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What could be causing these star shapes?


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Hi guys,

 

I'm struggling with deformed stars with my EdgeHD 9.25 SCT. It's far more pronounced on the right half of the frame. They look like this.

image.png.36731dcd191ba877d4fce63dbb5540f7.png

 

The left side looks like this.

image.png.b04648eec83095fd140e75f44d28d37c.png

 

Here's an aberration inspector image.

image.png.a8862a69278d86b870ff1993d1f52927.png

 

SCT imaging is fairly new to me, so I'm not experienced with the quirks that I need to deal with. I saw the same shapes in every sub so it wasn't a one-off guiding issue. Guiding was between 0.40 and 0.50 throughout the session.

Collimation? Camera tilt? Backfocus? Something else? What do you guys think?

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One explanation I’ve read is thermal tube currents. I have it occasionally with my 190MN, which is also a closed tube reflector.

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4 hours ago, wimvb said:

One explanation I’ve read is thermal tube currents. I have it occasionally with my 190MN, which is also a closed tube reflector.

Do you think it might be consistent enough to exhibit exactly the same patterns over multiple nights? I'm imaging as we speak and I'm seeing identical deformations.

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4 hours ago, Elp said:

How is the camera connected to the scope?

I'm using a .7 focal reducer which is connected to a Celestron OAG, then a couple of spacers going in to a ZWO 2" filter wheel which is connected directly to my ASI 2600MM.

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2 hours ago, Peter Drew said:

Could be a temperature plume though this would depend on the camera orientation.   🙂

I hope it's not this, because although I'm not familiar with temperature plumes, it sounds like it might be something which is hard to mitigate! 😆

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7 hours ago, Peter Drew said:

Could be a temperature plume though this would depend on the camera orientation.   🙂

What is a temperature plume, if I may ask?

5 hours ago, Mark_D said:

Do you think it might be consistent enough to exhibit exactly the same patterns over multiple nights? I'm imaging as we speak and I'm seeing identical deformations.

It was in my images. Also, I always have the same camera orientation.

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I get these flares occasionally on the edge of my RASA 11 images. I did read that heat could cause the problem, and that if your dew control was too high you could cause it. Not sure whether that's right or not - just passing on what others say. But I have dialled back my dew control aggressiveness a bit, and so far so good.

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4 hours ago, wimvb said:

What is a temperature plume, if I may ask?

It was in my images. Also, I always have the same camera orientation.

During the period when a SCT is reaching thermal equilibrium warm air rises from the tube system supporting the sliding mirror facility.  This can manifest itself by distorting a star image similar to that of your image.  It shouldn't persist eventually once equilibrium is attained.   🙂

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9 hours ago, Mark_D said:

.7 focal reducer

Just for process of elimination I assume it's the 0.7x reducer built for the 9.25 EHD?

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Meant to add...  not totally clear from your aberration inspector image, but looks a bit like the stars are pointing in towards the centre, rather than all in one direction. This can be a sign that you need to increase your backfocus distance a bit. Possibly a bit of tilt too.

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1 hour ago, Peter Drew said:

During the period when a SCT is reaching thermal equilibrium warm air rises from the tube system supporting the sliding mirror facility.  This can manifest itself by distorting a star image similar to that of your image.  It shouldn't persist eventually once equilibrium is attained.   🙂

 

4 hours ago, Fegato said:

I get these flares occasionally on the edge of my RASA 11 images. I did read that heat could cause the problem, and that if your dew control was too high you could cause it. Not sure whether that's right or not - just passing on what others say. But I have dialled back my dew control aggressiveness a bit, and so far so good.

Thanks, both. That explains why the problem disappeared once I installed a Pegasus power box with automatic dew control.

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On 08/07/2024 at 10:15, Peter Drew said:

During the period when a SCT is reaching thermal equilibrium warm air rises from the tube system supporting the sliding mirror facility.  This can manifest itself by distorting a star image similar to that of your image.  It shouldn't persist eventually once equilibrium is attained.   🙂

Thanks for the clarification Peter. The scope was definitely at equilibrium so I think we can rule that one out 🙂

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On 08/07/2024 at 11:57, Fegato said:

Meant to add...  not totally clear from your aberration inspector image, but looks a bit like the stars are pointing in towards the centre, rather than all in one direction. This can be a sign that you need to increase your backfocus distance a bit. Possibly a bit of tilt too.

I had thought that backfocus was correct, but this is definitely a motivator to recheck it!

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1 hour ago, Mark_D said:

Thanks for the clarification Peter. The scope was definitely at equilibrium so I think we can rule that one out 🙂

The scope itself may be at "equilibrium" (though if there are heat currents, it isn't really), but if a dew strip warms the front element, warm air will rise and affect the stars' airy disc. This is what I experienced. My 190MN has a massive piece of glass as a correcting element. This meniscus attracts dew very easily. I use both a dew strip and a dew shield. Before I installed a proper controller for the dew strip, I had it at full power. This caused the very same star profile that I also see in your image, also on one side of the image. Last year I installed a Pegasus power box with dew strip controller. I also recollimated the scope (although it was already decently collimated), and got much better stars. At that time I researched the problem extensively, and the only explanation that fit and made sense was heat currents, but I thought what was meant was currents within the tube. Since the problem persisted long after the scope had cooled down, I wasn't really convinced by that explanation. Now I understand that in reality, the combination of dew strip and dew shield acted as a stove and chimney. A warm air current above the scope, rather than inside.

In this image, you can see the heat plumes on the left hand side. (This is a processed image where I had tried to fix the worst stars.)

https://www.astrobin.com/full/97pq9e/0/?mod=&real=

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Thanks so much for all of your replies here guys! I really appreciate the help.

It looks like there may be a couple of hours of vaguely clear skies where I am tonight, so I'm going to test the Celestron dew ring theory and leave it off. I'll report back once I have the results.

Edited by Mark_D
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Ahh also I forgot to ask. On the topic of the Celestron dew heater being a little aggressive with its heating unless controlled properly. Can anybody with a 9.25 SCT who's using a dew heater band share their experiences with one? Have you found that it's powerful enough to keep dew off the element in cold weather? I'm specifically thinking about UK winters and other cold/colder climates.

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