Newtonian stars get worse during the night - Forums

Topic starter

Dec 11, 2022 · Edited: Dec 11, 2022

Hello my friends! I'm using a skywatcher 10'' F/4 newt.
I use OCAL to do the collimation.
Here's my image took in October 29th.
At 0:33 it looks like this

294mm,BIN1,5min single frame )，which seems acceptable .

However after a few moments,it becomes like this,and getting worse and worse
frame in 01:13

frame in 01:29

frame in 01:45

frame in 02:00

frame in 02:50

The stars are getting terrible.

After a month,November 25th I clean up the primary mirror,secondary mirror, redo the collimation. I shoot M81 for 2 hours, the stars are good all the time
(294mm 2min single frame)

But after a few days, I meet the same problem like in October

almost round stars at 4:04

04:24

04:44

05:00

stars with astigmatism

I think it's thermal problem but not for sure. There's a fan in the ass of the newt.Should I turn it on when imaging?

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andrea tasselli

Frequent contributor

Dec 11, 2022

Your primary cell is too tight around the mirror is the most likely explanation for your issue. I cannot advise on the solution as I have no idea how your cell is built.

六年之后

Topic starter

Dec 11, 2022

andrea tasselli:
Your primary cell is too tight around the mirror is the most likely explanation for your issue. I cannot advise on the solution as I have no idea how your cell is built.

thank you my friend, that's one reason I'm considering.
I recheck my frames and find that the stars are getting worse very quickly. I add the time in every frames

Tim Hawkes

Dec 11, 2022

Hello,

Just wondering about the thermal equilibration of the telescope and whether the conditions were different when you got the nice M81 image?

Also whether the attitude of the telescope is altering collimation? So - for example - if you use a laser to line up the focuser and secondary mirror onto the centre spot of the primary mirror with the telescope on the horizontal it can be that when you turn the telescope through 180 degress you will find that it is no longer aligned. This can be - for example - because of the primary mirror sliding slightly because the edge supports in the mirror cell are not tight enough (although overtightening causes trefoil mishapen stars?) - or because the secondary mirror is moving and the spider needs tightening up. Big F4 Newts are just so sensitive to small shifts. - or tube/ focuser flex etc.

However if such movements are the cause of the problems it is rather strange because they would have to be occuring rather slowly?!

Tim

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atlejq

Dec 11, 2022 · Edited: Dec 11, 2022

As Andrea points out you have pinched optics. This results in stars with a sharp, usually somewhat triangular look.

I had the same problem on my Skywatcher 200 PDS f/5. I sold that one, but you may try two things:

1) Heat the primary with a dew heater. This helps the problem on my current setup, probably because the aluminium does not contract as much.
2) If heating does not solve the problem, try to loosen the primary mirror holders SLIGTHLY before the scope cools down. Or use extremely thin spacers of some type.

The mirror cell is probably rather similar to this one:

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p10710_Skywatcher-adjustable-primary-mirror-cell-for-8--Newtonian-mirror.html

You see the 3 primary mirror holders in the middle picture. I guess these contract with low temperature so you get a triangular pinch. Hence, if they are slightly loose before the scope has cooled, they will contract to the desired shape.

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六年之后

Topic starter

Dec 12, 2022

Thank you all guys，I really appreciate your help
I'm trying to do the follow things:
1、Completely finish the thermal equilibration before imaging
2、Putting a card between the primary mirror holders and mirror,then remove the card when they are tighten
3、Do the collimation when the telescope is vertical ( I did this before imaging M81)
4、Close the primary heater or set the temperature as low as possible (maybe just a little higher than the environment temperature?)

I‘m hoping for the next imaging to see if the problem is solved

Tim Hawkes

Dec 12, 2022

) Heat the primary with a dew heater. This helps the problem on my current setup, probably because the aluminium does not contract as much

That is an interesting comment. I'd not heard of a dew heater for the primary mirror on a Newt - only the secondary. Even with a fan running doesn't this just result in tube currents and thermal gradients - or at least heating the edge of the primary more likedly a cause of problems than a cure ?!

andrea tasselli

Frequent contributor

Dec 12, 2022

Tim Hawkes:
) Heat the primary with a dew heater. This helps the problem on my current setup, probably because the aluminium does not contract as much

That is an interesting comment. I'd not heard of a dew heater for the primary mirror on a Newt - only the secondary. Even with a fan running doesn't this just result in tube currents and thermal gradients - or at least heating the edge of the primary more likedly a cause of problems than a cure ?!

There are but might not fit all primary cells out there, beside needing a controller with ambient sensor to adjust the heat input just a couple of degrees above the dew point (which means a feedback mechanism, i.e., temperature probes on the mirror).

Having said that, aluminium (alloys) do contract much more than any glass substrate (think plate glass/BK7/SiO2) for any drop in temperature, that's why you get pinched optics in many telescopes.

Earle Waghorne

Dec 12, 2022 · Edited: Dec 12, 2022

Tim Hawkes:
) Heat the primary with a dew heater. This helps the problem on my current setup, probably because the aluminium does not contract as much

That is an interesting comment. I'd not heard of a dew heater for the primary mirror on a Newt - only the secondary. Even with a fan running doesn't this just result in tube currents and thermal gradients - or at least heating the edge of the primary more likedly a cause of problems than a cure ?!

Hi,

I get dew formation on my Newt mirror in winter (Ireland humidity is typically >80%) and installed a Kendrick dew heater on the primary and it doesn't cause problems with thermals but I do turn early to let it get equilibrated. If you don't get dew formation it's probably not worth doing unless it gets very cold where you are. On my RC I use a long strip dew heater around the outside of the rear of the scope (effectively around the outside of the mirror cell) and this also works.
CS

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atlejq

Dec 12, 2022

My arrangement is a strip heater on the outside.

I have also worried a bit about thermal currents, but at least for deepsky imaging this problem seems to be far less pronounced than potential pinching.

Tim Hawkes

Dec 12, 2022

Earle Waghorne:
Tim Hawkes:
) Heat the primary with a dew heater. This helps the problem on my current setup, probably because the aluminium does not contract as much

That is an interesting comment. I'd not heard of a dew heater for the primary mirror on a Newt - only the secondary. Even with a fan running doesn't this just result in tube currents and thermal gradients - or at least heating the edge of the primary more likedly a cause of problems than a cure ?!

Hi,

I get dew formation on my Newt mirror in winter (Ireland humidity is typically >80%) and installed a Kendrick dew heater on the primary and it doesn't cause problems with thermals but I do turn early to let it get equilibrated. If you don't get dew formation it's probably not worth doing unless it gets very cold where you are. On my RC I use a long strip dew heater around the outside of the rear of the scope (effectively around the outside of the mirror cell) and this also works.

Thanks Earle, I was curious because I get the same problem in the Thames valley - my approach when it is down near the dewpoint has just been to occasionally get a hair dryer to the secondary but tbh I just give up once I can see that the primary has also misted over. I have seen the Kendrick but wasn't sure that I trusted the concept -- but yes I can see how it could work if switching that and the fan on is the first part of your set up and then tweak collimation once it has all settled down . Tim

atlejq

Dec 12, 2022 · Edited: Dec 12, 2022

My location (Eastern Norway next to a big lake) often has humidity up to 80-90%. Interestingly, my primary gets dew far more often than the secondary. So I often need to use this external heater strip anyway. From defocusing and looking at the diffraction rings around a star it does not seem like the tube currents are too bad once the scope has thermalized.

EDIT: Typo.

六年之后

Topic starter

Dec 19, 2022

hello my friends！I'm back again,and I'm so glad to say I've solved this problem.
The problem is thermal equilibration,caused by the primary mirror heater.
The heater makes the primary a bit hot，causes themal boundary layer near the mirror.
Now I close it when I'm imaging,and let the fan run for a whole night.
Yesterday I got a nice M81 picture, no terrible stars.
Thank all my friends for yours reply

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Alan Brunelle

Dec 19, 2022 · Edited: Dec 19, 2022

六年之后:
hello my friends！I'm back again,and I'm so glad to say I've solved this problem.
The problem is thermal equilibration,caused by the primary mirror heater.
The heater makes the primary a bit hot，causes themal boundary layer near the mirror.
Now I close it when I'm imaging,and let the fan run for a whole night.
Yesterday I got a nice M81 picture, no terrible stars.
Thank all my friends for yours reply

Thanks for posting your problem and the solutions. I just ran into the same problem with my rig for the first time, with stars that looked a lot like yours. It ruined a good night of imaging in what is a very rare night of clear skies all winter for my location. I also heat my primary. But very little. However, the night was colder by a few degrees than I had ever tried this new scope and no doubt heat or cold was the problem.

I will add that I have definitive effects optically from the heating of my primary with my home made heaters. My heaters are arranged in a triagonal pattern and I can see that pattern show up on defocused star images. (Heck, even the Webb images show the mirror pattern in bright star images), so image is very sensitive to this sort of thing. So I heat very lightly. I will lightly take to heating just the secondary or maybe not at all until the heavy dew sets in. The whole inside of my carbon tube was frosted lightly, but none of my optics. Well, I did have dew on the center of my coma corrector! Maybe another heater there is warrented?

With all the time I will have with winter clouds, I will likely also tweak the tightness of my primary cell just a tad. Seeing the effects I got with just the slightest of heating tells me that pinched optics can occur with very little extra tension on the primary mirror clamps. I now understand why many Dobsonians use sling-style mirror cells.

Great picture!

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