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Planetary imaging rule of thumb - which Barlow?

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#1 Shiraz

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Posted 02 April 2013 - 10:31 PM

With the ever increasing range of cameras coming onto the market, you will probably be asked at some stage "what Barlow should I get for my scope?" The attached JPEG from my word processor (couldn't put equations in the post) hopefully shows why the following simple rule of thumb can be used to work out an appropriate Barlow for any combination of scope and camera.

finalFNo = pixelsize(microns) * 5

would be grateful for any feedback based on your experience or if you are aware of anything similar being suggested elsewhere.

Regards ray

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  • 5774438-Barlow3.jpg

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#2 WarmWeatherGuy

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Posted 03 April 2013 - 01:55 AM

I use f/10 Cats for imaging the planets. I have used the NexImage (5.6 micron) and NexImage 5 (2.2 micron) and I found that my 2.5x Powermate (f/25) worked best with the NexImage and no Barlow worked best for the NexImage 5 (f/10). 5.6 * 5 = 28 (close to f/25) and 2.2 * 5 = 11 (close to f/10).

I would say this formula is spot on!

#3 Kokatha man

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Posted 03 April 2013 - 02:08 AM

.....just my 2-bob's worth Ray ($2 for today's person and all you folks in the US! :grin:)

Noting the provisos at the end of your article I allways try and adjust image scale/focal length for the seeing with my system, which is why I devised the variable unit shown in my current thread, with the added necessity to attempt to maintain collimation without needing to go to a star again when varying/adjusting said - seeing (especially good) seems to be at a premium a lot of the time and I've lost good seeing a few times faffing around.....and my one and only test so far seems to indicate that the device does maintain collimation. :)

Maybe it's the "old school" in me (I'm over 60) but I usually can tell pretty soon if the seeing can support the scale I'm using by the focusing.....the other week we went from just over 5000mm to 6000mm (quite significant with the ASI120MM's pixel-size) and focus demonstrated the benefit of a larger scale - but most importantly in that focusing was enhanced because we were actually resolving more detail in the onscreen image, which demonstrated that we were operating at a better/more appropriate focal length.

I realise this doesn't add much to the theoretical aspect of your thread but thought it worth posting..... :question: :)
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#4 Shiraz

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Posted 03 April 2013 - 03:45 AM

thanks Steve - good to find that it matches your experience with Neximage cameras. The majority of 618 users seem to work pretty close to the suggested f28 with both SCTs and Newts and I am aware of one imager who uses f30 with a 120mm refractor and 618 camera.

Hi Darryl. thanks for the comments. You are using about f17 and the rule of thumb suggests f19, so I guess that is close.

The intention of the theory stuff was mainly to show that there is a clear linear relationship between FNo and pixel size when you optimise performance - the multiplier suggested by sampling theory is about 5x, but that is of course determined by the seeing - however, it does seem to match pretty well with what experienced people do in practice. The rule of thumb is great for answering the oft asked question "what Barlow should I use with my xxx scope?". this will allow you answer that question with enough precision to get someone productive without them having to purchase a range of Barlows in order to get something that works. It applies to any scope and camera and is easy to understand.

Your variable Barlow is very neat and a great solution if it maintains collimation as well.

regards ray

#5 Space99

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Posted 03 April 2013 - 05:10 AM

Hi guys,
interesting topic :)

I bought the Baader "Q-TURRET" 2.25x Barlow. It's a short assy
.... I can unscrew the lens-group and attache directly to the ASI120MM .... perfect fit :) I guess the mag is about 1.3x.... then if the seeing is OK I cannibalized an old Meade damaged Barlow bottom section and use that as an extension of about 20mm the mag is about 1.6x perhaps.

Darryl's contraption is awesome .....

Now, what would it take for a company to make a continuously adjustable Barlow with a motor,
With built in linear encoders for repeatability and calibrated mag. and USB interface ....

Am I asking too much :question: :tonofbricks:

Mick

#6 Shiraz

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Posted 03 April 2013 - 09:58 PM

thank Mick.

assuming that you use an f10 scope, you work at a final FNo of around f16, which is reasonably close to the f19 suggested by the rule of thumb for the ASI120.

If you can make your USB Barlow, I will buy one. regards ray

#7 Kokatha man

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Posted 03 April 2013 - 10:42 PM

Darryl's contraption is awesome .....

Now, what would it take for a company to make a continuously adjustable Barlow with a motor,
With built in linear encoders for repeatability and calibrated mag. and USB interface ....

Am I asking too much :question: :tonofbricks:

Mick


Heh-hee Mick: I'm a sort of "Gyro Gearloose" which probably only makes sense to readers of Donald Duck comics from the 50's & 60's.....but this is one of the projects I have on the drawing board..! :shocked: :roflmao:

It's waiting for that little bit of lateral thinking/inspiration that might never come, but in essence I just need to duplicate the focuser mechanism "outboard" of the existing Moonlite unit & EFW as far as I can see: imo a vernier readout gauge seconded into this apparatus would really be all that is needed for repeatability of image scale settings.....no B**#S&#! - I'm seriously looking at that or a much simpler alternative, a helical mechanism that incorporates a digital vernier gauge display cannibalised from a pair of calipers....! :grin:

Of course a simple mechanical unit with vernier gradations would probably suffice: any old camera lens focuser employs this.....and there are other fittings that could be used except for their limited amount of travel, such as the Borg unit below - when I did a bit of researching for this project awhiles back I came across this and other units, but what gives me the gripe about a lot of this stuff (even though it isn't suitable for this specific purpose...) is that supposedly high-quality fittings seem to almost invariably utilise crude "pinch-screw" clamps without a brass compression band.....they're easy to fabricate/install and I've done it many times, but you'd think they'd be a bit more sophisticated in the first place!!!

The unit I posted images of in the other thread is actually only a "Mk2" and I was going to progess to the "Mk3" where I turned the unit down from some solid bar making a much smaller diameter than this one using a 2" fitting.....this Mk2 was predicated upon using mostly off-the-shelf gear but the Mk3 will be much suaver and minimalist, and I hope to incorporate one of the above variable mechanism ideas to make it all very "smooth & schmick." :)

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  • 5776785-BorgHelicalFocuser.png


#8 Shiraz

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Posted 04 April 2013 - 01:55 AM

what about a coarse helical mechanism along the lines of the one Televue put on top of their Paracorr - surely you don't need fine measurement of the exact magnification?

#9 Kokatha man

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Posted 04 April 2013 - 04:39 AM

what about a coarse helical mechanism along the lines of the one Televue put on top of their Paracorr - surely you don't need fine measurement of the exact magnification?


.....you're right Ray - that's why I suggested the simple mechanical vernier arrangement would be ok.....probably what I'd go for; the basic idea was to just scribe a line with 1mm markings in that shiny aluminium section of my Mk2 unit that the outer body slides along.....rub some white paint into this line and hey presto! :)

.....then I got to thinking it'd be dead simple with a screw pitch and locking ring...right through to the outer component having a gradated collar to ensure accurate measurement/adjustment settings.....that bit of inspiration I mentioned above is often when I get my feet back on the ground and make the simplest & easiest/most effective option....! :lol:

My only real requirement is a reasonable amount of travel which in fact isn't very much at all - about an inch of travel increases the f/l about 800mm at the start.....but Mars at the end of the year and next year might require something more than 1"-2". :question:

#10 moxican

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Posted 01 May 2015 - 05:42 PM

I personally calculated my needed focal length for my C9.25 and ASI120MM-S from this site http://www.frankbran.../htm/webcam.htm By this calculation I should be around F18, thus I have bought a Siebert 1.8x for the recommendation of Brian (BK Brown) and it has been working for me great. Seeing has to be really good (rarely happens) in order for me to stick my TV 2x in instead. So by that 3.75*5=18.75. It works for me, I think the rule of thumb gets you in the ball park.




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