C14 Recommendations

[matt_baker]

I have access to a C14 in an observatory (not mine). I've recently been attempting to use it again, particularly to collimate, but the image I'm getting is that awful. The stars are dancing and flailing around so much even basically overhead that I can't really make out the concentric rings to get a proper gauge what the collimation is actually like. I tried using metaguide and using seeing view but it's still awful. I did forget to take a sample video of what the star looked like but I do have a couple of examples with Jupiter and the Moon

I've heard things like wrapping it in Reflectix to slow down the cooling to prevent heat plumes which is something I might consider. Also the idea of using a Lymax cooler but I do wonder how much better that would be considering the cost.

Has anyone got any recommendations or tips to help?

Here's some footage of what Jupiter and the Moon looked like, although taken at 27 degrees.

Matt

 

[CraigT82]

If the image is dancing around then you’ve certainly got an issue which could be atmospheric in nature or something to do with the tube currents in the scope. 

Looking at a an out of focus star can help with diagnosing tube currents, so if that’s ruled out then it’s probably awful seeing. 

If it is tube currents then yes insulating the scope may help, could think about how the scope is stored and sited too, but if it’s permanently mounted in a dome then not much you can do about that.

 

 

[Cosmic Geoff]

With a C8, I found that a star looks like a badminton shuttle if the collimation is really bad.

[matt_baker]

Forgot to mention also that since it's located on a school site, there's quite a few buildings within where I'd be pointing the scope to the south so I can only assume the heat coming from them is only going to hinder the view further. Although again saying that, when I pointed the scope almost overhead, it was still very bad. Next time it's clear and I'm there, I'll take a quick video on an out of focus star.

[riklaunim]

5 hours ago, matt_baker said:

I've heard things like wrapping it in Reflectix to slow down the cooling to prevent heat plumes which is something I might consider. Also the idea of using a Lymax cooler but I do wonder how much better that would be considering the cost.

Has anyone got any recommendations or tips to help?

Depending how fast the temperature is rising/falling C14 may not be able to follow it - thus never reaching ambient and always having air current/boundary layer around the primary mirror.

With all SCTs I had, and especially C14 I used an active fan to push air through the whole OTA so when I started using it it was forced at ambient - but if the temperature change was to steep then around 40-60 minutes and it would start to drag again.

As for collimation you can start with looking visually to see if the error is large or what's the thermal state of the telescope.

[Froeng]

On 19/07/2022 at 17:36, matt_baker said:

I have access to a C14 in an observatory (not mine). I've recently been attempting to use it again, particularly to collimate, but the image I'm getting is that awful. The stars are dancing and flailing around so much even basically overhead that I can't really make out the concentric rings to get a proper gauge what the collimation is actually like. I tried using metaguide and using seeing view but it's still awful. I did forget to take a sample video of what the star looked like but I do have a couple of examples with Jupiter and the Moon

I've heard things like wrapping it in Reflectix to slow down the cooling to prevent heat plumes which is something I might consider. Also the idea of using a Lymax cooler but I do wonder how much better that would be considering the cost.

Has anyone got any recommendations or tips to help?

Here's some footage of what Jupiter and the Moon looked like, although taken at 27 degrees.

Matt

 

Looking at the video of Jupiter you posted, the turbulence is directional, which in my experience indicates it is related to atmospheric seeing. This could be as far away as the Jetstream or it could even be quite close by, ie nearby rooftops. Is the observatory on a flat roof or is it standing on the ground surrounded by buildings? Also, is it a dome with a slit, or does the roof roll off completely?

Tube currents are typically not as directional, and more "swirly" in a SCT, but then all of this is dependent on so many factors, it all depends. Does the telescope cool down after opening the observatory, and if yes, for how long? Or is it air-conditioned to match the outside temperature? Is there a dew shield installed? Are there people in the observatory? You see, there are many questions.

Let's see what the answers are so we can all help to assess the situation.

A first thought might be to use an artificial star during the day to "test" the collimation with less atmospheric interference...

Best Regards

Frank

[matt_baker]

On 20/07/2022 at 21:32, Froeng said:

Looking at the video of Jupiter you posted, the turbulence is directional, which in my experience indicates it is related to atmospheric seeing. This could be as far away as the Jetstream or it could even be quite close by, ie nearby rooftops. Is the observatory on a flat roof or is it standing on the ground surrounded by buildings? Also, is it a dome with a slit, or does the roof roll off completely?

Tube currents are typically not as directional, and more "swirly" in a SCT, but then all of this is dependent on so many factors, it all depends. Does the telescope cool down after opening the observatory, and if yes, for how long? Or is it air-conditioned to match the outside temperature? Is there a dew shield installed? Are there people in the observatory? You see, there are many questions.

Let's see what the answers are so we can all help to assess the situation.

A first thought might be to use an artificial star during the day to "test" the collimation with less atmospheric interference...

Best Regards

Frank

Observatory is on a flat roof and there are buildings 40m in front, which I can only guess is not helping at all. It's a dome with a slit too.

I'm fairly sure that video was taken a couple of hours after opening, there's only me inside, there's no air con inside there and the dew shield was on.

[Froeng]

Okay, are there concrete pavers around the observatory on that flat roof? If yes, are they all around or are there less in one direction.

I would try to look at the direction where there is less thermal mass present, ie concrete or any other man-made things/buildings. The less the better! Then try to use a star about 60deg from the horizon. The scope has to be as acclimated to ambient as possible. 
if the seeing is still bad, it’s either another day of bad seeing (which do happen about 80% of the time), or it’s the rising heat from the roof/houses. 🤔

[matt_baker]

18 hours ago, Froeng said:

Okay, are there concrete pavers around the observatory on that flat roof? If yes, are they all around or are there less in one direction.

I would try to look at the direction where there is less thermal mass present, ie concrete or any other man-made things/buildings. The less the better! Then try to use a star about 60deg from the horizon. The scope has to be as acclimated to ambient as possible. 
if the seeing is still bad, it’s either another day of bad seeing (which do happen about 80% of the time), or it’s the rising heat from the roof/houses. 🤔

It's on a school site so concrete is pretty much surrounding the observatory and there's a lot of buildings facing south where I'm looking most of the time.

I'd say it's probably 50/50 with poor seeing from the atmosphere and buildings. Saying that I did have a look at a star above 60 degrees and it was still dancing all over the shop, I still could barely make out the rings even with it being outside for a couple of hours but that could've been very poor seeing