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Imaging Saturn with a Barlow

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#1 hdoraisamy

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Posted 18 August 2022 - 05:44 PM

Hi

 

My imaging train consists of a ZWO224 connected to a Celestron 2x barlow which in turn directly goes into my Celestron Nexstar 8SE. I do not use a diagonal. If I try to view Saturn, i don't see anything despite moving the focus from one end to the other. I do not have this problem when the barlow is taken out. 

 

I am using Sharpcap and tried adjusting exposure and gain, but still no joy... I am using the highest FOV on my camera with a solar system alignment.

 

Lunar imaging with the barlow works fine, but not saturn.. Any ideas what i should do different? 

 



#2 KiwiRay

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Posted 18 August 2022 - 05:53 PM

I have an ASI224, image with a Celestron 2x Barlow, and with both Nexstar 6SE and Evo 9.25 have never had this problem. If Lunar is fine, then it looks like adding the Barlow throws Saturn out of the field of view. I'm not sure why that would happen, but try aligning the finder with the Barlow in. Use a spot on the Moon if you have to - I guess if you can't find Saturn, you're not going to find a star either.

 

Wait...are you doing a solar system alignment on the Moon and then slewing to Saturn? That's not going to be very accurate. Try a star alignment (eg, auto 2-star).


Edited by KiwiRay, 18 August 2022 - 05:56 PM.


#3 hdoraisamy

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Posted 18 August 2022 - 06:02 PM

 

Wait...are you doing a solar system alignment on the Moon and then slewing to Saturn? That's not going to be very accurate. Try a star alignment (eg, auto 2-star).

Thank you! I usually do a solar system alignment on saturn itself. I have a Telrad mounted on my scope. I verify that Saturn is centered visually in my eyepiece when I do the alignment and then swap the eyepiece for the barlow/ZWO. 



#4 cinenosin

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Posted 18 August 2022 - 06:12 PM

My thoughts:

  1. With a 2x Barlow, you will need 2-stops (4x) more exposure.  For instance, if your exposure without the Barlow is 20ms, it will need to be 80ms with the Barlow.  With ZWO cameras, a gain difference of 60 is a stop.  A 2-stop difference is a gain change of 120.  I often use a gain of 300 to 400 at 20 to 40ms when capturing Saturn.
  2. Not all Barlows are parfocal.  You may need to refocus.  Test this on the moon.
  3. The Barlow may cause the image to move (uncentered optics).  Again, find a recognizable crater on the moon, and switch the Barlow to see if the crater is still centered, or if it moved, which direction and how much.
  4. Planets are very difficult to find and keep centered.  It is not unusual to spend 10 to 20 minutes just finding it.

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#5 DirtyRod

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Posted 18 August 2022 - 06:13 PM

I generally start by aligning the planet first without the barlow because sometimes it's just outside the FOV depending on how well I aligned the scope. Monday night it was barely off to the side so I had to move the scope around a bit to find it. 

 

When I start, I generally make sure I'm not zoomed in, set the maximum ROI, and turn up gain. If the planet is just out of the FOV, often I can see the brightness and know which direction I need to move the scope. Once I find it and make sure it's center, then I add the barlow. 


Edited by DirtyRod, 18 August 2022 - 06:32 PM.

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#6 hdoraisamy

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Posted 18 August 2022 - 06:20 PM

 

My thoughts:

  1. With a 2x Barlow, you will need 2-stops (4x) more exposure.  For instance, if your exposure without the Barlow is 20ms, it will need to be 80ms with the Barlow.  With ZWO cameras, a gain difference of 60 is a stop.  A 2-stop difference is a gain change of 120.  I often use a gain of 300 to 400 at 20 to 40ms when capturing Saturn.
  2. Not all Barlows are parfocal.  You may need to refocus.  Test this on the moon.
  3. The Barlow may cause the image to move (uncentered optics).  Again, find a recognizable crater on the moon, and switch the Barlow to see if the crater is still centered, or if it moved, which direction and how much.
  4. Planets are very difficult to find and keep centered.  It is not unusual to spend 10 to 20 minutes just finding it.

 

Thank you. I did not know this. I will try this tonight



#7 DirtyRod

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Posted 18 August 2022 - 06:22 PM

 

I often use a gain of 300 to 400 at 20 to 40ms when capturing Saturn.

What size scope is that? With my C8 and barlow at 300 gain, anything over 14ms will start to overexpose the planet. My best images are 10-12ms for Saturn at 300. That gives me the best images while still getting high frame rates. 


Edited by DirtyRod, 18 August 2022 - 06:22 PM.

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#8 KiwiRay

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Posted 18 August 2022 - 06:34 PM

I also center with Barlow and eyepiece, then add the camera. I wouldn't add the camera and Barlow together as a unit. For Saturn I start with 400 gain and 20 ms exposure, which makes it bright onscreen and easy to see, even if out of focus. I'll reduce exposure for capture.


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#9 Sunspot

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Posted 18 August 2022 - 06:38 PM

In my case, once I put the camera/Barlow combination on my scope I don't remove it until the imaging session is over, even if I'm imaging three planets.


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#10 dcaponeii

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Posted 18 August 2022 - 07:11 PM

Hi

 

My imaging train consists of a ZWO224 connected to a Celestron 2x barlow which in turn directly goes into my Celestron Nexstar 8SE. I do not use a diagonal. If I try to view Saturn, i don't see anything despite moving the focus from one end to the other. I do not have this problem when the barlow is taken out. 

 

I am using Sharpcap and tried adjusting exposure and gain, but still no joy... I am using the highest FOV on my camera with a solar system alignment.

 

Lunar imaging with the barlow works fine, but not saturn.. Any ideas what i should do different? 

If you can focus on the Moon with the system you can focus on Saturn. Either your not on the chip OR it is too faint for you to detect.  Aligning your finder fixes the 1st, increasing gain and exposure fixes the 2nd.


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#11 hdoraisamy

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Posted 18 August 2022 - 08:15 PM

Thank you everyone! I was able to get it to work! Here's what I did differently from last time based on your feedback:

 

1. Solar system alignment with Saturn centered exactly in my eyepiece with the barlow in place

2. Set my exposure to 20 ms and a gain of 400

 

It took me a while but I was able to get my images! Thank you so much!


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#12 cinenosin

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Posted 19 August 2022 - 07:06 AM

What size scope is that? With my C8 and barlow at 300 gain, anything over 14ms will start to overexpose the planet. My best images are 10-12ms for Saturn at 300. That gives me the best images while still getting high frame rates. 

Thanks for pointing that out.  Come to think of it, I use a 2.5x PowerMate.  By the way, exposure is not a function of the telescope size, but of the focal ratio.  An f/10 SCT with a 2x magnification is f/20 regardless of the telescope aperture.  I have an article on my website to help set optimal exposure times: https://texasstarcav...exposure-times/


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#13 RedLionNJ

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Posted 19 August 2022 - 08:11 AM

Hi

 

My imaging train consists of a ZWO224 connected to a Celestron 2x barlow which in turn directly goes into my Celestron Nexstar 8SE. I do not use a diagonal. If I try to view Saturn, i don't see anything despite moving the focus from one end to the other. I do not have this problem when the barlow is taken out. 

 

I am using Sharpcap and tried adjusting exposure and gain, but still no joy... I am using the highest FOV on my camera with a solar system alignment.

 

Lunar imaging with the barlow works fine, but not saturn.. Any ideas what i should do different? 

How confident are you Saturn is actually in the (narrower) FOV?  If not 100%, that's your problem right there.



#14 kevinbreen

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Posted 19 August 2022 - 01:16 PM

Thank you everyone! I was able to get it to work! Here's what I did differently from last time based on your feedback:

1. Solar system alignment with Saturn centered exactly in my eyepiece with the barlow in place
2. Set my exposure to 20 ms and a gain of 400

It took me a while but I was able to get my images! Thank you so much!


Well done. Gain of 400 seems a bit high, for me anyway. I try to be not much higher that 300, maybe 320. It's probably a personal thing.
Your kilometerage might fluctuate.
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