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  #21  
Old 08-02-2019, 10:03 AM
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Camelopardalis (Dunk)
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Btw, just to clarify, I wasn’t explicitly advocating a 12” newt, they are a beast(!), although Glen’s mount would be fine with it I’m sure.

My gut feeling is a little more mid-size, say a 10” f/5, as Glen already has an ASI1600 camera, which combined will give a resolution of about 0.6”/pixel.

May sound like a compromise, but an economical, easy(ish) to handle, easy(ish) to collimate compromise
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  #22  
Old 08-02-2019, 11:08 AM
glend (Glen)
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Quote:
Originally Posted by Camelopardalis View Post
Btw, just to clarify, I wasnt explicitly advocating a 12 newt, they are a beast(!), although Glens mount would be fine with it Im sure.

My gut feeling is a little more mid-size, say a 10 f/5, as Glen already has an ASI1600 camera, which combined will give a resolution of about 0.6/pixel.

May sound like a compromise, but an economical, easy(ish) to handle, easy(ish) to collimate compromise
Thanks Dunk, you may recall I had a 10" f5 truss Newt that I built some years ago, and used for imaging. It was pretty light, sadly I let it go (parted out in the end). Re a 12" I have considered buying the bare carbon tubes that TS sells and using a GSO 12" as a donor to build a light weight 12". The tubes are bored for GSO components. With an upgraded focuser it would be nice. Of course I could do the same with a 10". My worry about that approach is over capitalising something I will have to sell down the track.


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Originally Posted by mental4astro View Post
Dew control is certainly a challenge for a corrector plate. But there is a change of thinking when it comes to dew control.

We seem to think that the only way of using a scope is to wait for it to cool, and then we stick a heater on the thing... I've always thought this was strange because of the thermal problems a heater creates... But that's beside the point here.

The new thinking is NOT to let the scope cool!

The problem of a scope cooling is not the heat per say, but the temperature differential that is created between the warm interior (mirror, baffle, cell), and the cold metal tube, and the heat plumes that are then generated inside the OTA.

So, if the heat differential is not allowed to occur in the first place, then the heat plumes won't be generated, the inside of the OTA is thermally stable, and you can start using the OTA immediately without waiting for the thing to cool.

I am not talking about keeping the OTA warm. Instead I am saying we should slow the rate of cooling. By insulating the OTA, the metal/CF tube is not allowed to get cold, and so the heat differential is reduced to the point where there are no heat currents being generated inside the OTA.

The insulative material is extended beyond the corrector plate of the SCT or Mak, and this acts as both a dew shield and an extension of the insulation to enhance the protection of the corrector. This won't totally prevent dew formation on the corrector in humid conditions, but it will extend the dew-free period, and if a heating strap is used it can be implemented sooner while the OTA is still warm, so to take advantage of the residual heat of the OTA so the strap is not needing to also heat a stone-cold metal tube at the same time.

Alex.
This is how I managed my MN190, it lived in the house and was carried out for any session, and went to work straight away. I used a foam yoga mat (black) as a dew shield with a heater strap underneath it to prevent corrector cool down. It tended to last all session without fogging.
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  #23  
Old 08-02-2019, 12:26 PM
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mental4astro (Alexander)
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Glen!

I received a PM from Dunk where he looked at explaining the reason behind having the focuser/corrector/camera (fcc) on the OTA orientated in the way you mentioned. In thinking about this between your posts, Dunk's PM and another experience I had with my Mak in seeking to balance it (yes, that bloody Mak again...), I've figured out why it works!

It is a mechanical relationship between the OTA, where the weight is distributed along the OTA, and how the OTA is attached to the mount. Though it has nothing to do with it becoming a part of the counterweight axis as you said. The reason is a geometric weight distribution one.

When you look at a Newt, you have the primary mirror in line with the optical axis. The fcc is stuck on the side of the OTA, and no longer in line with the optical axis. The weight geometric axis of the OTA is off to one side running parallel (roughly) to the optical axis, but between the primary mirror and the fcc. Once the OTA is coupled to the mount, if the fcc is located above say the long axis of the dovetail plate, the weight along the scope is not in a symmetrically distributed, and it becomes impossible to balance the OTA.

Now, if the fcc is orientated below the dovetail plate, all of a sudden the weight distribution along the OTA becomes a whole lot more symmetrical as the weight axis is in line with the dovetail plate and most importantly crosses directly over the pivot point of the dec axis of the mount. The mechanics of the OTA balances out! This is still not a perfectly balanced situation as the weight distribution axis is not exactly a balanced situation, and some fine tuning can be done by adding some small counterweights just above the fcc if this system is too light, or down at the primary mirror if the fcc system is heavier than the primary mirror system. The ideal situation would be having the weight distribution axis running directly through the dovetail plate, and then the entire OTA is balanced along the dec axis and attachment point!

Below is a little diagram I prepared to help explain things.

Click image for larger version

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Glen, thank you so much for pointing me in the right direction here. I wasn't quite following what you were saying, and Dunk's PM helped me put all the pieces together!

I'll be looking at my own 8" f/4 Newt again with this new insight and look at ways to really fine tune its balance. It's also helped me figure out why I've also been struggling with balancing my bloody Mak both on the NEQ6 and CPC mount. I now understand the weight mechanics at play!

Alex.

PS: Dunk! I nearly forgot to thank you too!!

Last edited by mental4astro; 08-02-2019 at 01:15 PM.
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  #24  
Old 08-02-2019, 12:44 PM
glend (Glen)
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Yes Alex that drawing is what I was suggesting, and not explain too well. I was looking for a photo of that sort of setup to link here, but your drawing gets it.
One thing to think about is the addition of a guidescope, which has to go on the top of the scope (otherwise it would be blocked), and just can't be mounted underneath. However, OAG setups will maintain that geometric architecture. A top mounted guidescope will move your geometric line upward above the dovetail if it weighs enough. Simple light guidescopes (like the ZWO 60mm) don't seem to offset too much, and centering it over the DEC rotation point helps.

https://goo.gl/images/BNVQ2Z
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  #25  
Old 08-02-2019, 09:06 PM
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Camelopardalis (Dunk)
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Yeah guidescopes don’t have to be big and heavy. An OAG concerns me on a newt because, at least on fast scopes, they are so sensitive to the alignment of the coma corrector in the focuser...although I guess it’s no worse than a mono camera and filter wheel

Glen, my sense is that you might be happier going with a smaller scope that is less beastly and requires less outlay. I’m entertaining an 8” f/5 myself, seems like a decent enough focal length on most nights and 4x the light gathering of my 4” apo...mono sensors have their own charms of course
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  #26  
Old 08-02-2019, 09:47 PM
glend (Glen)
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Quote:
Originally Posted by Camelopardalis View Post
...
Glen, my sense is that you might be happier going with a smaller scope that is less beastly and requires less outlay. Im entertaining an 8 f/5 myself, seems like a decent enough focal length on most nights and 4x the light gathering of my 4 apo...mono sensors have their own charms of course
Yes good points. I need to digest all this good advice. Thanks to everyone for their input here. I will retire to consider.
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  #27  
Old 09-02-2019, 09:03 AM
garymck (Gary)
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Hi,
a bit late to this, but my suggestion would be a Skywatcher 12" f5 newt. I own one. I had to do he following mods:

added heavy springs to the mirror cell
Removed mirror clips and spot siliconed the mirror in place
replaced the focuser with Baader Diamond Steeltrack (superb focuser, craps all over Moonlights)
Rounded off the ends of the push screws of the mirror cell and the secondary adjusters
Put a teflon ring between the secondary adjustment screws and the secondary holder
Replaced the double sided tape that holds the secondary on with 3 dots of silicon

After these mods the scope works extremely well and holds collimation well.

Collimation is MUCH less critical than f4 newts.

I have found that Skywatchers cheap .9x coma corrector works very well (although can have the odd reflection when very bright stars like Alnitak are in the field). This drops the f ratio to f4,5. The Baader MPCC similarly works well at F5 but without the reflections.

Dirt cheap at under 2K and works well. I have only used it for testing purposes as when I purchaed it I miscalculated where it would balance and found that it hit the wall of my tiny obs. I liked it so much I have stored it for the future, won't sell it off.

My positive experience with newts means that if I had the cash I would love one of these:
https://www.teleskop-express.de/shop...tomizable.html

FWIW
Gary
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  #28  
Old 09-02-2019, 10:17 AM
morls (Stephen)
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(A bit off-topic, sorry...)


Great to hear of your experience with the Baader Diamond Steeltrack Gary, I'm waiting for mine to arrive.


Stephen
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  #29  
Old 09-02-2019, 01:11 PM
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Andy01 (Andy)
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Hi Glen,

Have you considered one of these?

The company is owned by two very good deep sky image makers and I'm delighted with my new 10"f4 CF Newt that came fully sorted by Diego himself.

Well within your budget too

Cheers
Andy
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  #30  
Old 18-02-2019, 09:47 PM
kosborn (Kevin)
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Quote:
Originally Posted by Andy01 View Post
Hi Glen,

Have you considered one of these?

The company is owned by two very good deep sky image makers and I'm delighted with my new 10"f4 CF Newt that came fully sorted by Diego himself.

Well within your budget too

Cheers
Andy

I'm very late to the discussion but I have one of the Sidereal 10" f/5 Newtonians that Andy mentions. It comes as tweaked as you need with a carbon fibre tube, new secondary and spider, Moonlight focuser, and as Diego said to me it was built with "lots of love and care". I managed to snag this image with it. Needless to say, I'm very happy with it.


Kevin
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