Question for Mewlon Owners

[dweller25]

2 minutes ago, bosun21 said:

My friend has the 210 and he allowed me to borrow it for 9 months while he was abroad working on a contract. He asked me to ensure that I placed the fan correctly. He explained that he used to get pollen depositing on the primary over the course of  summer evenings. I have actually noticed a similar thing with a classical cassegrain  regarding pollen while using a fan for cooling. He had small elasticated paper socks that fitted over the secondary mirror to protect it while cooling.

Aah, good idea 👍

[MalcolmM]

I had another go last night and the scope was cooling for 2 hours. The hairyness did improve a bit, but definitely not as good as my refractor (though I did not have the 100DC out to do a direct comparison).

I did a star test on Polaris and tried to get photos, but my phone lacks the ability to alter the exposure much so the pics were either way over exposed or totally blank. However, the defocused star was moving around a bit; sometimes it looked spot on, sometimes it looked a bit off center. Again lacking experience here so I don't know if this is to be expected, or if the seeing was poor. I was able to split Alnitak, a gap between the two components, but again, a more difficult split than in the 100DC, and I'm not always able to split it.

This scope has shown me stunning views of Jupiter, Saturn and the Moon; better than the 100DC, so I suspect if the collimation is off, it's not off by much. That being said, I really need to learn how to tweak the collimation, so I'll probably give it a go! 

Thanks again everyone for your help and advice.

Malcolm

[dweller25]

From what you say about Polaris and the views of the planets I suspect your collimation is very good and perhaps not worth worrying about too much.

I think seeing and that final bit of cooldown where the boundary layer (where residual heat hovers on the primary mirror surface) may be the issues.

You should be able to see the airy disk of a star at perfect focus at x300 if your scope is cooled and seeing is decent. I see this regularly in the M180 but I always use a fan.

In fact I will get the Mewlon out now and give it a try…

Edited March 2 by dweller25

[JeremyS]

3 hours ago, MalcolmM said:

I had another go last night and the scope was cooling for 2 hours. The hairyness did improve a bit, but definitely not as good as my refractor (though I did not have the 100DC out to do a direct comparison).

I did a star test on Polaris and tried to get photos, but my phone lacks the ability to alter the exposure much so the pics were either way over exposed or totally blank. However, the defocused star was moving around a bit; sometimes it looked spot on, sometimes it looked a bit off center. Again lacking experience here so I don't know if this is to be expected, or if the seeing was poor. I was able to split Alnitak, a gap between the two components, but again, a more difficult split than in the 100DC, and I'm not always able to split it.

This scope has shown me stunning views of Jupiter, Saturn and the Moon; better than the 100DC, so I suspect if the collimation is off, it's not off by much. That being said, I really need to learn how to tweak the collimation, so I'll probably give it a go! 

Thanks again everyone for your help and advice.

Malcolm

If the star was moving around the seeing must be poor. Wait for a better night to check collimation 

[dweller25]

@MalcolmM

Took the M180 out of the house at 5:40pm, started the fan running on the front and left it until 6:10pm.

Looked at Jupiter at x178 and it was ok but not super sharp,  could make out the GRS and several bands but a bit soft. Jupiters moons were trying to show an airy disk but it was not steady. 

Turned the scope onto Bellatrix and defocussed a little, the outer ring shows small protruding spikes - this could be the hairiness you see ?

The scope had clearly not cooled down.

But there was no lateral air movement across the out of focus image - so seeing was good.

Fan put back on and waited until 6:30pm.

Back onto Jupiter at x178, much better views and the moons were showing airy disks.

Back onto Bellatrix and that was much better too, the outer ring showing much more stability and slight spikes/hairs. In focus showed perfect collimation then slight mis collimation - scope still not cool.

Fan back on until 6:45pm, back onto Bellatrix and it was almost stable at x178, very slight spikes when defocussed and perfect collimation.

Fan back on until 7:05pm, back onto Bellatrix at x178 - stable.

Turned the scope onto Al Kab at x321, stable, no spikes, perfect airy disk, good seeing.

So even with active cooling it took 1 hour and 25mins to cool from the house !

BUT - an obstructed telescope will never quite match a refractor for sharpness 👍

Hope that helps.

 

 

Edited March 1 by dweller25

[The60mmKid]

Glad to hear that the view is improving.

It's helpful to learn to differentiate thermal issues from poor seeing at the eyepiece because then we know how to address it. If it's a thermal issue, then more/different acclimation is in order. If it's poor seeing, then the Mewlon won't be able to strut its stuff until the air is steadier (but will still provide nice lower-magnification views).

If it's not thermally acclimated, you'll see a distinct plume on a defocused star, and the diffraction pattern will be warped. The final picture in this post shows it: https://www.cloudynights.com/topic/812857-so-what-is-it-about-insulating-cats/?p=11776530

If it's poor seeing, the whole defocused star image will be weak and swaying.

Of course, it could be both a thermal issue and poor seeing.

And the Mewlon will always be more susceptible to poor seeing than a refractor. The central obstruction causes the light to be concentrated beyond the first diffraction ring... pushes it outward so that the "target" shape of a defocused star is more diffuse than in a refractor. Since the light is less concentrated, star images sway around more with atmospheric turbulence.

I used to use a 180C with an FC-100DC. On nights of poor to average seeing, the refractor always provided a more satisfying view. But on the nights of good seeing, and when I'd taken care to thoroughly acclimate the Mewlon, the Mewlon would provide views far beyond the capability of the refractor. Some of the views I had through that scope are still so vivid in my mind... Mewlons are lovely scopes.

[MalcolmM]

1 hour ago, dweller25 said:

@MalcolmM

Took the M180 out of the house at 5:40pm, started the fan running on the front and left it until 6:10pm.

Looked at Jupiter at x178 and it was ok but not super sharp,  could make out the GRS and several bands but a bit soft. Jupiters moons were trying to show an airy disk but it was not steady. 

Turned the scope onto Bellatrix and defocussed a little, the outer ring shows small protruding spikes - this could be the hairiness you see ?

The scope had clearly not cooled down.

But there was no lateral air movement across the out of focus image - so seeing was good.

Fan put back on and waited until 6:30pm.

Back onto Jupiter at x178, much better views and the moons were showing airy disks.

Back onto Bellatrix and that was much better too, the outer ring showing much more stability and slight spikes/hairs. In focus showed perfect collimation then slight mis collimation - scope still not cool.

Fan back on until 6:45pm, back onto Bellatrix and it was almost stable at x178, very slight spikes when defocussed and perfect collimation.

Fan back on until 7:05pm, back onto Bellatrix at x178 - stable.

Turned the scope onto Al Kab at x321, stable, no spikes, perfect airy disk, good seeing.

So even with active cooling it took 1 hour and 25mins to cool from the house !

BUT - an obstructed telescope will never quite match a refractor for sharpness 👍

Hope that helps.

 

 

That's a very helpful post. Thanks @dweller25

Malcolm 

[MalcolmM]

42 minutes ago, The60mmKid said:

but will still provide nice lower-magnification views

I think this is why I have not paid much attention to collimation to date. I mostly use the Mewlon for low(ish) power DSO and open clusters. I did a star test very early on and reckoned (to my inexperienced eyes) it was spot on, and never questioned it further. I have only recently started looking at double stars!

Malcolm 

[bosun21]

On 28/02/2024 at 23:27, Stu said:

. Agreed about centring the star each time, that is essential for an accurate result.

This is crucial when doing a star test for collimation. I also use Polaris and centre the star with an illuminated reticle eyepiece then switching to a standard high magnification eyepiece. After each tweak of the collimation I recentre with the reticle eyepiece again. It can become tedious swapping back and forth but it eliminates any errors in star placement.