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Trouble finding planet when using a barlow

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9 replies to this topic

#1 bluesilver

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Posted 17 September 2021 - 04:00 AM

Hi, I am trying do some planetary imaging.

I am using a ASI 244mc camera and i am using a Skywatcher dobsonian that has goto tracking.

Software for imaging is FireCapture.

So say i am working on Jupiter for example.

I can get an image on my laptop if i just use the 224mc camera by itself, So camera is straight into the eyepiece.

But when i go to use a barlow, either 2X , 3X, 5X i can't seam to get an image or find the planet.

 

I start out at the beginning, camera straight into the eyepiece, move the scope around slightly to get the planet dead center in the screen ( it is fairly small at this stage )

I then take the camera out, install a barlow, install the camera and no planet is visible.

Try slightly moving the scope around a bit and still no sign of the planet.

 

Is there something that i am obviously doing wrong here?

I figured that i should at least see some part of the planet when i installed the barlow.

 

And advise would be greatly appreciated.

 

 

 

 



#2 Ittaku

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Posted 17 September 2021 - 04:22 AM

You are probably way out of focus with the barlow in place and the planet is massively defocused which is why you can't find it. On a dob it can be difficult to get the right amount of backfocus movement to get focus. When you put the barlow in, turn the gain and exposure up to huge amounts and see if there isn't a big white ring there instead of a planet.


Edited by Ittaku, 17 September 2021 - 04:22 AM.

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#3 bluesilver

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Posted 17 September 2021 - 04:47 AM

Thanks for the reply,

Never thought of those things, I just basically just get the black screen as if nothing is there.

I will try out as you mentioned with the gain and exposure.

I am wondering if also just winding the focus in and out would bring an image up?

Ideally i need to go to 5X to get the optimum numbers for imaging, but i think the planet wouldn't fit on the screen with that much magnification.

I am thinking 3X might be a most of a realistic barlow to go with.



#4 KpS

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Posted 17 September 2021 - 04:59 AM

I assume you have the camera in focus without the barlow. But after inserting the barlow, the camera will no longer be in focus. Try the necessary shift on a distant object during the day.

 

P.S. Before I could write an answer with my English, Ittaku was faster.


Edited by KpS, 17 September 2021 - 05:02 AM.


#5 Quopaz

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Posted 17 September 2021 - 05:03 AM

Yes if you're way out of focus you won't see anything. Also with the barlow you will have a narrower field of view, meaning you could
be missing it. The moon is a good big target to sight in on and get your focus right, then you need to get your finderscope aligned
with what you see on screen. I do it roughly on the moon, then move to say Jupiter and get it on screen. Once you have it on screen
then you can adjust the finderscope to be right on.

#6 TOMDEY

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Posted 17 September 2021 - 07:06 AM

Those Barlows squeeze the field down to a soda straw. Forget the planets and just get on a bright field of stars or better yet the moon. Establish the proper focus for all three of your Barlows, marking those on e.g. the barrel of your focuser with a Sharpie. Now get on Jupiter or Saturn sans Barlow; center it; immediately switch to the Barlow without screwing up pointing (try the 2x first) and focus to that Sharpie mark. Jupiter will be in the field. Touch up the focus a wee bit and image. The 3x will be tougher and the 5x very challenging, with that miniscule field.

 

Anyway... practice at 2x, then 3x, then 5x to get your established good technique down. That's really all there is to it!

 

Another useful aid is a good additional higher-power big finder with crosshair.   Tom

 

~click on~ >>>

Attached Thumbnails

  • 22 Toms early TeleVue Genesis 100mm F-5 focuser retrofit focus marks Sharpie.jpg


#7 kevinbreen

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Posted 17 September 2021 - 07:21 AM

The planet might be slightly off centre with the Barlow installed even if it's centred without. As you change focus nudge the scope gently with your other hand and you should see evidence that you're coming into focus - you'll see movement of a cloudy blob*. That's your planet. Make note of how many turns of focus knob is needed for next time.

* "cloudy blob" is not a technical term.

#8 KiwiRay

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Posted 17 September 2021 - 09:22 AM

I first focus with an eyepiece and the Barlow inserted, then replace the eyepiece with the camera. The onscreen image will then already be close enough to in focus that it's easy to see (especially with longer exposures selected initially). Some folks find that replacing the eyepiece with the camera throws the planet off centre, so this might not work for everyone.


Edited by KiwiRay, 17 September 2021 - 09:23 AM.

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#9 bunyon

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Posted 17 September 2021 - 09:25 AM

I do what KiwiRay does. But if you want to keep doing it the way you are, you can increase gain and exposure (a lot). If the out of focus planet is on the chip, you'll see the donut and can focus on it. If you don't see the donut with the gain all the way up and a long exposure, you lost the planet from the chip when you inserted the Barlow.


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#10 bluesilver

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Posted 17 September 2021 - 05:53 PM

Thanks for the replies,  a lot of very good ideas there and all of which i never thought of trying.

I really like the idea of focusing it on the moon to get it close,  but then the best time to image is when there is no moon,  so a star focus could be the way to go.

Might give trying to focus during the day on a distant object also,  That could help out a bit.

 

Marking the draw tube with a sharpie is an excellent idea,  will defiantly be doing this.

Now just have to wait until i get another clear night to put it all back into practice.

 

Excellent information and advice from all,

Very much appreciated.




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