2 x Barlow with a CMOS?, finding focus or just a bad idea

[Legion Of Andromeda]

Ok, i have a W.O. G.T.81 and a colour ZWO CMOS, used mostly with field flattener and sometimes with a focal reducer / flattener.

I recently imaged the moon, ( i was bored / curious) but it came out very well but i was only using 20% of my sensor area, so i wondered about using a barlow...

Can i use my existing optical 2"  2x Barlow in the imaging train?... and if so, where does it go ?... into the focus tube or nearer the sensor?...or half-way maybe?

I'm taking the flattener out of the equation for simplicity.....

Is there an optimal back-focus between the barlow and the sensor, then a simple matter of moving the two items away from the objective lens until sharp? like with a FF or FR?

Ive tried today with various permutations and the images are very out-of-focus... roughly how much am i to expect to come out? 5, 10, or even 30 cm?

TIA.

[michael.h.f.wilkinson]

I often use a Barlow or for preference a tele-centric lens like  PowerMate with my CMOS cameras. I don't use a flattener combined with either. Typically, the order I use with a tele-centric is telescope, tele-centric, filter wheel , camera, but I could swap filter wheel and tele-centric around readily. A tele-centric lens has an almost constant magnification, irrespective of the distance between camera and lens. For a Barlow, the longer the distance between the Barlow and the image plane of the sensor, the larger the magnification. A typical Barlow might be designed for e.g. 2x magnification, but that just means that at the magnifcation the aberrations are minimal. In many cases (but not necessarily for "shorty" Barlows) the most likely order in which the optimum magnification is reached is probably telescope, filter wheel, Barlow, camera. I would simply try which configuration works best for you

[vlaiv]

With barlows you want to be on exact distance to sensor (not so with telecentric amplifiers).

Barlow magnification depends on barlow sensor distance. Required focus change also depends on this - so if you over do it - you won't be able to get proper focus.

Usually you don't need field flattener as x2 barlow will only show inner half of the field (or quarter by surface) - and field is pretty flat there.

Proper configuration would be scope/focuser - barlow - camera

where barlow - camera has certain distance to achieve wanted magnification (around what is "prescribed" for that barlow).

In order to know exact distance - you need to know focal length of barlow - but you can treat camera as eyepiece and adjust as you see fit. So screw in nose piece onto camera and slide that into barlow - that is a good starting position.

[MarkAR]

1 hour ago, michael.h.f.wilkinson said:

I often use a Barlow or for preference a tele-centric lens like  PowerMate with my CMOS cameras. I don't use a flattener combined with either. Typically, the order I use with a tele-centric is telescope, tele-centric, filter wheel , camera, but I could swap filter wheel and tele-centric around readily. A tele-centric lens has an almost constant magnification, irrespective of the distance between camera and lens.

This is of great interest to me, I've been mulling over a 2x powermate for imaging. 

With this one https://www.firstlightoptics.com/barlows/tele-vue-powermates.html I take it you remove the reducer at the top and it threads directly (M48) to the filter wheel.

Also is it the silver lower section that unscrews to thread mount it to the focuser ? I take it that is M48 as well.

[Merlin66]

Mark,

All the Powermates are similar design. I've used them all on solar imaging......

They are constructed in two parts - the top part where the eyepiece would sit is removable, and for imaging is usually replaced with a TV T2 thread adaptor. Your camera then fits to this T thread.

The bottom part with the chromed barrel is fixed. There's no option but to use the Powermate with a 1.25" (or 2") focuser.

 

Edited May 11, 2020 by Merlin66

[MarkAR]

Thanks @Merlin66, I was hoping that it unscrewed so that it could be threaded to the focuser M68 thread via a M68/M48 adapter.

[Legion Of Andromeda]

Results are in! Some of this will confirm any previous replies..

Using a W.O. gt81 with a ZWO 1600 mc pro-cooled and a cheapo £50 2" optical 2x Barlow..............

I tried two extreme distances between the sensor and the centreline of the barlow lens, the first was 40mm which gave the least amount of magnification, required 163mm from the centreline of the barlow to the base of the focuser, as a reference, to get focus.

The second was 120mm (by adding 2 x 40mm M42 extenders) which gave the most amount of magnification, this required 143mm from the centreline of the barlow to the base of the focuser, as a reference, to get focus. (20mm less) The ratio of 80mm  / 20 mm looks noteworthy, maybe just a coincidence.

This first experiment was pretty time consuming, so I didn't get into any decent data gathering... However, I did a single 2 min shot at M51 straight overhead without the barlow and again with the barlow set at 120mm... Apart from a dimmer image, it was very well zoomed-in but the optics looked pretty screwy... So i would appreciate any recommendations for good quality 2 x... Anything, unless the degradation is always an issue.

More updates to follow..

[MarkAR]

I think that Powermates are generally supposed to be better that barlows for imaging.

2 x focal length but also 2 x f ratio, so longer exposures needed.

[vlaiv]

2 hours ago, Legion Of Andromeda said:

Results are in! Some of this will confirm any previous replies..

Using a W.O. gt81 with a ZWO 1600 mc pro-cooled and a cheapo £50 2" optical 2x Barlow..............

I tried two extreme distances between the sensor and the centreline of the barlow lens, the first was 40mm which gave the least amount of magnification, required 163mm from the centreline of the barlow to the base of the focuser, as a reference, to get focus.

The second was 120mm (by adding 2 x 40mm M42 extenders) which gave the most amount of magnification, this required 143mm from the centreline of the barlow to the base of the focuser, as a reference, to get focus. (20mm less) The ratio of 80mm  / 20 mm looks noteworthy, maybe just a coincidence.

This first experiment was pretty time consuming, so I didn't get into any decent data gathering... However, I did a single 2 min shot at M51 straight overhead without the barlow and again with the barlow set at 120mm... Apart from a dimmer image, it was very well zoomed-in but the optics looked pretty screwy... So i would appreciate any recommendations for good quality 2 x... Anything, unless the degradation is always an issue.

More updates to follow..

At first I thought you were asking about planetary imaging so using barlow makes sense, but I'm not sure it will make sense for DSO imaging.

Your scope has 480mm focal length (or there about) - with ASI1600 and 3.8um pixel size - that gives you 1.63"/px. That is really about as high as you should go with 80mm scope.

Adding barlow will simply capture no additional detail. You can check this by examining your current subs. If you take a sub that you shot with that scope and camera and measure FWHM of stars and it is lower than about 2.5" FWHM - then you can go with higher resolution - else there simply is no point in doing so.

In case you still want to use barlow and image at high resolution regardless - this is probably best photographic barlow - not cheap though:

https://www.firstlightoptics.com/barlows/baader-vip-modular-2x-barlow-lens-125-and-2.html

Element is wide enough to illuminate full frame sensor and there is exact spec on focal length so you can dial in wanted magnification. Barlow element has T2 connection - easy adaptation to camera.

[Janusz]

Hello All, I am wrestling with how to properly use my 2xBarlow for planetary imaging.

My refractor has a 420mm focal length and focal plane falls 140mm behind the focuser draw tube end.

How do I know that? I subtract OTA length (as measured 280mm) from the Focal Length. 

That is where the camera (be it CMOS or DSLR) sensor should be placed. My CMOS came with just enough spacers to make up the 55mm and

all I need is another 85mm. So I add 2x30mm extension rings and extend focuser's draw out tube by the remaining 25mm. This gives me almost equal

travel of the focuser in and out. (It works the same with a reducer. I multiply telescope focal length by reducer number (0.8 in my case) and repeat the above, (the reducer is providing 30mm of space by itself.) So far this is simple geometry. Just to dispel any notion that 55mm is somehow a magic number.

After this lengthy  introduction, enter the 2xBarlow. My first question - Barlow as I understand it, is placed between an eyepiece and the telescope to double total  power (magnification) without causing eye strain associated with high magnification eyepieces. So do I need an eyepiece in my imaging train? 

Or Barlow should work on the telescope's prime focus alone? Or one could have it either way? 

Interestingly, I do not see my Barlow's focal length listed in its Specs, nor on others either. Does it mean, it behaves like a reducer except it elongates

the FL? Then in my case it would mean 840mm? Do I need an extra 420mm of extension tubes? Sounds ridiculous.

 

Assuming no eyepiece is involved, my first instinct would be to place Barlow's lens glass at the prime focus plane of the telescope (140mm) from the end

of the OTA. Then position camera's sensor at the focal length of the Barlow's glass. With no FL given is one destined to hunt for it moving the camera

back and forth?  Or, maybe try to focus a light source through it and measure the distance? There should be a better way, I would think. Please help in as plain language as possible , since this

is so new to me. Hope to find something that works before December 21st! Thank you. Janusz