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.

[MalcolmM]

On 29/02/2024 at 07:42, dweller25 said:

@MalcolmM

I have the same two scopes as you and know how long the Mewlon takes to fully cool down, particularly when compared to the FC100.

I aid the Mewlon cooldown by resting a cooling fan with a USB powerbank on the front of the spider blowing air onto the primary and down the primary baffle and OPEN visual back.

Even with this active cooling it takes a good hour to fully cool the primary and primary baffle !

The fan solution is not expensive and works well.

 

Once fully cooled, to get the best out of the Mewlon it must be well collimated, I use this method - always making sure the star is perfectly central in the eyepiece before tweaking the collimation screws at x300, step 4 should be done at perfect focus.

 

Just to close the loop on this one ... I finally got some steady skies and had the Mewlon outside for 4 or 5 hours observing the moon as the sky got darker. Contrast and sharpness seemed very good so I tried a star test. No hairyness  and perfect airy disk and diffraction rings, just as the PERFECT example in @dweller25's diagram.

I suspect the scope simply hadn't cooled enough. Thanks again everyone for all the very helpful replies.

Couple of moon pics; phone just hand held at eyepiece (Maxbrights plus 1.25GPC plus Tak Abbe 18mm). Stupidly didn't take one of the star test!

Malcolm 

[John]

I'm interested by Mewlons, probably the 180 more than the 210. My only hesitation is the potential cool down time. My LZOS 130mm triplet takes around 40 minutes to cool on a cold night. It's kept in a centrally heated room as are my other scopes.

I could deal with a little longer time to cool but would not fancy having to wait a couple of hours. 

Other than that doubt, a Mewlon 180 is a very attractive prospect 🙂

 

Edited May 15 by John

[PatientObserver]

On 29/02/2024 at 03:45, MalcolmM said:

Harry Potter reference? Very clever 🙂 I was actually using Alnitak, purely because I was doing the Orion doubles. Sounds like it's much to low in the sky to be a good judge of collimation. 

Thanks everyone for some brilliant replies and links. I've had this scope for a couple of years, but it seems I still have much to learn. It sounds like I'm maybe still underestimating the cooling time and I'll definitely check out Polaris. @dweller25, you have mentioned active cooling before, I really must look into that!

Malcolm 

Daniel Mounsey on CN has suggested lymax coolers on multiple occasions for the Mewlon. It is on my wish list of gear to try out:

https://www.lymax.com/#purchase

[JeremyS]

1 hour ago, John said:

I'm interested by Mewlons, probably the 180 more than the 210. My only hesitation is the potential cool down time. My LZOS 130mm triplet takes around 40 minutes to cool on a cold night. It's kept in a centrally heated room as are my other scopes.

I could deal with a little longer time to cool but would not fancy having to wait a couple of hours. 

Other than that doubt, a Mewlon 180 is a very attractive prospect 🙂

 

I store mine in a plastic storage box and leave it outside to acclimate. And/or put it on the mount with the covers off. Not a big deal considering the performance you can get 👍🏻

[John]

30 minutes ago, JeremyS said:

I store mine in a plastic storage box and leave it outside to acclimate. And/or put it on the mount with the covers off. Not a big deal considering the performance you can get 👍🏻

Thanks Jeremy.

Is yours the 180 or 210 ?

How long does it take before it's worth observing with ?

Thanks 🙂

 

[JeremyS]

2 minutes ago, John said:

Thanks Jeremy.

Is yours the 180 or 210 ?

How long does it take before it's worth observing with ?

Thanks 🙂

 

I have the M210. With the above preacclimation, I would say an hour.

[MalcolmM]

9 hours ago, MalcolmM said:

I suspect the scope simply hadn't cooled enough

Thinking about it overnight, I now suspect my original problem was seeing, not cool down.

@John, I've mostly used my Mewlon for deep sky in the last 6 months, so have not been aware of cool down times, but before that, when using it on planets and the moon, I would agree with @JeremyS. When acclimatising in a plastic box before hand, I would get fantastic views (in my opinion!) in less than an hour.

@dweller25 uses active cooling to help acclimatise. 

Apart from my wobble which caused me to start this thread, I have had nothing but enjoyment from my Mewlon. I can't speak for the 210, but the 180 is very portable and really easy to set up. The finder handle is quite simply brilliant

Malcolm 

[dweller25]

I have now bought a plastic box that I will use to acclimate the M180 for a few hours before use, hopefully instead of using the front fan, will test in the Autumn……

Either way the M180 is a keeper as it’s easy to set up and optically excellent.

Edited May 16 by dweller25

[Dr Strange]

On 28/02/2024 at 12:47, MalcolmM said:

I have been touring Orion's double stars with various scopes. I have found the Mewlon 180 less satisfying than my 100DC. In the DC, the stars are the typical small dots surrounded by diffraction rings. In the Mewlon, the stars are slightly hairy, for want of a better word. Sure I can see deeper in the Mewlon, Trapezium E being easy in the Mewlon and invisible in the 100DC. But despite giving up 80mm to the Mewlon, the 100DC splits close doubles easier.

When I do a star test in the Mewlon and defocus, the donut is ever so slightly off center. It's not much, but if you look hard, it's there. I'm wondering if I need to tweak collimation, or is this (hairy stars) simply the difference between a refractor and a reflector?

I'm a great believer in 'if it ain't broke, don't fix it' and I have a history of fixing things and making them worse!

So, I really don't want to touch the collimation, despite people saying it's not as hard as it sounds, unless I really really need to. 

Are slightly hairy stars to be expected in a Mewlon?

Thanks, 

Malcolm 

The hairy stars are likely because the Mewlon hasn't cooled down. It can take a while. I would suggest covering the front of it with a light cloth like a sheet then pointing a fan at the front. It will blow air into the tube and cool it faster but keep the dust and dirt out. Alternatively you could put it outside, wait an hour or so, then observe. And splitting close doubles is always going to be better in the FC-100 because there is no CO nor are there any spider vanes.

Collimation sounds a bit off. Collimation is very straight forward BUT small adjustments of about 1/8th turn. The screws will likely screech but don't be alarmed. That is normal. A trick is to put it on Polaris, defocus, put your finger in front of it pointing at the part that is off center. This will tell you what screw to adjust. 

It comes down to horses for courses. The two scopes compliment each other very well and are really for two different things. I would recommend you use the Mewlon for smaller DSO, planetary, and lunar. Use the FC-100 for wide(er) field stuff, quick sessions, open clusters, and doubles.  

[Doctor D]

Here's what you can do if dust is a concern. Go to https://www.lymax.com/ and purchase a Lymax that's designed for a C9.25" or whichever one they recommend for the M180. Place M180's dust cover over the front of the tube and use the Lymax exactly as you would an SCT. 

[John]

Would insulating the tube work with Mewlons ?

Or does that only work with closed optical tubes such as SCT's and mak-cassegrains ?

[Doctor D]

Just an FYI. The open tube of the Mewlon's means it will be harder to acclimate them passively because cooler air falls down the tube and makes immediate contact with the warmer primary substrate and as a result, a boundary layer can occur which becomes more problematic if observers are setup in a climate with high deltas from day to night. 

[Doctor D]

5 minutes ago, John said:

Would insulating the tube work with Mewlons ?

Or does that only work with closed optical tubes such as SCT's and mak-cassegrains ?

Sorry, no you don't need insulation.

Edited May 16 by Doctor D

[F15Rules]

I have read all of the above with interest.

I think I will stick with refractors.😉

Dave

[Doctor D]

Yes, unless observers want to dedicate themselves to Mewlon's, they are easier off just going with a refractor. 

[JeremyS]

Hang on a mo. I think myths are being perpetuated. Well have the one about them being difficult to collimate next. 😊

Mewlons aren’t that difficult. In UK the temperature swings aren’t that difficult to acclimate to. I’m a refractor man, but the Mewlons are great (if you don’t mind diffraction spikes). The resolution on planets with my M210 is significantly greater than with my Tak TSA 120.