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Barlow Lenses for Astrophotography

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#1 Jireh

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Posted 12 June 2019 - 02:11 PM

Can someone give me the basics of how barlow lenses are used in AP? I've never used, touched or seen one. How do they fit in the image train? Is there any other hardware needed to use them? I'm using an ES 127ED APO and wanted to at least try my hand at imaging Jupiter while the opportunity is there. I image with an ASI1600MM Pro but I do have an ASI290MM Mini I use as a guide cam. Which ones do you recommend? Do I need 1.25" or 2"? 2x, 3x, 5x?



#2 cfosterstars

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Posted 12 June 2019 - 02:41 PM

I have several of them of all different flavors. I do not do an planetary imaging and that is there primary purpose. They fit in the train like a focal reducer, between the focuser and the camera. Most are targeted for 1.25" format eye pieces or cameras since for planetary you only are looked at small objects are high mag. I have a 2" format one also.  I got them early on when I was just buy stuff to buy stuff. The only place I really use them is for my guide scopes. I used them to increase the F/L of the guide scope to lower the guider pixel scale to get better measure of the mount corrections. What one you need depends on what you want to do with it. For planetary imaging you want very long focal lengths - 5000mm like F/L to make the very small planets appear as a disk on you camera.  Go to https://astronomy.to.../field_of_view/ and look at how the planet would fit on your camera with you scope and then since it is planetary you would be running in video mode so that is a subframe of you camera. Which one you want depend on these factors. That is assuming that is what you want to do with a barlow.


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#3 Kaydubbed

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Posted 12 June 2019 - 02:42 PM

From my understanding, as I am also interested in doing this with the same gear, put the barlow closer to the focuser for more magnification [i.e. your 2x barlow now acts like a 2.5x] or closer to the sensor for less [that 2x now acts like a 1.5x]. The backspacing requirements will change also [increase?] but someone else will need to chime in about that.

 

Don't use a FF/FR because you won't get any benefit from it. You might have to get rid of the filter wheel if you have one and just put the filters at the end of the barlow if you do RGB. You might just want to shoot video and stack from that. You won't be able to do lucky imaging with either camera [effectively] because the mini is USB2 and the ASI1600 has like, 17MB frames [brain  fart] and you aren't realistically going to be able to stack thousands of those.

 

 


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#4 bobzeq25

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Posted 12 June 2019 - 02:48 PM

I've done a tad of planetary/lunar.  You'll get _far_ better answers on the solar system imaging forum.  Just a few very basic points.

 

As you probably know, you definitely shoot video.

 

The way either camera works is that you define a small Region of Interest to speed up the frame rate.  Speed is king.  USB2.0 is not best, but it will work fine.

 

2X or 3X could both work.  I'd go 3X.  Focal length involves tradeoffs, it's not just that longer focal length is better. 

 

People do use filter wheels.

 

The planets are so low that this would be helpful (I have one).

 

https://astronomy-im...product/zwo-adc

 

I've used PI to process the stack, but just using Registax (possibly combined with Astrostakkert) is likely easier.

 

Go ask on the SSI forum.


Edited by bobzeq25, 12 June 2019 - 02:54 PM.

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#5 MalVeauX

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Posted 12 June 2019 - 02:56 PM

Can someone give me the basics of how barlow lenses are used in AP? I've never used, touched or seen one. How do they fit in the image train? Is there any other hardware needed to use them? I'm using an ES 127ED APO and wanted to at least try my hand at imaging Jupiter while the opportunity is there. I image with an ASI1600MM Pro but I do have an ASI290MM Mini I use as a guide cam. Which ones do you recommend? Do I need 1.25" or 2"? 2x, 3x, 5x?

Hello,

 

The barlow is used to control imaging scale so that you can optimally sample based on your aperture, focal-length and pixel size of your camera. Since you are using a 5" refractor with a fairly short focal length, I would use your ASI290MM for planetary imaging because its pixels are small and you will have fewer pixels on target anyways, and these are smaller pixels (2.9um) and there are fewer of them so it will be fast for high FPS. When imaging planets you want thousands of frames to stack. So you will region of interest around the planet and grab 60~90 seconds of video (Jupiter for example) and stack a bunch of them. Put a UV/IR block filter in there (your luminance filter usually) to increase contrast due to long IR sensitivity and blocking that.

 

I assume this is the F7.5 version?

 

So your 127mm aperture, 2.9um pixesl (ASI290MM) will result in critical sampling of green wavelength at F14. So all you need is a 2x barlow for your F7.5 scope and you're pretty much there.

 

If you used your ASI1600MM (3.8um pixels) it would be F18 to properly sample, so a 2.5x barlow/powermate would be appropriate. But this camera is slower than the 290, so I suggest you use the 290.

 

Very best,


Edited by MalVeauX, 12 June 2019 - 02:58 PM.

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#6 Jireh

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Posted 12 June 2019 - 03:30 PM

I've done a tad of planetary/lunar.  You'll get _far_ better answers on the solar system imaging forum.  Just a few very basic points.

 

As you probably know, you definitely shoot video.

 

The way either camera works is that you define a small Region of Interest to speed up the frame rate.  Speed is king.  USB2.0 is not best, but it will work fine.

 

2X or 3X could both work.  I'd go 3X.  Focal length involves tradeoffs, it's not just that longer focal length is better. 

 

People do use filter wheels.

 

The planets are so low that this would be helpful (I have one).

 

https://astronomy-im...product/zwo-adc

 

I've used PI to process the stack, but just using Registax (possibly combined with Astrostakkert) is likely easier.

 

Go ask on the SSI forum.

Thanks, I'll go ask on the other forum.



#7 Jireh

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Posted 12 June 2019 - 03:34 PM

Hello,

 

The barlow is used to control imaging scale so that you can optimally sample based on your aperture, focal-length and pixel size of your camera. Since you are using a 5" refractor with a fairly short focal length, I would use your ASI290MM for planetary imaging because its pixels are small and you will have fewer pixels on target anyways, and these are smaller pixels (2.9um) and there are fewer of them so it will be fast for high FPS. When imaging planets you want thousands of frames to stack. So you will region of interest around the planet and grab 60~90 seconds of video (Jupiter for example) and stack a bunch of them. Put a UV/IR block filter in there (your luminance filter usually) to increase contrast due to long IR sensitivity and blocking that.

 

I assume this is the F7.5 version?

 

So your 127mm aperture, 2.9um pixesl (ASI290MM) will result in critical sampling of green wavelength at F14. So all you need is a 2x barlow for your F7.5 scope and you're pretty much there.

 

If you used your ASI1600MM (3.8um pixels) it would be F18 to properly sample, so a 2.5x barlow/powermate would be appropriate. But this camera is slower than the 290, so I suggest you use the 290.

 

Very best,

Thanks - all very good info!

So help me understand the basics regarding connections, etc. Yes, my 127ED is the f/7.5 version with a 2.5" hex focuser. So the focuser compression tube is 2". Does that mean I need a 2" barlow? I'm completely lost on how the barlow fits into the imaging train.



#8 MalVeauX

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Posted 12 June 2019 - 03:40 PM

Thanks - all very good info!

So help me understand the basics regarding connections, etc. Yes, my 127ED is the f/7.5 version with a 2.5" hex focuser. So the focuser compression tube is 2". Does that mean I need a 2" barlow? I'm completely lost on how the barlow fits into the imaging train.

Heya,

 

You do not need 2" anything. None of your sensor sizes are larger than 1.25" barrels inner diameters. So you can use all 1.25" equipment essentially here, especially for planets. Imaging train will be telescope -> whatever you want here (usually an extension tube to help achieve focus) -> 1.25" 2x Barlow/Powermate/FocalExtender -> 1.25" nose of your camera with a 1.25" UV/IR block filter threaded onto it. It's no different than your typical imaging train for DSO with your cameras, but you put the 2x barlow on the nose of the camera to increase imaging scale and it will move where the focus point is, so you may need an extension tube unless your focuser has a lot of out travel.

 

Very best,


Edited by MalVeauX, 12 June 2019 - 03:41 PM.

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#9 Kaydubbed

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Posted 12 June 2019 - 03:44 PM

Thanks - all very good info!

So help me understand the basics regarding connections, etc. Yes, my 127ED is the f/7.5 version with a 2.5" hex focuser. So the focuser compression tube is 2". Does that mean I need a 2" barlow? I'm completely lost on how the barlow fits into the imaging train.

Do you have a 2" to 1.25" adapter on your focuser? Put a 1.25" barlow in that. With Region of Interest Cropping, you won't have any vignetting as long as your subject is centered in the frame. 


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#10 Jireh

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Posted 12 June 2019 - 04:16 PM

Heya,

 

You do not need 2" anything. None of your sensor sizes are larger than 1.25" barrels inner diameters. So you can use all 1.25" equipment essentially here, especially for planets. Imaging train will be telescope -> whatever you want here (usually an extension tube to help achieve focus) -> 1.25" 2x Barlow/Powermate/FocalExtender -> 1.25" nose of your camera with a 1.25" UV/IR block filter threaded onto it. It's no different than your typical imaging train for DSO with your cameras, but you put the 2x barlow on the nose of the camera to increase imaging scale and it will move where the focus point is, so you may need an extension tube unless your focuser has a lot of out travel.

 

Very best,

Maybe I'm dense but I'm not quite getting how the barlow connects to the focuser. At the end of the 2.5" focuser there is a 2" opening. For DSO imaging this is where I would insert my Hotech 2" FF. I know I don't use the FF in this scenario, I just mention it to help illustrate. How does the 1.25" barlow work with the 2" opening?

 

Thanks for your patience!



#11 Jireh

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Posted 12 June 2019 - 04:17 PM

Do you have a 2" to 1.25" adapter on your focuser? Put a 1.25" barlow in that. With Region of Interest Cropping, you won't have any vignetting as long as your subject is centered in the frame. 

No, just the 2".



#12 vio

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Posted 12 June 2019 - 04:47 PM

You mentioned having a mini 290, which is 1.25”, how do you plan to use it (Barlow or no Barlow) with your scope without a 2”-1.25” adapter? If this is your constraint, it leaves just one option, using only 2” stuff - camera, Barlow etc. There was a suggestion earlier to use the 290 for captures for its higher speed. If taking that route, use an adapter (eyepiece adapter 2”-1.25”) to insert the Barlow, then camera (290) in the Barlow lens. With all on 2”, a 2” Barlow first, then the 1600 in the Barlow lens.


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#13 Jireh

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Posted 12 June 2019 - 05:55 PM

You mentioned having a mini 290, which is 1.25”, how do you plan to use it (Barlow or no Barlow) with your scope without a 2”-1.25” adapter? If this is your constraint, it leaves just one option, using only 2” stuff - camera, Barlow etc. There was a suggestion earlier to use the 290 for captures for its higher speed. If taking that route, use an adapter (eyepiece adapter 2”-1.25”) to insert the Barlow, then camera (290) in the Barlow lens. With all on 2”, a 2” Barlow first, then the 1600 in the Barlow lens.


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Something like this?

https://www.highpoin...nch-adapter-fad



#14 Kaydubbed

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Posted 12 June 2019 - 07:44 PM

Yes that. If you ask around one of these guys might be able to send you one if they live in your area. A lot of people like myself have quite a few of these lying around in toolboxes.

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#15 Jireh

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Posted 12 June 2019 - 08:07 PM

Yes that. If you ask around one of these guys might be able to send you one if they live in your area. A lot of people like myself have quite a few of these lying around in toolboxes.

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Thanks, I just wanted to confirm I was looking in the right direction!




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