Question for Mewlon Owners

[MalcolmM]

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 

[Nik271]

On top of the central obstruction you have 6 diffraction spikes, perhaps they create the hairy look (when they are too dim to be easily seen as spikes)?

Are you comparing the two scopes at the same magnification? Seeing starts to mess the star images a lot at high mags. Also if the Mewlon is not completely cooled down the in focus star image will not be very clean. I see this in my 180 Skymax often.

The doughnut being off center indicates a slight miscollimation, the question is how much. A picture through the EP will be the best way to measure it.

Edited February 28 by Nik271

[MalcolmM]

Thanks @Nik271, next clear night I'll try and get a photo. The scope had been out for over an hour so I'm pretty sure it should have been cooled.

I don't think it's diffraction spikes, but that's an interesting thought. I assume diffraction spikes would be static? I'll bear that in mind next time I'm looking though. 

Malcolm 

[JeremyS]

Hairy stars sounds like a seeing issue.

I use Polaris to check collimation. It’s at a decent altitude and doesn’t move. It shows diffraction rings.

Now, on the donut. It should be central when defocussed. But only on stars at the centre of the field. This is key. When tweaking collimation, you need to re-centre Polaris before the next adjustment.

More here https://www.cloudynights.com/topic/283204-mewlon-210-finally-arrived/

and https://www.cloudynights.com/topic/354729-best-ever-collimation-video/?p=4548838

[The60mmKid]

A major consideration with the Mewlon is thermal acclimation. I would pay close attention to that (especially during the winter) since a lack of thermal acclimation can be mistaken for poor seeing or collimation. Here's some essential reading: https://www.cloudynights.com/topic/530068-seeing-vs-thermals/

I'd bet money that that's the main culprit behind your hairy stars. The solution is to keep the scope in a cool place before you use it and put it outside with the eyepiece opening pointing up after sunset for a couple of hours. The baffle, and not just the mirror, in the Mewlon traps heat, so positioning it in that way may help with acclimation.

I share this based on plenty of first-hand experience with a 180C. When that Mewlon is acclimated and the seeing is good, the views will blow your socks off.

Edited February 28 by The60mmKid

[The60mmKid]

9 minutes ago, The60mmKid said:

A major consideration with the Mewlon is thermal acclimation. I would pay close attention to that since (especially during the winter) a lack of thermal acclimation can be mistaken for poor seeing or collimation. Here's some essential reading: https://www.cloudynights.com/topic/530068-seeing-vs-thermals/

I'd bet money that that's the main culprit behind your hairy stars. The solution is to keep the scope in a cool place before you use it and put it outside with the eyepiece opening pointing up after sunset for a couple of hours. The baffle, and not just the mirror, in the Mewlon traps heat, so positioning it in that way may help with acclimation.

I share this based on plenty of first-hand experience with a 180C. When that Mewlon is acclimated and the seeing is good, the views will blow your socks off.

PS - Really.

[Stu]

I would agree that it is potentially a cooling/acclimatisation issue. I used to find the same with my 210. As for collimation, I think it is worth having a go. The screws are very tight, and should remain so; I just backed one off and took the tiny amount of slack up with whichever one needed adjusting. Very small adjustments, and keep everything tight and it should hold very well once done. Agreed about centring the star each time, that is essential for an accurate result.

[The60mmKid]

But wait until you have the thermals sorted and good seeing before adjusting the collimation! 

[Stu]

14 minutes ago, The60mmKid said:

But wait until you have the thermals sorted and good seeing before adjusting the collimation! 

Why so sad?

[The60mmKid]

7 hours ago, Stu said:

Why so sad?

Because it would be sad to make changes to the collimation if it isn't actually an issue 😭 That's why I think the thing to do would be to address the thermals and wait for good seeing. That may solve the hairy stars problem. And if the collimation is the problem, adjusting it would still require a thermally acclimated scope and good seeing since a steady defocused star image with a clear diffraction pattern is needed for the job (assuming that's the method of collimation being used). So, in either case, I think it would be a mistake to touch those collimation screws right now.

Edited February 29 by The60mmKid

[Stu]

4 minutes ago, The60mmKid said:

Because it would be sad to make changes to the collimation if it isn't actually an issue 😭 That's why I think the thing to do would be to address the thermals and wait for good seeing. That may solve the hairy stars problem. And if the collimation is the problem, adjusting it would still require a thermally acclimated scope good seeing since a steady defocused star image with a clear diffraction pattern is needed for the job (assuming that's the method of collimation being used vs. an artificial star). So, in either case, I think it would be a mistake to touch those collimation screws right now.

Well it is Malcolm’s choice, and there are other ways of going about it such as an artificial star. Cooling is a problem we all face, and all deal with when collimating scopes. He has said the collimation looks slightly off, and that can have quite an impact on planetary detail. I don’t think it is the cause of the hairy stars as I said, but do think it is worth looking at, carefully as I’ve said. It is actually not as hard with these scopes as it seems.

[dweller25]

@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.

 

Edited February 29 by dweller25

[The60mmKid]

Just now, Stu said:

Well it is Malcolm’s choice, and there are other ways of going about it such as an artificial star. Cooling is a problem we all face, and all deal with when collimating scopes. He has said the collimation looks slightly off, and that can have quite an impact on planetary detail. I don’t think it is the cause of the hairy stars as I said, but do think it is worth looking at, carefully as I’ve said. It is actually not as hard with these scopes as it seems.

Of course it's his choice, but I'll respectfully repeat my opinion that adjusting the collimation isn't the best action to take at this point 👍 I doubt that my opinions are robbing him of free will ☺️

Another point worth mentioning is the inherent coma of the DK design. Stars have to be dead center in the FOV for the diffraction pattern to appear concentric. So, if a star is a little bit off to the side, it may appear as though the collimation is off.

There is a benign learning curve when using a Mewlon after refractors. I think it's most helpful to point out the possible variables first rather than, at this point, suggesting fixing collimation that may not need fixing.

[Stu]

6 minutes ago, The60mmKid said:

Of course it's his choice, but I'll respectfully repeat my opinion that adjusting the collimation isn't the best action to take at this point 👍 I doubt that my opinions are robbing him of free will ☺️

Another point worth mentioning is the inherent coma of the DK design. Stars have to be dead center in the FOV for the diffraction pattern to appear concentric. So, if a star is a little bit off to the side, it may appear as though the collimation is off.

There is a benign learning curve when using a Mewlon after refractors. I think it's most helpful to point out the possible variables first rather than, at this point, suggesting fixing collimation that may not need fixing.

I have used a Mewlon 210 for some time, so am well aware of the characteristics. I have already pointed out the need for a centred star and agreed with the need for a cooled scope. I have said that I don’t think the hairy stars are related to collimation but are likely cooling or seeing related. To get the best out of these scopes, they need to be bang on with collimation so that is certainly something well worth doing as David has verified, carefully and with research. I really don’t see what your beef is?

[The60mmKid]

Just now, Stu said:

I have used a Mewlon 210 for some time, so am well aware of the characteristics. I have already pointed out the need for a centred star and agreed with the need for a cooled scope. I have said that I don’t think the hairy stars are related to collimation but are likely cooling or seeing related. To get the best out of these scopes, the need to be bang on with collimation so that is certainly something well worth doing as David has verified, carefully and with research. I really don’t see what your beef is?

No beef, Stu! I'm a vegetarian! And especially not with you! You're one of my SGL faves!

But it's permissible to make a point with a certain degree of respectful adamance when it feels helpful, no? And maybe also with a certain playfulness? Although I admit that my attempts at humor usually fall flat... Respectful, playful disagreement isn't too beefy, I hope 🤞

In any case, no insult or injury intended. Please PM me if further communication on the matter would help.

Wishing you a good day ❤️

[The60mmKid]

Ok, I'll say one more thing, and then I vow to be less asinine for a while.

 

Maybe the stars looked hairy because Malcolm was looking at SIRIUS 🤣🤣🤣

[dweller25]

14 minutes ago, The60mmKid said:

Ok, I'll say one more thing, and then I vow to be less asinine for a while.

 

Maybe the stars looked hairy because Malcolm was looking at SIRIUS 🤣🤣🤣

Don’t give your day job up 👍🙂

[MalcolmM]

13 minutes ago, The60mmKid said:

Ok, I'll say one more thing, and then I vow to be less asinine for a while.

 

Maybe the stars looked hairy because Malcolm was looking at SIRIUS 🤣🤣🤣

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 

[dweller25]

2 hours ago, 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 

Yes Alnitak is too low to assess collimation and it’s a double star - I aim for something at least 70’ high, I aim for Al Kab in Auriga this time of year, which means you are looking through a bit less atmosphere - it all helps !

PS - I have seen a scope that was not fully cooled showing it was out of collimation. Later when it was fully cooled it had perfect collimation 👍

Edited February 29 by dweller25

[Stu]

3 minutes ago, The60mmKid said:

No beef, Stu! I'm a vegetarian! And especially not with you! You're one of my SGL faves!

But it's permissible to make a point with a certain degree of respectful adamance when it feels helpful, no? And maybe also with a certain playfulness? Although I admit that my attempts at humor usually fall flat... Respectful, playful disagreement isn't too beefy, I hope 🤞

In any case, no insult or injury intended. Please PM me if further communication on the matter would help.

Wishing you a good day ❤️

Good good. As my wife frequently says to me when I tell her I was joking about something, ‘I would work on that’ 🤪

Absolutely, friendly disagreement is fine and an essential part of the forum. I guess I just personally work on the basis of presenting opinions and letting the OP take all the info in and decide themselves. Much like when people ask for advice on which scope to buy, I prefer to see people discussing the pros and cons of scopes they think are suitable, rather than a ‘you must buy this because it’s what I’ve got and think is good’ type approach.

I will drop you a PM later, but all is good 👍 

[The60mmKid]

2 hours ago, 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 

A Dog Star reference, actually 😊 But I'm a big fan of Animagus Gary Oldman, too.

 

 

[John]

When I have owned cassegrain type scopes (not Mewlons alas) I have felt that they had the potential to show excellent star definition, sharpness and contrast but the designs, with their relatively large central obstruction, needed to be accurately collimated to enable this. Even small misalignment of the mirrors can scrub a noticeable edge off performance. There is a well known web page somewhere illustrating this but I can't recall where just now.

I know that the Mewlon secondary obstruction is not quite as large as an SCT's or the primary as fast but they are still instruments with relatively large CO's and relatively fast (F/3 ?) primary mirrors so I would think that accurate collimation is still quite important to getting the best from them, as is cool down of course.

I've read quite a few reviews of the 180 and 210 Mewlons (I may well be interested in one someday 🙂) and I think they all mentioned both the need to tweak collimation following some initial slight concerns with the quality of views and to allow quite long cool down periods, depending where the scope has been stored prior to observing. Once these issues had been addressed the reports are consistently of excellent performance I'm pleased to say 🙂 

[bosun21]

4 hours ago, dweller25 said:

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.

I have found it better to draw the air up and out of open end of the scope as it prevents the chance of blowing any dust or particulate matter onto the mirror. Both will have the same cooling properties.

[dweller25]

1 hour ago, bosun21 said:

I have found it better to draw the air up and out of open end of the scope as it prevents the chance of blowing any dust or particulate matter onto the mirror. Both will have the same cooling properties.

Which Mewlon did you have and can I ask In what way was it better ?

I’m interested because I tried that but even with filters the secondary got dirty and its harder to clean the secondary than the primary.

And the cool rate of the primary was much slower.

Edited February 29 by dweller25

[bosun21]

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.