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shifting focus during the night

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#1 mtn_nut

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Posted 02 March 2021 - 11:44 AM

Hi, 

 

I went out last night to shoot the Orion nebula, using a bahtinov mask (sharpstar 2) to get critical focus. When I went out an hour later to check on it, all of the photos had gotten out of focus. It was around freezing, so I'm guessing that this happened because of the temperature change. When I went to refocus, my lens (Sony 200-600 G) at max infinite focus couldn't quite get the cross hairs caused by the bahtinov mask to line up, but it was close enough to get good photos.

 

I have a few questions. First did the focus shift because of the temperature change (house was ~70 degrees F, outside was ~30 degrees F) or did something else possibly cause that. 

Second, if the focus shift was caused by a temperature change, would a dew heater strap help keep focus? I'm in Colorado, and its pretty dry here, so I normally don't have to deal with dew too much.

Lastly, if the lens couldn't quite focus to infinity after the focus shifted, is there a way to fix that? I know macro tubes allow lens to focus closer to the subject than the lens normally can, but there doesn't seem to be a way to make it focus past infinity more without some extra optics.

 

EDIT - This was on an A7iii, not a dedicated astro cam, so I don't think an electronic auto focuser would be an option. the standard helical focuser did not slip (it was taped in place)

 

-Ted


Edited by mtn_nut, 02 March 2021 - 01:22 PM.


#2 Pokemoncrusher1

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Posted 02 March 2021 - 11:48 AM

your focuser could not be adjusted correctly and could be slipping due to the weight


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#3 AtlantaAstro

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Posted 02 March 2021 - 11:57 AM

You can’t focus past infinity. It’s infinity. Where Infinity comes to focus changes based on focal length. This is why some camera lenses you can focus “past infinity” because as you zoom in or out, or change focal length, focus at infinity changes. But you literally cannot focus past infinity. But I’m a n00b so maybe don’t listen to me haha

 

Most likely what happened was that the temp change may have cause condensation on your objective lens to give it the impression it was out of focus. How long did you acclimate your scope before imaging?

 

Or, what Pokemoncrusher1 stated, your focus draw tube may have slipped slightly because of weight. What focuser do you use? How heavy is everything attached to the focuser tube?



#4 AstroVagabond

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Posted 02 March 2021 - 11:57 AM

I'm a beginner and I just added a ZWO electronic autofocuser. I did so because it is my understanding that along the course of the evening temperature changes could cause contraction of elements within the imaging train creating a change in focus.

 

I don't really know one way or the other at this point. I thought the ZWO EAF was an inexpensive way to remove the possibility from the equation by allowing me to set parameters based upon temperature changes that would cause a refocus event. Also using a monochrome camera and filters an autofocuser made sense.


Edited by AstroVagabond, 02 March 2021 - 12:16 PM.


#5 wrnchhead

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Posted 02 March 2021 - 12:33 PM

Focus changing due to temperature is part of the process. You either get autofocusing (I personally re-focus with every 2 degrees change in C, and/or every 2 hours) or have to babysit and refocus yourself. 



#6 charlieb123

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Posted 02 March 2021 - 12:38 PM

You can skip the temp sensor and have a program like APT check focus every 10th or 20th exposure and adjust accordingly before continuing on with imaging.



#7 mtn_nut

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Posted 02 March 2021 - 01:28 PM

Focus changing due to temperature is part of the process. You either get autofocusing (I personally re-focus with every 2 degrees change in C, and/or every 2 hours) or have to babysit and refocus yourself. 

Do you think using a dew heater would help keep the focus stable?

 

I'm a beginner and I just added a ZWO electronic autofocuser. I did so because it is my understanding that along the course of the evening temperature changes could cause contraction of elements within the imaging train creating a change in focus.

 

I don't really know one way or the other at this point. I thought the ZWO EAF was an inexpensive way to remove the possibility from the equation by allowing me to set parameters based upon temperature changes that would cause a refocus event. Also using a monochrome camera and filters an autofocuser made sense.

I'm not using a dedicated astro camera, so I don't think an EAF is an option

 

You can’t focus past infinity. It’s infinity. Where Infinity comes to focus changes based on focal length. This is why some camera lenses you can focus “past infinity” because as you zoom in or out, or change focal length, focus at infinity changes. But you literally cannot focus past infinity. But I’m a n00b so maybe don’t listen to me haha

 

Most likely what happened was that the temp change may have cause condensation on your objective lens to give it the impression it was out of focus. How long did you acclimate your scope before imaging?

 

Or, what Pokemoncrusher1 stated, your focus draw tube may have slipped slightly because of weight. What focuser do you use? How heavy is everything attached to the focuser tube?

You can focus past infinity. Focus has to do with the backfocus distance (from the optics to the sensor) so macro rings elongate this distance, and to focus past infinity, I would have to shorten the backfocus, which I don't think is possible with my camera/lens combo, but i figured i'd see if anyone has an idea.

 

your focuser could not be adjusted correctly and could be slipping due to the weight

It was taped in place, and its an internal focusing system, so it didn't slip

 

You can skip the temp sensor and have a program like APT check focus every 10th or 20th exposure and adjust accordingly before continuing on with imaging.

Can you explain. I have no idea what you are talking about with APT.



#8 wrnchhead

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Posted 02 March 2021 - 01:39 PM

The focus shift is affected by lens cooling for sure, but I think it's a greater effect on the tube itself. It is effectively shortening the tube, bringing you out of focus. 

 

A smarter guy than me, James Lamb has an interesting video on this, and more of his videos are really good if you are inclined. 

https://www.youtube....h?v=9tMaeX_8kR8



#9 schmeah

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Posted 02 March 2021 - 01:48 PM

A dew heater will prevent dew from forming, but will have no beneficial effect with respect to maintaining focus.

 

Derek



#10 MikiSJ

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Posted 02 March 2021 - 02:10 PM

I have a C11EdgeHD and a Celestron Focuser. Over the past weekend when I started to image SGPro gave me a focus solution of "X". After imaging my first target I ran another SGPro focus solution and the focus changed about 35 points. Another subsequent focus solution saw movement around 25 points.

 

The focus point should not change with the C11EdgeHD due to a mechanical issue (slippage) but it will change due to temperature fluctuations during an imaging session. In my case the change in focus was a result of a 0.1% change in focal length

 

SGPro does not always provide a focus solution and sometimes needs to be re-run several times for it to come up with what SGPro thinks is focus. Because of the hit or miss answer for focusing with SGPro, I do not let the program perform automated re-focusing routines. SGPro is also very slow when it comes to getting a focus solution.

 

I have recently acquired TheSKYX Imaging Edition and will be trying TheSKYX's focus routines to see how fast it can come to a solution.



#11 Ken Sturrock

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Posted 02 March 2021 - 02:59 PM

Yeah. It'll depend on the nature of the instrument.
 

The focus shift is affected by lens cooling for sure, but I think it's a greater effect on the tube itself. It is effectively shortening the tube, bringing you out of focus.

 
More precisely, the lens cools and contracts, but that changes the shape of it which causes the shift in focus. For example, you'll (probably) notice that during the night, the focus on a typical refractor shifts INWARD. If, on the other hand, the focus shift was caused more by the tube length contraction then you'd expect the focus point to shift OUTWARD.
 
A number of designers have recommended aluminum tubes over CF tubes because the shortening of the aluminum tube can mitigate the inward shifting of the optical focus point due to lens shape changes. Others have found this not to be the case.

 

Regardless, focus shift definitely happens and it's related to a mix of lens material and shape as well as OTA materials.


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#12 freestar8n

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Posted 02 March 2021 - 06:37 PM

You can’t focus past infinity. It’s infinity. Where Infinity comes to focus changes based on focal length. This is why some camera lenses you can focus “past infinity” because as you zoom in or out, or change focal length, focus at infinity changes. But you literally cannot focus past infinity. But I’m a n00b so maybe don’t listen to me haha

 

Most likely what happened was that the temp change may have cause condensation on your objective lens to give it the impression it was out of focus. How long did you acclimate your scope before imaging?

 

Or, what Pokemoncrusher1 stated, your focus draw tube may have slipped slightly because of weight. What focuser do you use? How heavy is everything attached to the focuser tube?

You can indeed focus "beyond infinity."  A nearby object has rays arriving at the lens that appear to be diverging.  An object at infinity has rays coming in parallel.  An object beyond infinity has rays coming in slightly converging.  The lens itself just needs to put the sensor a bit inside the focal distance and it will be focusing "beyond infinity."  Normally lenses allow going past infinity just to be sure there is plenty of room to find exact focus - particularly with autofocus that needs to sample on both sides of focus.

 

As for the focus shift happening in this thread - if it appears slow and gradual it is probably thermal, but if there are any sudden jumps it is probably mechanical.  Even with a dslr lens it may help to remove backlash in focus by pushing up against gravity, like with an sct mirror.  Finishing focus with small clockwise or counter-clockwise motion may end up holding focus better over time.

 

Frank




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