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1CM69

Usefulness of Flats when imaging via auto focuser

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Hi,

I am considering purchasing an auto focuser for my setup but I’m stuck on one aspect of its use. 

While imaging throughout the night the auto focuser will be continuously adjusting focus as it should due to temp change etc.... 

Now I have always been under the impression that one use of Flats is to remove any dust specs that may be in the image train & that to capture Flats, the focus & camera position needs to be in the same place as it was when capturing the Light frames. 

So how can Flats created after imaging using an auto focuser be of any use if the focus has been constantly updated while capturing the Lights?

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Hi

In theory you are right , but in practice the few thou of a mm you move the focuser has no real effect

go get a electric focuser , do not know how I managed before :)

Harry

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15 minutes ago, harry page said:

Hi

In theory you are right , but in practice the few thou of a mm you move the focuser has no real effect

go get a electric focuser , do not know how I managed before :)

Harry

OK, so even after a full night capturing Lights and then taking Flats there should be no issue in dividing out the motes from say the 1st Light frame taken?

Seems counterintuitive as everyone says, ‘Lock Down Focus, Do Not Move Anything in Image Train’, when talking about Flat capturing

12 minutes ago, steppenwolf said:

Wot Harry said :D

??

 

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4 minutes ago, 1CM69 said:

OK, so even after a full night capturing Lights and then taking Flats there should be no issue in dividing out the motes from say the 1st Light frame taken?

Seems counterintuitive as everyone says, ‘Lock Down Focus, Do Not Move Anything in Image Train’, when talking about Flat capturing

??

 

Hi

Yup I just take my flats at the end of a run , always worked to date :)

unless you are very unlucky and gain dust during the night :hmh:

Harry

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I would argue the focuser is keeping the sensor - main mirror distance constant to compensate for any change in tube length with temperature. The impact of the secondary mirror's effective position in the imaging train can be ignored as (a) it is a plane mirror so no optical changes and (b) far enough from the sensor for any dust bunnies not to create a shadow.

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3 minutes ago, harry page said:

Hi

Yup I just take my flats at the end of a run , always worked to date :)

unless you are very unlucky and gain dust during the night :hmh:

Harry

Great, thanks

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The auto focuser will just be finding perfect focus, or best compromise, so if it really worried you, take the flats immediately after a focusing run at the end of a session.

I believe some focusers can operate via temperature changes too which would make it easier, as they'll adjust the focus to be right for the current temperature.

Personally, I enjoy manual focusing, it maintains a hands on relationship with each image and gives me a reason to stay awake and with my gear, albeit it in a warm room or from inside. I check it every hour, or every sub in extreme temperature changes.

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On 2017-12-04 at 21:49, 1CM69 said:

Seems counterintuitive as everyone says, ‘Lock Down Focus, Do Not Move Anything in Image Train’, when talking about Flat capturing

"Wot" Harry and Steve said. The thing is to not rotate the camera or move focus all the way in or out before doing flats. As Harry wrote, the tiny adjustments that are made during the night, have no impact on flats.

Btw, when you have a motorised focuser, you don't lock the focus down. The motor's torque works as the lock. Locking down a motorfocuser will fry either the motor or the electronics.

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Another for 'Wot Harry said' though I don't use motorized focusers myself. That doesn't mean I'm not happy to fiddle endlessly with the ones I host in our robotic sheds which all work perfectly except when they don't... :evil4::evil4:

Olly

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On 04/12/2017 at 23:16, Tim said:

The auto focuser will just be finding perfect focus, or best compromise, so if it really worried you, take the flats immediately after a focusing run at the end of a session.

I believe some focusers can operate via temperature changes too which would make it easier, as they'll adjust the focus to be right for the current temperature.

Personally, I enjoy manual focusing, it maintains a hands on relationship with each image and gives me a reason to stay awake and with my gear, albeit it in a warm room or from inside. I check it every hour, or every sub in extreme temperature changes.

 

6 hours ago, wimvb said:

"Wot" Harry and Steve said. The thing is to not rotate the camera or move focus all the way in or out before doing flats. As Harry wrote, the tiny adjustments that are made during the night, have no impact on flats.

Btw, when you have a motorised focuser, you don't lock the focus down. The motor's torque works as the lock. Locking down a motorfocuser will fry either the motor or the electronics.

 

3 minutes ago, ollypenrice said:

Another for 'Wot Harry said' though I don't use motorized focusers myself. That doesn't mean I'm not happy to fiddle endlessly with the ones I host in our robotic sheds which all work perfectly except when they don't... :evil4::evil4:

Olly

Great, thanks all.

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