View Full Version here: : DSLRs and Barlows?
10-04-2007, 12:58 PM
Anyone used a Barlow with their DSLR whist taking images?
10-04-2007, 01:36 PM
Depends on what you are imaging Andrew. Barlow up the DSLR if you are going to shoot the moon or sun (properly filtered of course :P ) or even a wide field of Jupiter and its moons. But if you are going for DSOs unless it's really bright like the trap in M42 then usually we are trying to go the other way. If you add a barlow your f/ratio goes up by the factor of the barlow (ie F5 scope ^ f/10 for 2x barlow)
The attached photo is a couple of years old, back in the time of filum. EOS500D with 2xbarlow+2Xteleconverter on F/10 Celestron refractor.
10-04-2007, 01:47 PM
I have a maxview DSL camera adapter and barlow made by scopetronics (alas I think they are now defunct) it could be what you are after - you can even add eyepieces in the train.
10-04-2007, 04:38 PM
Well they aren't defunct. I found their website and they still make adaptors.
Go to www.scopetronix.com.
I've been after something similar and will be purchasing very soon.
10-04-2007, 04:40 PM
Just check that scopetronix is still functioning. I've ordered things from them and either never received emails or when purchase completed items take months to turn up, if at all.
10-04-2007, 04:43 PM
Thanks for the warning. I'll check it out.:thumbsup:
10-04-2007, 05:00 PM
Thanks for the info guys, I knew about the F ratio increase with the Barlow, so no worries there, its more to do some experiments with which I may write something up later :P
10-04-2007, 07:51 PM
i have tried using a 2 x barlow to get a closer view of M16. doesn't work for me :( well not for DSO stuff. F ratio increase therefore longer exposure time.
10-04-2007, 08:00 PM
What was wrong Eric, guiding or something else? Was that with the current 10" or the 8"? I understand image size will be a problem, but if you never give it a go you never know. I will hopefully use a 2x on my 8"f6 so focal length will be 2400mm @ F12
10-04-2007, 08:25 PM
Hi Andrew. it was from my 8" F.4 scope trying to image at F8 with a 2" ED 2 x barlow. Thats why i ended up selling it because the barlow didn't work. Nothing got to do with guiding actually (i was thinking it might), got to do with lack of photons.
10-04-2007, 08:27 PM
here's a pic of M16 using the 2 x barlow from my old website. exposure time was 5 minutes at iso 800. tried processing extremely and nothing came out.
10-04-2007, 08:28 PM
Okay thanks Eric for the advice, something I will seriously take on board. Nothing like learning from the PRO's :)
10-04-2007, 08:32 PM
:ashamed: :ashamed: well it didn't work for me doesn't mean it won't work for you :P just think twice the F-fatio means twice the exposure time ;)
10-04-2007, 08:39 PM
Yep, if not more LOL, still working on getting the EQ6 and EDMOD to work properly at this stage, at least a 2x Barlow will show any tracking problems
10-04-2007, 09:21 PM
Actually guys it doesn't quite work that way. If you go up one f/stop on a camera lense (same thing applies to telescopes), say f/4 - f/5.6 then you double the exposure time. The next step up is f/5.6 to f/8, which is double again. So if you go from f/4 to f/8 you will need 4 times the exposure time f/8 to achieve the same exposure at f/4.
Sorry to be the bearer of bad tidings.
10-04-2007, 09:28 PM
whoops sorry. my mistake. Pauls correct.
10-04-2007, 09:29 PM
If you take the square roots of 2, 4, 8, 16, 32, 64, 128, 256 you will get you standard lense f stops of 1.4, 2, 2.8, 4, 5.6, 8, 11.3, 16. So to work out the exposure increase (or decrease) you take your f ratios, square them and if you are up then divide the lower into the higher, if you are going down then the higher into the lower.
So in your 2x barlow case. 4^2 into 8^2 = 16 into 64 = 4 times.
If you were using a 2x reducer then it would be going down so the forumula would be 8^2 into 4^2 = 64 into 16 = 1/4 times the exposure.
I hope that makes sense. :P
10-04-2007, 09:33 PM
Ahhh... yep makes sense thanks Paul.
11-04-2007, 11:57 AM
Well I guess the best Barlows would be the extenders made by the DSLR brand maker. At least what I think is that as with all optics the distance from the back element to the chip must be exactly in the correct place in order to get a flat field.
I use my Canon telextenders 2x and 1.4x together with my Canon EOS cameras. They can be also combined and you get a 2.8x telextender.
If you have one test it. Be sure to tape off the 8 contacts. If you do not that your camera will not wrok and you get an error. Use some electric insulation tape. A small stripe over the contacts is enough. There is absolutely no problem. For daytime imging just take the tape off and it will work again.
16-04-2007, 04:08 PM
Actually it's worse than that: twice the focal ratio means four times the exposure. A 1mm by 1mm (area 1 sq mm) square goes to a 2mm by 2mm square (area 4 sq mm), so your light is spread over 4 times the area and hence 4 times the exposure.
Woops! should have read the whole thread before replying.
16-04-2007, 04:46 PM
So on brighter objects of pretty good seeing and a mount that is cooperating I should be able to get something at least.
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