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Stacking Barlow lenses

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#1 jborko

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Posted 30 November 2020 - 08:54 AM

Hi everyone,

 

I have been trying to search for an answer on the forums but after couple of hours of research I decided I would rather attempt asking my specific question here.

 

I own Skywatcher ED80/600 (f7.5) and EQM35, 2x GSO ED Barlow and mirrorless Sony A6000.

 

I have recently developed an interest in astrophotography and I have been successful (read satisfied) in making a few beginner photos of Andromeda, Pleiades, Neowise (still far from great, but I got that heartbeat skip when I got some of my end results), so that is good enough for me for now. Now, being cloudy for almost a month I am eager to continue as soon as the sky clears. My next target is the conjunction of Jupiter and Saturn in December. (I will be in a dark area in the days of the biggest proximity)

 

Obviously I never took a closer shot at planets previously, so I am having few thoughts (perhaps completely incorrect) on how to achieve this.

 

I run a simulation in Stellarium. The approximate results of how I imagine the result to be, requires 6x Barlow magnification equivalent. Stacking 2x and 3x onto the scope apparently should give the effect of 6x Barlow. Obviously that introduces the problem the light/brightness loss, which will force me to use longer shutter speeds. And to make things even more complicated I read that you need to shoot video and then stack the images from the video to get more details on the planets.

 

So my questions coming out of this reasoning:

 

1. Is it realistic to use 2 stacked Barlows (2x and 3x)? Has someone tried it? (reason for asking is whether I should go through the effort of obtaining/barrowing one or just ditch the whole idea)

2. If yes, will I be able to achieve focus behind the stacked Barlows without an excessive length of extenders? (if I will need an extender is there a way to calculate the length?)

3. Am I correct in my assumptions that I would need to shoot video and then use identify sharp frames from the video for stacking?

 

Thanks for all the help.

Borko


Edited by jborko, 30 November 2020 - 09:01 AM.


#2 ismosi

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Posted 30 November 2020 - 10:26 AM

Welcome to Cloudy Nights!

 

Most of us imaging the planets use 'lucky imaging', i.e. capturing a large number of frames and then using software to identify the best of those and stack them to create the final image. I am not familiar with your camera, but, if it can take images at a high frame rate it could suffice.

 

If you do stack Barlow lenses to a nominal 6x, you'll have an effective focal length of 3600mm, which can be a challenge for your mount in and of itself. Again, I do not know your gear so maybe it won't be an issue for you.

 

Jupiter and Saturn will be low in the sky where the seeing is likely to be suboptimal. I expect the view to be mushy at best and I may just opt to take in the view with Eyeball Mark I instead :) 

 

If you decide to try imaging a few 'dry runs' beforehand wouldn't hurt.



#3 RedLionNJ

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Posted 30 November 2020 - 11:32 AM

I'm trying to find something encouraging to type in response to your eager willingness to capture this occasion while still (very) steeply in the planetary astrophotography learning curve. I don't have very much.

 

As Luigi already pointed out to some extent, the particular conditions of this conjunction, along with your (proposed) equipment are far, far from optimal.

 

In planetary astrophotography in general, we need practically EVERYTHING to be "just right" to get excellent results.  There is a lot on your list which is not.

 

In no particular order:

The skies will not be dark (very shortly after sunset)

The elevation will be extremely low (poor seeing, color dispersion)

80mm is not a class of scope generally used for planetary work

I am not familiar with the A6000, but a review of the online specs does not reveal any uninterpolated, high-speed video modes - a huge negative

 

 

On the POSITIVE side -

With the ED80 & A6000, you could shoot the conjunction as if it were a short-exposure DSO (no barlows). This would nicely frame the two planets and possibly the Galilean moons, within a large area of (relatively) dark space. This could be potentially more dramatic than any mushy, rainbow-ridden closeups.


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#4 ismosi

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Posted 30 November 2020 - 12:57 PM

 

On the POSITIVE side -

With the ED80 & A6000, you could shoot the conjunction as if it were a short-exposure DSO (no barlows). This would nicely frame the two planets and possibly the Galilean moons, within a large area of (relatively) dark space. This could be potentially more dramatic than any mushy, rainbow-ridden closeups.

That is a good suggestion ;)

I might just try that ...


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#5 Kiwi Paul

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Posted 30 November 2020 - 05:34 PM

Just off the top of my head, I would guess there’s no way you would get the whole field containing the two planets in you shot using the two barlows. In fact, if I have a go at it I may use a focal reducer (0.75) to widen the field in my C8 SCT. From memory they will be about 6 minutes of arc apart?
Cheers Paul
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#6 Kokatha man

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Posted 01 December 2020 - 01:29 AM

There is never any reason to use more than one barlow! Extension tubes or increasing the distance from the back-end of a single barlow will create all the image scales/focal lengths ever required!

 

My addition to the Televue graph page, also showing how reducing the distance by using just the lens element itself without the rest of the body can achieve lower multiplication factors. Most barlows (not telecentrics aka "power mates" et al) follow the Televue plot to a large extent btw.

 

Televue2XGraphModified.jpg

 

 


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#7 jborko

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Posted 01 December 2020 - 12:26 PM

Folks, thanks a lot for your thoughts and suggestions.

 

Before I go into some of your responses...

 

I spent some time yesterday testing my idea of the 2 barlows (I used one that I had for the scope plus one old camera lens (M42 mount) 2x teleconverter (it really is a very old teleconverter and I guess of a lower quality one that I bought on a garage sale). I believe that the teleconverter is the same optical construction as the Barlow lens). So the setup required some tries and fails. Eventually I was able with the extenders and the adapters to get 3 successful configurations.

 

1. ED80 Scope + 0.85 Coma corrector on the focuser + 2x Barlow + adapter + 2x Teleconverter + camera adapter + camera - long setup but I managed to find focus

2. ED80 Scope + 2x Barlow + adapter + 2x Teleconverter + camera adapter + camera - shorter setup with the highest "magnification" (if I can call it magnification) and I was able to reach focus too

3. ED80 Scope + 2x Barlow + camera adapter + camera - I tried this first, but was not able to achieve focus, so I took out the Barlow extension tube (the adapter for the eye pieces) and "voila". Focusing ok...

 

In terms of perceived Magnification:
it is there, and quite a significant one for that matter. But that is where it really ends... Resolution dropped significantly with the introduction of the second barlow (teleconverter) in the stack (maybe because of the weak optical performance of the old one)... I also noticed/experience what ismosi and RedLionNJ suggested. High air turbulence making almost any exposure over 1 second unusable. And I understand I need to shoot high framerate video. So that is that...

 

Today is cloudy again but I will attempt some tests during daylight to see the real optical resolution performance differences.

 

I will attempt the DSO scenario for sure. I think that is the most I can pull out of this situation...

 

On your last note Kokatha - thanks a lot... That is really helpful and simplifies my initial idea. I will just need to find an extension tube to test and make sure I've got the right focusing distance...Might need to think of a way how to shorten/lengthen the position of the focus.



#8 Kokatha man

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Posted 01 December 2020 - 06:07 PM

<"On your last note Kokatha - thanks a lot... That is really helpful and simplifies my initial idea. I will just need to find an extension tube to test and make sure I've got the right focusing distance...Might need to think of a way how to shorten/lengthen the position of the focus.">

 

Realise that the light cone from any scope (refractor, reflector, SCT etc) is intercepted just short of the native focal length of that scope by the front lenses of any barlow. (ie, just before 600mm for your ED80)

 

As long as the barlow can be positioned at that point, then the focused image (after the barlow) can be obtained at any position beyond the rear lenses of any barlow...the further behind the barlow, the greater the image scale/focal length - hence the extension tubing to vary this. ;)



#9 jborko

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Posted 02 December 2020 - 08:31 AM

Thanks, Kokatha, so I did some more tests and I think I can get close to the "magnification" that I need with just the one barlow I have. There is one caveat though...

 

The Barlow lens for my scope cannot be closer than 520mm from the front element. If it is I cannot achieve focus. And the coincidence is that the focuser extends to 520mm. In my tryouts, I have the stacked the layman's way (I have reversed the tube from the barlow and inserted the lens in the place where you usually put the eyepiece (that gave me some 550mm distance from the front element.

 

So I figure I will need:

  • Focuser Extension tube (thread 56mm, length ±30mm) - If that is a regular accessory that can be found (will browse the shops for that)
  • Adapter from focuser 56mm to 48mm (that is to properly attach the Barlow) - If that is a regular accessory that can be found
  • Barlow lens - ✓ OK
  • Extension tube/s behind the Barlow - 48mm - length will depend on the scaled magnification I want to get - I guess this should be a normal accessory
  • Adapter from M48 to camera - ✓ OK

The way I see it, while doable it will put some strain on the focuser and the whole system.


Edited by jborko, 02 December 2020 - 09:11 AM.


#10 Kokatha man

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Posted 02 December 2020 - 06:59 PM

If I understand you correctly you need a bit more back focus on the scope/focuser to achieve any focus with the barlow you are using..?

 

This is an absolutely common issue - surely just using a 2" or 1.25" extension tube inserted into the ED80's focuser will provide you with the extra back focus distance required to focus after the barlow..?

 

These tubes come in various lengths, just make sure when you measure up you account for the distance your focuser travels to get the focus behind the barlow with the focuser racked out no more than half its' travel.

 

It's been a long time since I owned an ED80 & it came with the R&P focuser which I swapped out for a Crayford style 10:1 - but I do have the simple achro SW equivalent which I use sometimes as a guider/spotter where the 1.25" R & P focuser appears very robust.

 

I see that the weight of the A6000 is 344gm or around 12oz without the lenses (which you won't be using) so I'd imagine that if you have any sort of decent focuser on your ED80 it's not going to be much of a load tbh. wink.gif


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