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Which barlow/powermate for planetary imaging?


Beardy Bob
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Hi there.

I currently use a QHY5 for guiding my deep sky setup and wanted to have a go at some planetary stuff during the summer when the nights are a little lighter.

The question is which Barlow to buy. I'd be using my current guide camera (QHY5) through a SW130p (f5 I think) and there is a fine line between spending a load of cash and not sticking with it and buying something shoddy and being put off by the poor results. If anything I think the former would be the best outcome of the two because at least the re-sell value of quality stuff is good!

As a starter, I am aware of the fantastic reputation that the Televue Powermates have, and a 5x one of those would I think give the best results. That said my camera probably isn't optimal. I also saw these:

http://www.ebay.co.uk/itm/370496121805?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

and wondered if anyone had any experience of them. I've heard good things about much of TS's other stuff.

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The TS offering may be good, but I doubt they can match powermates. You could have a look at Meade Series 5000 TeleXtenders. I have 2x and 3x types, plus a 2.5x powermate, which gives F/25 on my F/10 scope. The results with an ASI130MM (same chip as QHY5) are very nice.

Jupiter 12 01 2013 RGB

The TeleXtenders are no longer made by Meade, but they can still be found under the Bresser brand, and there is a 5x:

http://www.astroshop.eu/bresser-1-25-5x-sa-barlow-lens/p,20891

(I do not know if this is the cheapest supplier, it just came up first in Google).

Explore Scientific also supplies similar tele-centric lenses

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Thanks for the advice, I'll look into that. I've also just realised that this forum is meant for visual eyepiece advice so I'm glad I've had any replies at all!

If any of the moderators notice this is in the wrong place and knows where it should go then please feel free to move it.

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Thanks John.

Wow, I've just looked at the price of those TeleXtenders! For that much I think I would just go for a Powermate instead.

I did have a Tal 3x, which i used for visual, but it cut out a huge amount of light so I wouldn't consider that for photography. Does anyone have any experience of cheaper brands such as the TS one in my original post?

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You also need to consider the small(ish) aparture that you are using - if you try and stretch the focal length too much the image will be so dim as to be useless for imaging purposes. You may be successful with a 2.5x increase but 2x is more likely to give you satisfactory results. If you double the focal length the image is four times fainter.

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At 5x focal length the image will be very faint - you may have difficulty getting a decent signal/noise ratio and therefore poor images. You may get away with it with the Moon but the planets are small and dim in comparison.

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With a smaller scope it is simply that you don't collect enough light to create a bright enough image. When I used my 180MAK - f/15 - on Jupiter I could easily get a decent image with a webcam or the much more sensitive QHYIMG132H - occasionally a 2x (f30) Barlow would just get an image but I never managed a decent image with a 2.5x Powermate.

The Moon was quite different - the extra light allowed the use of the 2.5x Powermate regularly.

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I think rather than getting a powermate or something like that I'd be more tempted to look at getting a second hand 127 Mak. With a 2x barlow you'll get an image little different from what you'd get with a 5x barlow or powermate and the 130P, but in all likelihood much more easily.

James

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Thanks. I certainly don't want to buy a new scope though. Even if I could justify the expenditure I don't have the space! At this stage it is just to dip my toe in the 'planetary imaging' water.

Does anyone have my experience of working with a 130mm f5 refractor so I have an idea of what magnification with which barlow I should be heading for with this setup?

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Ah. I only suggested it because it's a small scope (only about a foot long by less than 6" diameter -- probably smaller than your existing one) and second hand would probably be cheaper than any new powermate. Not sure how you'd fit it on your existing mount as I don't know which you have, but I don't imagine that would be a hard problem to solve.

James

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Hi Bob,

I have the following barlows, Skywatcher free 2X that came with the 127 mak, hopeless. Revelation astro 2" 2X ED, good but nothing to write home about. Revelation astro 2.5X ED barlow, excellent for the price, compact but not really 2.5X, it is more like a 2.2 and although good it is not a patch on the Televue. Televue 2.5X powermate, superb, expensive, nearly transparent but long and bulkier than the Revelation. As there is about £120.00 difference in price only you can be the judge. Revelation is also marketed under GSO amongst other names. I use the powermate mostly on my SW 100 ED, really superb but I would not push the ED100 any further than 2.5X, with smaller apertures you really are limited to 1.6X~2.0X max. If you have a really large Newt swith A+ optics then you can start to think about 3X and upwards. I would not also take any notice of these ED or Apo branding, all it means is that they have used "high index" glass ( 3 elements design ) to control the focus and colour fringing. Celestron ultima is also supposed to be good. I was also given a so called 3X ED barlow, the less said the better. I really think in this case you get what you pay for. Hope this helps

Regards,

A.G

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At 5x focal length the image will be very faint - you may have difficulty getting a decent signal/noise ratio and therefore poor images. You may get away with it with the Moon but the planets are small and dim in comparison.

That is not correct. The focal ratio is what determines the number of photons per unit of surface area (or per pixel for a given camera). My F/10 scope with a 2.5x powermate works just fine on Saturn with the SPC900. An F/5 scope with a 5x powermate will give the same illumination per pixel.

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That is not correct. The focal ratio is what determines the number of photons per unit of surface area (or per pixel for a given camera). My F/10 scope with a 2.5x powermate works just fine on Saturn with the SPC900. An F/5 scope with a 5x powermate will give the same illumination per pixel.

In theory you are correct, however, a 5" Newtonian of F5 just does not have the resolution that is obtained by using a 12" Dob of F5, the brightness will be same but the available detail will be vastly different, you will get a bright round planet with not enough detail to develop during digital manipulation in post capture. The use of a high power barlow will only make the problem worse as the aperture gets smaller. A case of less=more.

Regards,

A.G

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Thanks for some barlow reviews AG. I think it is down to a Revelation or a Powermate. I will keep an eye out for prices and bargains and hopefully try and have a look through each of them to compare.

In the meantime, seeing as I will need to 'stretch' the focal length of my f5 newt further than someone using a mak would (at, say, f8+) does this put a lot more emphasis on the quality of the barlow? Or does a cruddy 2x barlow cut out just as much light as a cruddy 3x barlow?

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