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Main objective cleaning

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#1 senzala

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Posted 15 September 2020 - 10:42 AM

I have a skywatcher Evostar 120mm refractor.
After using it the other night I noticed some small stains on the backside of the main objective.

Is it possible to get into the backside of the lens?

I dont know if its possible to remove the objective "house" from the telescope tube.

Thanks you.

/Ricky

Attached Thumbnails

  • 20200915_174444.jpg

Edited by senzala, 15 September 2020 - 10:45 AM.


#2 ButterFly

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Posted 15 September 2020 - 10:50 AM

"Some small stains" are nothing to worry about.  The cell can't be collimated on these, so it is better to leave well enough alone.  Just cap the end of the focuser tube at the end of the night to prevent more staining (and growing of existing stains).  If it goes from a cold house to warm outside, give it time to warm up.  Back of the objective staining usually forms from the stuff left behind in condensed dew as it evaporates.

 

If it gets to "several large stains", it is time to proceed with great caution.  Just avoid that.



#3 senzala

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Posted 15 September 2020 - 12:31 PM

"Some small stains" are nothing to worry about.  The cell can't be collimated on these, so it is better to leave well enough alone.  Just cap the end of the focuser tube at the end of the night to prevent more staining (and growing of existing stains).  If it goes from a cold house to warm outside, give it time to warm up.  Back of the objective staining usually forms from the stuff left behind in condensed dew as it evaporates.

 

If it gets to "several large stains", it is time to proceed with great caution.  Just avoid that.

Thank you.

 

Last time i was out at night, there was a lot af moist in the air. The telescope was wet from dew. 

 

I keep the telescope in a small house with no heat. 

I usually keep the dust cap of until the dew has evaporated. 

 

I just wondered if stain would effect the image (use ZWO camera) or the ability to focus properly.



#4 ButterFly

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Posted 15 September 2020 - 01:03 PM

Take flat frames for imaging.  Stains are out of focus, but they affect how much light gets to different parts of the image.  Flat frames will take care of that.  Goolge that to see what they are and how to do it.  Small stains are very hard to notice visually.

 

I like mine for imaging and visual.  But the lack of a collimatable cell gets me worried sometimes.  Everything gets fixed at the focuser, so I'd rather not touch it (or drop it).

 

Generally speaking, trapped hot air has a lot more water in it.  When that hot trapped air cools, the water condenses out.  If hotter moist air hits a cooler surface, it will also condense out.  Just keep that in mind to prevent dew.  Preventing dewing is beneficial for lots of reasons, so a dew heater strip is a good idea if it's a problem in your area.  The objective is big and has lots of area to radiate off its heat to reach the dew point.



#5 gnowellsct

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Posted 15 September 2020 - 02:18 PM

Sooner or later these optics do need to be cleaned....



#6 senzala

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Posted 15 September 2020 - 02:25 PM

Sooner or later these optics do need to be cleaned....

Thats my thoughts. Just wondering how to reach it



#7 senzala

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Posted 15 September 2020 - 02:31 PM

Take flat frames for imaging.  Stains are out of focus, but they affect how much light gets to different parts of the image.  Flat frames will take care of that.  Goolge that to see what they are and how to do it.  Small stains are very hard to notice visually.

 

I like mine for imaging and visual.  But the lack of a collimatable cell gets me worried sometimes.  Everything gets fixed at the focuser, so I'd rather not touch it (or drop it).

 

Generally speaking, trapped hot air has a lot more water in it.  When that hot trapped air cools, the water condenses out.  If hotter moist air hits a cooler surface, it will also condense out.  Just keep that in mind to prevent dew.  Preventing dewing is beneficial for lots of reasons, so a dew heater strip is a good idea if it's a problem in your area.  The objective is big and has lots of area to radiate off its heat to reach the dew point.

 

Thank you. 

Any idea how to clean the backside if the stains get to many?



#8 gnowellsct

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Posted 15 September 2020 - 03:00 PM

It's a popular scope.  Someone out there has done it.  

 

On the ones I have done....they just disassemble if the dew shield is not sliding.  Unscrews off the tube.

 

On the the dew shield slides, the usual routine is a front outermost ring  which actually serves to provide a stop to the dew shield.  

 

Unscrew that ring and the dew shield slides back farther, allowing you to grip the lens cell to remove it.  The whole lens cell (all three lenses) unscrews.

 

When you are cleaning you can dampen your kim wipes (or unscented kleenex if you're following Televue instructions) with lens cleaner and use that to clean the back.  You don't want to super saturate the lens cell in such a way that liquid gets in past the lenses into the cell.  That is a very big negative event.  It is the main hazard of which to be wary.  If you want the lens cell back in exactly where it was before you took it out, put a small piece of painter's tape (will come off easily) joining lens cell to tube.  Then sever it.  When you are screwing it back on stop when the two halves are lined up again.

 

Greg N



#9 SteveG

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Posted 15 September 2020 - 05:06 PM

I have a skywatcher Evostar 120mm refractor.
After using it the other night I noticed some small stains on the backside of the main objective.

Is it possible to get into the backside of the lens?

I dont know if its possible to remove the objective "house" from the telescope tube.

Thanks you.

/Ricky

The main objective lens cell simply screws off the tube. You don't even have to unscrew the dew shield, but I recommend doing so first. Be very careful putting the cell back on, because you can cross-thread it and still screw it all the way down, resulting in misscollimated optics. Ask me how I know this...LOL.


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#10 k5apl

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Posted 15 September 2020 - 07:57 PM

I have cleaned a couple of objectives' backsides without taking them out of the tube.  I used a long dowel rod of wood and taped pure natural cotton ball to it.  I used a spritz of cleaner on the cotton ball, worked it down through the baffles, and gently and lightly wiped the back of the objective.  It worked for me.  I was especially careful to not apply a lot of pressure and not soak the cotton balls to prevent streaking..  

I also have taken cells off the tube, cleaned the lenses, and screwed the cells back on the tube.  Proceed with moderation and caution.  Mark cell orientation if possible.  The collimation held on my previous attempts.

Wes



#11 gnowellsct

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Posted 15 September 2020 - 08:31 PM

Warning to Tak owners: there are very tiny grub screws that screw into the threads on the tube to keep the lens cell from unraveling, I suppose.   You need a very small screwdriver to loosen these up or you'll shred the threads on the tube.

 

This is for the FS128 and probably all the FS series.  

 

I would like to think that Tak has stopped doing this ghastly practice.  

 

But I've never heard someone say "I know exactly what you're talking about and they don't do it any more."

 

Greg N


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#12 senzala

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Posted 16 September 2020 - 01:51 AM

It's a popular scope.  Someone out there has done it.  

 

On the ones I have done....they just disassemble if the dew shield is not sliding.  Unscrews off the tube.

 

On the the dew shield slides, the usual routine is a front outermost ring  which actually serves to provide a stop to the dew shield.  

 

Unscrew that ring and the dew shield slides back farther, allowing you to grip the lens cell to remove it.  The whole lens cell (all three lenses) unscrews.

 

When you are cleaning you can dampen your kim wipes (or unscented kleenex if you're following Televue instructions) with lens cleaner and use that to clean the back.  You don't want to super saturate the lens cell in such a way that liquid gets in past the lenses into the cell.  That is a very big negative event.  It is the main hazard of which to be wary.  If you want the lens cell back in exactly where it was before you took it out, put a small piece of painter's tape (will come off easily) joining lens cell to tube.  Then sever it.  When you are screwing it back on stop when the two halves are lined up again.

 

Greg N

Thank you. 

 

This is my first telescope, so I'm a bit concerned about damaging the telescope. 

 

So I remove the outer ring(see attached picture)

and the house with lenses can be removed?

Attached Thumbnails

  • 20200916_085113.jpg

Edited by senzala, 16 September 2020 - 05:05 AM.


#13 senzala

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Posted 16 September 2020 - 04:17 AM

The main objective lens cell simply screws off the tube. You don't even have to unscrew the dew shield, but I recommend doing so first. Be very careful putting the cell back on, because you can cross-thread it and still screw it all the way down, resulting in misscollimated optics. Ask me how I know this...LOL.

What do you mean by "you can cross-thread it"?

 

As I understand, you cant collimate a refractor, right?

 

Thanks

 

/Ricky



#14 Rutilus

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Posted 16 September 2020 - 06:34 AM

Thank you. 

 

This is my first telescope, so I'm a bit concerned about damaging the telescope. 

 

So I remove the outer ring(see attached picture)

and the house with lenses can be removed?

Only remove the lens retaining ring (shown in your photo) if you intend to the take the lenses out of the cell.

However it is best to remove the lens cell from the main telescope  by un-screwing the cell as mentioned in other posts.

Once the lens cell is removed you can clean the back of the lens without taking the lenses out of the cell.


Edited by Rutilus, 16 September 2020 - 06:34 AM.


#15 Rutilus

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Posted 16 September 2020 - 06:48 AM

Photo of lens cell un-screwed from main tube, as you can see you can now get to the back of the lens for cleaning.

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  • lens-back-cn.jpg

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#16 Rutilus

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Posted 16 September 2020 - 07:01 AM

Removing the lenses from a cell is a bit more complex with the added danger of chipping one the lenses

if the lens jams when trying to remove. Here is a photo of an i-Star lens I removed from cell and later 

re-fitted into a sky-watcher lens cell.

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  • Istar-remove-cn.jpg


#17 senzala

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Posted 16 September 2020 - 07:25 AM

Only remove the lens retaining ring (shown in your photo) if you intend to the take the lenses out of the cell.

However it is best to remove the lens cell from the main telescope  by un-screwing the cell as mentioned in other posts.

Once the lens cell is removed you can clean the back of the lens without taking the lenses out of the cell.

 

Oh, I see. The front ring (Yellow arrows) holds the lenses in the cell 'house'?

So I can unscrew the cell 'house' without removing the outer ring?

Do it usually come of using hands only or do you use some tool?

 

Thank you. Really appreciate it! 

 

/Ricky 

Attached Thumbnails

  • 20200916_141507.jpg
  • 20200916_142335.jpg


#18 Rutilus

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Posted 16 September 2020 - 07:30 AM

Yes. The cell might at first seem a bit tight if it has not been removed before.

I find it helps to put the tube on top of a bed, then you have a soft landing if the cell 

suddenly pops off out of your hand.

If the cell is stuck, then it helps if you can find another person, one to hold the cell and twist while the 

other holds the telescope tube.

 

Or another method is to hold the tube between your legs and twist the cell to break any paint seal.


Edited by Rutilus, 16 September 2020 - 07:32 AM.

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#19 senzala

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Posted 16 September 2020 - 07:42 AM

Yes. The cell might at first seem a bit tight if it has not been removed before.

I find it helps to put the tube on top of a bed, then you have a soft landing if the cell 

suddenly pops off out of your hand.

If the cell is stuck, then it helps if you can find another person, one to hold the cell and twist while the 

other holds the telescope tube.

 

Or another method is to hold the tube between your legs and twist the cell to break any paint seal.

 

Will give it a try tomorrow:-) 

 

If I remove the lenses from the cell, is there anything special to consider with reattached them back into the cell? I assume there are pretty much in a fixed position, so adjustment isn't necessary afterwards. 

 

Thank you. 🙏🙏

 

/Ricky 


Edited by senzala, 16 September 2020 - 07:50 AM.


#20 doctordub

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Posted 16 September 2020 - 07:58 AM

If it is too tight, (the lens cell) a large rubber "strap wrench" can loosen it.

CS

Jonathan


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#21 Rutilus

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Posted 16 September 2020 - 08:55 AM

Will give it a try tomorrow:-) 

 

If I remove the lenses from the cell, is there anything special to consider with reattached them back into the cell? I assume there are pretty much in a fixed position, so adjustment isn't necessary afterwards. 

 

Thank you. 🙏🙏

 

/Ricky 

Usually, the Evostar 120 lens does not have any markings on them for alignment. So you might be best to mark 

the edge of the lens with something like a pencil. You will find that there is a spacer ring in between the lenses.

Here is a photo of the markings on my 6 inch version of the scope and the spacer ring from my Evostar 120mm.

Just be very careful if removing the lenses, I have found the Sky-watcher cells to be a bit tight and it can be easy to

jam the lenses against the side of the cell and risk chipping one of the lenses. 

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  • 150-CNb.JPG
  • mico-cn.jpg


#22 senzala

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Posted 16 September 2020 - 10:48 AM

Usually, the Evostar 120 lens does not have any markings on them for alignment. So you might be best to mark 

the edge of the lens with something like a pencil. You will find that there is a spacer ring in between the lenses.

Here is a photo of the markings on my 6 inch version of the scope and the spacer ring from my Evostar 120mm.

Just be very careful if removing the lenses, I have found the Sky-watcher cells to be a bit tight and it can be easy to

jam the lenses against the side of the cell and risk chipping one of the lenses. 

 

From your picture it seems like the main objective is two lenses?



#23 Rutilus

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Posted 16 September 2020 - 03:55 PM

Yes it has two lenses. This is way it is best to mark them with a pencil, so that they go back together in the correct way.



#24 senzala

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Posted 17 September 2020 - 07:35 AM

Managed to remove cell from tube. 

 

How do I loosen the outer ring? 

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  • 20200917_143517.jpg


#25 doctordub

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Posted 18 September 2020 - 11:37 AM

I would try cleaning outer surfaces of lenses before taking lenses out of the cell!

CS

Jonathan


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