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Old 18-08-2015, 09:48 AM
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rustigsmed (Russell)
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Question 1.25 or 2" barlow/powermate for dslr?

Hi IIS'ers,

I've been thinking I might try and make use of the resolution of my scope and nab some extra focal length, i want to make planetary nebulas a bit larger. the scope is 12" f4 so am thinking I wouldn't want to push it past f8 which means pretty much a 2x barlow or powermate? I have a televue 5x which the quality is quite impressive but then it has to be for 5x the focal length.

my first question is in regard to whether I should be going for a 1.25" or 2" - will the 1.25 result in excessive vignetting on the dslr?

the second question is really relating if the answer is 2" whether something like the Televue big 2x barlow is good enough (or overkill) or should I really be going for a powermate? (~$460)

Pretty much a 1.25" powermate is the same price as 2" barlow, and then a 2" powermate is becoming more than I want to spend... unless its really going to make a significant difference.

In summary,
1. 1.25" too much vignetting?
2. balancing pricing options up 2" gso ed $89; 2" televue barlow $300, 1.25 tv powermate $300, 2" powermate $460 eeek!

thoughts and experiences or even other brands welcome. obviously it needs to be able to thread into oag/dslr somehow. I am thinking 2" is probably a better option.

cheers

Rusty
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  #2  
Old 18-08-2015, 10:59 AM
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PRejto (Peter)
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Here are 2 solutions. One is very expensive ($793!)but looks very interesting:

Follow links from here: http://www.astro-physics.com/index.h...ducts/products

Baader Fluorite Flatfield Converter Barlow (BPFFC)

or

2" Advanced Convertible Barlow (BARADV) ($250)

I've used the 2" AP barlow with my KAF8300 CCD and it works very well. You can adjust the power somewhat by moving in/out. I took this photo of M83 using it with my TEC140:

http://www.pbase.com/prejto/image/152287993
and M16

http://www.pbase.com/prejto/image/149151685

TAK also has a good photographic barlow at 1.6X.

I would think you need 2" to avoid vignetting. Personally I'd go with one of these over your standard barlows.

Peter
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Old 18-08-2015, 01:31 PM
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rustigsmed (Russell)
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Quote:
Originally Posted by PRejto View Post
Here are 2 solutions. One is very expensive ($793!)but looks very interesting:

Follow links from here: http://www.astro-physics.com/index.h...ducts/products

Baader Fluorite Flatfield Converter Barlow (BPFFC)

or

2" Advanced Convertible Barlow (BARADV) ($250)

I've used the 2" AP barlow with my KAF8300 CCD and it works very well. You can adjust the power somewhat by moving in/out. I took this photo of M83 using it with my TEC140:

http://www.pbase.com/prejto/image/152287993
and M16

http://www.pbase.com/prejto/image/149151685

TAK also has a good photographic barlow at 1.6X.

I would think you need 2" to avoid vignetting. Personally I'd go with one of these over your standard barlows.

Peter
thanks for that Peter, I did kind of ignore whether coma correction was required i was thinking perhaps f8 might be fairly insignificant but I wasn't aware of these barlow-field correctors which would obviously do a better job as the coma would be still there just less obvious.

I think with the baader it would need be setback around 57mm (TSOAG9 44+11+2mm and magnifications and the back spacing required: 2x = 42 mm, 3x = 96.2 mm, 4x = 150 mm,) which would give a magnification of about 2.3 which would make the scope f9.2 ish - a little slower than I was after but not diabolical.

if I interpret the AP barlow info correctly a setback from the chip needs to be 90mm (for 2x) so I've got more room to play with (plus its in my price range) so i'd need about a 33mm spacer which is doable... well a 10mm spacer think my existing variable spacer is 17-23mm (for my TSOAG9 + RCCi combo) so that's easy enough.

thanks for the tips (again) that's exactly the type of info I was after.

Rusty
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Old 18-08-2015, 03:57 PM
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jjjnettie (Jeanette)
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Seeing how you'll be hanging this extended imaging train off the side of your scope (don't forget to add in the weight of your extender) I'd be going for the lightest possible solution.
I wouldn't worry too much about vignetting or coma as the targets will still be small enough that you can crop the worst out.
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Old 19-08-2015, 04:49 PM
britgc (Bret)
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I use the Baader FFC with my WO FLT98 and Canon 450D. Very happy with it.

Bought mine from Teleskop Service who have it for about 502 Euros which, when all the conversions are done at bank rates, works out to be around 800 AUD currently. Still expensive, but it is pretty nice

Image of NGC 3372 taken with Baader FFC here:http://www.iceinspace.com.au/forum/s...37#post1186637

Bret

Edit: Just to expand a bit more. I was contemplating the same question as you Rusty a few months back but decided to go with the Baader FFC over a barlow or powermate because of the flexibility you get with magnification simply by adding/removing extension tubes. In the case of using a DSLR, that means anywhere from around 2.5x - 8x. Handy for all sorts of imaging/viewing.

Last edited by britgc; 19-08-2015 at 05:27 PM.
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Old 20-08-2015, 02:08 PM
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rustigsmed (Russell)
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Quote:
Originally Posted by jjjnettie View Post
Seeing how you'll be hanging this extended imaging train off the side of your scope (don't forget to add in the weight of your extender) I'd be going for the lightest possible solution.
I wouldn't worry too much about vignetting or coma as the targets will still be small enough that you can crop the worst out.
thanks jjjnettie I wasn't sure about vignetting and coma, I may even just buy a cheap CC first as a trial. not too worried about the weight of the imaging train as it already is super long with the baader RCCI. this would make it shorter I think!

Quote:
Originally Posted by britgc View Post
I use the Baader FFC with my WO FLT98 and Canon 450D. Very happy with it.

Bought mine from Teleskop Service who have it for about 502 Euros which, when all the conversions are done at bank rates, works out to be around 800 AUD currently. Still expensive, but it is pretty nice

Image of NGC 3372 taken with Baader FFC here:http://www.iceinspace.com.au/forum/s...37#post1186637

Bret

Edit: Just to expand a bit more. I was contemplating the same question as you Rusty a few months back but decided to go with the Baader FFC over a barlow or powermate because of the flexibility you get with magnification simply by adding/removing extension tubes. In the case of using a DSLR, that means anywhere from around 2.5x - 8x. Handy for all sorts of imaging/viewing.
Thanks for the input Bret. that is a great image with good stars. yes the baader FFC does have a lot of flexibility there with magnification variation. I probably would have gone with that if I knew about it earlier on, I already have a televue 5x powermate, so just looking for 2x at the moment (want to keep the f ratio as close to f8 as possible - as a imaging sensitivity/fov sweetspot) but it should be something someone considers if they are looking at buying so the don't end up with multiple purchases (like me)
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Old 20-08-2015, 02:23 PM
glend (Glen)
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Why not use the 2" GSO ED Barlow which is 2x but customised based on extension used. The Baader MPCC MkIi will fit inside the extension ahead of the barlow. Low cost option if you have the coma corrector which already has the right spacing for the Canon.
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Old 20-08-2015, 02:32 PM
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if you haven't already come across it, this gives a good summary of the various types of Barlows and some of the things that you could consider when using them. http://www.brayebrookobservatory.org...BarlowLens.pdf

re quality etc, the GSO 2" that Glen mentions is worth a look - mine is almost as good all round as the much more expensive TV Big Barlow.

Powermates are just another Barlow, but they have extra lenses to turn them in telecentric systems - for your application, that means no extra field curvature. That might be worth having if you can find a way to control coma.

Last edited by Shiraz; 20-08-2015 at 02:44 PM.
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Old 20-08-2015, 02:59 PM
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Quote:
Originally Posted by glend View Post
Why not use the 2" GSO ED Barlow which is 2x but customised based on extension used. The Baader MPCC MkIi will fit inside the extension ahead of the barlow. Low cost option if you have the coma corrector which already has the right spacing for the Canon.
Quote:
Originally Posted by Shiraz View Post
if you haven't already come across it, this gives a good summary of the various types of Barlows and some of the things that you could consider when using them. http://www.brayebrookobservatory.org...BarlowLens.pdf

re quality etc, the GSO 2" that Glen mentions is worth a look - mine is almost as good all round as the much more expensive TV Big Barlow.

Powermates are just another Barlow, but they have extra lenses to turn them in telecentric systems - for your application, that means no extra field curvature. That might be worth having if you can find a way to control coma.
thanks guys,


Ray, I guess good flats would take care of the curvature?
if I was going a low cost option then going a barlow with a cc could work. but would that be

Camera - OAG - MPCC - barlow

or

Camera - OAG - barlow - RCCi

not knowing the connections is makes it harder to visualise. Glen did you mean the GSO would connect after the coma corrector or before?

I would have thought it be better to 'correct' for some reason I would have thought the second option be more likely to deal with the coma correctly?

If it were in my f5 scope I don't think i'd worry too much f4 might require some fairly severe cropping.

Russ
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Old 20-08-2015, 03:11 PM
glend (Glen)
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The camera/coma corrector inserts into the barlow. Re your OAG, the Lacerta OAG is designed for Canon sensor spacing and will not affect the coma corrector spacing as it is also the camera bayonet fitting. If I was going to use OAG it would be with the Lacerta. Watch out for the cheaper OAGs which allow flex under that sort of load. Teleskop-express carries the Lacerta.
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Old 20-08-2015, 04:49 PM
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Originally Posted by rustigsmed View Post
thanks guys,


Ray, I guess good flats would take care of the curvature?


Russ
Flats will correct for vignetting, but field curvature will give out-of-focus away from the centre of the fofv. I reckon that the best approach would be to try a Barlow and see what you get - work out what is important from that.

I don't have any feel for how to combine a Barlow and coma corrector - or even if a CC will be necessary over the fields of view that you will be using. Maybe just take JJJ's advice and crop.
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