, with good seeing, it just works, but at f9 it is really pushing it. I have managed to use it with the 80ED, and did get an image, but it was pretty poor.

So, my hunch is that the x5 with the two scopes you have listed will be a waste of time. The x2, or even the x3, might be of use. You can work it out for yourself, although I don't have a link to the maths (I could use an accurate reference for this myself). You can work out the practical maximum magnification of your scope based on its dimensions (aperture & focal length), which will not be the same as the manufacturer's stated maximum (unless they also also list the practical maximum). Then you work out a figure based on the scopes dimensions, the EP magnification, and the multiplying factor of the barlow. If it is beyond the practical limits of the scope, then there's no point buying it.

If anybody has the formula for this, I'd appreciate it - although I will probably go hunting for it now anyway. I did know it at one time, but have forgotten it.

M.

[MishMich]

Looks like you have a pretty comprehensive range of eyepieces already, do you really need a barlow, especially a 5x barlow? The only ep that you could really use it with would be the 32 mm which would make your images very dim and probably very fuzzy.

Allan

This is my recollection. On a good night, with a very bright object, I could use the x2 with a 15mm EP, but normally, the 32mm was about as far as it would go.

The other thing to take into account is that a good 6mm EP will work better than a 12mm with barlow anyway, especially with something like an achromat - the barlow will intoduce more CA, which with basic Achro's is an issue anyway.

For imaging, it can make more sense. The sensor on a camera is roughly equivalent to a 6mm EP, I believe. With visual use, you can alter the magnifying effect by placing it between the focuser and the diagonal, rather than between the diagonal and EP. With the 102, placing it between the focuser and diagonal was hopeless, and is something I've not tried with my current refractors. A diagonal and eyepiece is asking a lot of a barlow that has a single thumb screw and no copper tensioning band, the weight pulls down on the barlow too much.

M.

[MishMich]

Oh yes, if using for imaging, you will probably need an extension with the barlow, to make up for the missing length of the diagonal - with the 102, you can get away with that without a barlow. Again, with the type of fitting on the supplied diagonal, I wouldn't bother trying to attach a camera to the diagonal, you'll probably end up with the sensor aimed at the wall of the tube unless you support the diagonal, which will make the assembly shake so much you won't get a worthwhile image. If you do get an extension tube, I'd go for a 2". They can cost as much as a barlow themselves.

M.

[MishMich]

Found them.

I've calculated the theoretical max for a variety of apertures, based on ideal seeing conditions, and the magnifications yeilded by a combination of EPs and barlows for different focal lenghs.

Max power = 60 times aperture in inches

aperture (inches) max power

3 180

4 240

5 300

6 360

8 480

Mag = focal length T / focal length EP

FL

no barlow

EP 32 25 15 13 9 6

500 16 20 33 40 56 83

600 19 24 40 48 67 100

750 23 30 50 60 83 125

1000 31 40 67 80 111 167

1200 38 48 80 96 133 200

1500 47 60 100 120 167 250

2x barlow

EP 32 25 15 13 9 6

500 31 40 67 80 111 167

600 38 48 80 96 133 200

750 47 60 100 120 167 250

1000 63 80 133 160 222 333

1200 75 96 160 192 267 400

1500 94 120 200 240 333 500

3x barlow

EP 32 25 15 13 9 6

500 47 60 100 120 167 250

600 56 72 120 144 200 300

750 70 90 150 180 250 375

1000 94 120 200 240 333 500

1200 113 144 240 288 400 600

1500 141 180 300 360 500 750

4x barlow

EP 32 25 15 13 9 6

500 63 80 133 160 222 333

600 75 96 160 192 267 400

750 94 120 200 240 333 500

1000 125 160 267 320 444 667

1200 150 192 320 384 533 800

1500 188 240 400 480 667 1000

5x barlow

EP 32 25 15 13 9 6

500 78 100 167 200 278 417

600 94 120 200 240 333 500

750 117 150 250 300 417 625

1000 156 200 333 400 556 833

1200 188 240 400 480 667 1000

1500 234 300 500 600 833 1250

[MishMich]

sorry, it lost the formating.

M.

[arad85]

Also worth bearing in mind that in the UK, you've got to have a special night where you can go over 250x visually...

[Mak the Night]

On 05/02/2010 at 11:16, arad85 said:

Also worth bearing in mind that in the UK, you've got to have a special night where you can go over 250x visually...

I can regularly get 300x on the Moon using a 6mm AH Abbe and a TeleVue 2x Barlow with my SW Explorer 130M. Although personally I think some of the less expensive Celestron Barlows are perfectly adequate.

[Stub Mandrel]

My scope (2nd hand) came with what appears to be a celestron x2 barlow. I haven't ever used a 'posh' one, but it gives me good results for planetary imaging.