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5x Barlow


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Hi everyone!

I have a 100ED type APO refractor and was just wondering if it would be worth purchasing a 5x barlow to try and get a few decent images of planets?

I have a toucam 740K which I have been told is about the equivelant of a 6mm eyepiece. So I calculated magnification with my scope (900mm focal length), camera and 5x barlow at approximately 750x magnification :)

I think I already know the answer to this one but just wanted to get some advice first - will I just end up with a huge sack of blur?

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The advantage of taking an avi with a webcam is that registax selects and stacks your best frames, so you can increase the magnification you would be able to get away with when visually observing.

As an alternative, have you considered something like Antares 1.25" combined T-mount camera adaptor and eyepiece projection unit (Yes, I know S'n'S have received terrible press, not least from me, but they do seem to have been getting their act together a bit). That would give you a range of magnifications you could use, depending on the e/p you put in it, depending on conditions.

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Ah yes, Scopes and skies as I like to call them. I recently promised myself I wouldn't buy from them again after getting burned but if what you say is true I might reconsider.

I think I would prefer going down the barlow route though as it would probably end up cheaper than buying the projection unit and high magnification eyepieces. But will 750x magnification really hold up, even with stacking?

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I've seen some good results with 5x Barlows and stacked avis in Registax. Might need really good seeing though (not had that for months!). I only go up to a 3x myself and my TAL 3x has been very good value for money. You might be able to pick one up cheap on fleabay but these days, after the recent TV programmes and huge numbers of new telescopes being sold, there are few bargains to be had. I've seen second hand eyepieces sell for just less than a new one of the same brand, but add on the postage etc. and they actually cost more!

Maybe some 5x Barlow owners can help you more.

Edited by Skybrowser
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If it's any help, in Martin Mobberley's 'Lunar and Planetary Webcam User's Guide' he says a good starting point is to increase focal ratio to at least f/20. And that eventually you will want to experiment "as high as f/30 or 40".

I think your scope is f/9? So your 2.5x turns it into f/22.5 (9x2.5) I think.

So with a 5x you'd be at f/45, sounds like it might be pushing it a bit going by the book's advice, but I don't know myself. I've had a few goes at planetary imaging and find it tricky at the higher magnifications just to get the planet on the chip and the focus seems to need to be a lot more bang on for me at higher mag!

Edited by Luke
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I have a 5x Antares barlow (got it for less than a talx2 from AB&S). It works and does the job pretty nicely although you will probably end up having to push the gain on the camera rather more than you'd like. I got away with it on my Jupiter image, as I used my QHY5v to capture a mono avi, and added colour with the SPC900 so the blue tinge of over pushed gain on the spc900 just sort of got lost...

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  • 3 weeks later...

so just to clarify, will the 5x barlow be too much for my scope? This is what I am getting with 2.5 and a single shot image (will try stacking soon)

saturn200311titled.jpg

The result is extremely blurred as you can see, and while this is most likely down to the fact that my R&P focuser may not be up to the task of reaching sharp focus I fear it may also be that my telescope just simply wont cope at any magnification above this.

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Hi Samir,

The problem is not with the scope (And I'm not just saying that because I used to own it!). The main problem is stack size. A single picture will always look terrible, for planets and the moon you MUST stack at least a few hundred frames. I have attached a single shot of the clavious region of the moon along with a stacked image using over 1000 frames, you can see the difference (These were taken with your C100ED and a 3x barlow so you can see what you could do!). I would also say that a DSLR is really not suited for planetary work, you should use your webcam and then save as an AVI file and process in registax. I used a DMK21 and with just 0.3 mega pixels it still massively outstripped my Canon 350D i(8MP) in planetary work.

Are yoy using a IR cut filter? Even the best APO's will show blurred images without one as they are configured to bring visable light to the same focus point. IR light, which CCD's are sensitive to, will be out of focus and blur the image.

Single shot

post-14636-13387754988_thumb.jpg

Stacked shots

post-14636-133877549884_thumb.jpg

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nice to hear from you again! Thanks very much for the reply. I bought my toucam second hand and it came with an unbranded violet filter which has no text on it but I assume it is some sort of IR filter.

The last time I tried making an avi through the toucam this was the result of one of my images:

dr77zb.jpg

I tried stacking the 15sec clip in registax and the result was horrendous! maybe the conditions were so bad that night that I shouldnt have bothered....or perhaps I need to get a baader IR filter?

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An IR filter should be clear as it only blocks IR wavelengths which are invisible to the naked eye anyway. My impression of this is that it is slightly out of focus. Whenever I imaged with the DMK I would always focus on a star as I found this much easier than a planet. How many frames did the avi have?

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I would stick with the webcam myself and make sure you have an IR filter, not sure what you have there, mine is clear. I'd hold off on the new barlow for a mo until you're happy with what you are getting at the lower mag, as for me it gets trickier to get a good result the more mag you do, so I would try to nail the low mag first before moving up.

If you haven't tried it I'd also try and record several different movies with different settings, try different levels of gain, gamma, etc.

Hope it helps. Good luck with the next go!

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One problem with the posted frame is that compression artifacts are prominent. You must capture the file in uncompressed format or you lose a lot. F/30 is generally stretching things with Saturn (even of Jupiter), because photon noise starts to cause trouble. On Mars and Venus you can get away with it. Don't forget that in relation to the sensor size, Saturn will appear much larger in the web-cam at F/30 than in the DSLR frame. You simply need sufficient photons per pixel to get a only a semi-horrendous image per frame, and then stack LOTS. You need to get hundreds if not thousands of frames (I try to get 3000 frames regularly = 5 min at 10 fps). A 15 s run is generally not enough. Your image also suggests offset and gain were not set properly: the background must be nearly (but not quite) black, and the brightest areas must be nearly saturating the ADC (i.e the brightest pixels in each channel must remain just below value 255).

If you have a barlow rather than a powermate, you could experiment with extension tubes to change the multiplication ratio.

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Re: the violet, wish I knew to help Samir, I've no idea what colours they could be!

Sorry I missed the bit that you were doing 15 seconds, I would recommend at least two minutes, so you can stack the best frames that look clearest.

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I tried this again last night with a 2 minute capture however I had to set the shutter speed to about 75 for me to even see anything on the screen.

Annoyingly I am using a mac to do this and the only software I've found (that's free) to capture the frames is a trial version of BTV - which seems to create extremely bizarre .mov files which no other software can read :D

Has anyone had any experience with this?

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