I'm trying to decide between a couple of different Cassegrain designs to replace my SCT. I have many years experience with the SCT design, but one Cassegrain design I have no idea about in terms of practical use, is the Takahashi Mewlon. I know they are a Dall Kirkham, so no corrector lenses, and have a superb image, but that is it.
My questions then are:
1, How do they deal with dew? Any particular precautions to take?
2, The mirrors are exposed, so are they overcoated to give more protection?
3, Being exposed, how susceptible are they to getting dirty? This I guess is an additional part to the dew factor. I live on the coast in Sydney, so I am concerned.
4, With regular use, how often is it necessary to clean the mirrors?
So you know, the Cassegrain I end up with will be principally as a visual scope for my sketching of the Moon and planets. Photography will only be of the planets and Moon too, and this will be minimal. I'm looking for a Cassegrain in the range of 7" to 8" for my purposes.
I think that's it for now that I can recall. Hopefully as discussion progresses here other questions will come to mind, or will be answered even though I didn't ask them...
I can't suggest anything about Mewlons, but seeing as you mentioned you'll be looking for a 7" to 8" Cassegrain I thought I'd jump in and say the 180 Mak I bought recently is GREAT. Sealed tube too...
I am familiar with Maks. Not a question about Maks over Dall Kirkham. As I am not familiar with the practical use of Dall Kirkham scopes like the Mewlon, I need to ask so I can make the best choice for me.
If I had a choice between an Intes 7" Mak, a kickarse good SW 7" Mak, and a 180 Mewlon, I want to know all about the ins and outs of both designs before deciding on which.
So please no more about Maks for now. I need to ask about Mewlons as I don't know enough about them.
Raymo/Morls - Alex is very familiar with the SW 180 f/15 Maks - and in particular an excellent one I sold a while back to a chap in the Blue Mountains. He's trying to locate examples of a few interesting commercial cassegrain/mak/SCT scopes in the 7-9" aperture range for a specific comparison. There's no shortage of Celestrons/Meade, the challenge is finding other unusual types which are in single digits here.
Scope of interest include Vixen VMC 200, Takahashi Mewlons, SW 180mm maks both the gold ProMak and "Black Diamond" series, Intes M715, M809 or M815 but not the 703/803; the Saxon 203mm mak, ideally a Questar 7 if we can find one (there is at least 1 in Sydney), Kunama's TEC (if only he would drop by on a good night or two) and ... there's my MK91.
And if someone has a working classical cassegrain around that size - even homemade - I'm sure he'd be interested.
I have an 8" VixenVC200L Cass. Similar optical layout to the Mewlon, no corrector, primary & secondary in open tube, correctors in baffle. Visually inferior to a Mewlon but I'd expect it to have very similar dew characteristics due to their similar optical geometry.
Quote:
Originally Posted by mental4astro
Hi all,
I'm trying to decide between a couple of different Cassegrain designs to replace my SCT. I have many years experience with the SCT design, but one Cassegrain design I have no idea about in terms of practical use, is the Takahashi Mewlon. I know they are a Dall Kirkham, so no corrector lenses, and have a superb image, but that is it.
My questions then are:
1, How do they deal with dew? Any particular precautions to take?
They are very resistant to dew. On nights when the exterior of the tube has been dripping wet, the mirror surfaces are dry. Only the finder and eyepiece dew up. All without any dewstraps.
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2, The mirrors are exposed, so are they overcoated to give more protection?
I think both Vixen and Mewlons are overcoated.
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3, Being exposed, how susceptible are they to getting dirty? This I guess is an additional part to the dew factor. I live on the coast in Sydney, so I am concerned.
4, With regular use, how often is it necessary to clean the mirrors?
Dirt of course is a factor of exposure to particulates. Sticky salt air is something I am unfamiliar with. Similar to your dobs? Mirrors can be removed and cleaned. Phil Robustellini cleans his Mewlon 300 every few years. It lives in a ROR shed in an agricultural community.
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So you know, the Cassegrain I end up with will be principally as a visual scope for my sketching of the Moon and planets. Photography will only be of the planets and Moon too, and this will be minimal. I'm looking for a Cassegrain in the range of 7" to 8" for my purposes.
The Mewlon 180, 210 or the CN212 set to cass configuration are probably your best bet. The Vixen is optimised for photography but not visual.
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I think that's it for now that I can recall. Hopefully as discussion progresses here other questions will come to mind, or will be answered even though I didn't ask them...
Alex the M210 is a very nice visual scope, mine was often dew soaked on the outside but the mirror never dewed up.
Things to consider:
It needs to be allowed to cool to ambient or you will be looking at woolly stars
Will the diffraction spikes bother your sketching pursuits, the M210 has 4 spikes, the M180 has 6 slightly fainter ones
DK scopes without correctors will show some coma
Focusing is by primary mirror unless you install a second R&P focuser
There is some minor mirror shift during focusing so always do the last bit of focusing by pushing the primary forward
The removal of the mirror and cell is easy but make sure you replace it in the same orientation or you will need to recollimate it
Do NOT change the collimation screws for Bob's Knobs etc
The collimation holds very well with the original screws
The secondary is not centre spotted so best way to finely collimate is by star but to do that the scope must be thermally at equilibrium.
The finder mounting is good enough for the scope to be carried by it
The finder is bolted on with 2 M6 screws from the inside so could be replaced with a RACi if needed
If you're going to wash the mirror don't wet the baffle tube while doing it...
The mirrors are very well made and coated, a 10 year old mirror was indistinguishable from a new one...
Did I mention that you must let it cool properly if you store it somewhere warm....
Hi Alex, I have the 210. Getting to use it more and more now. I pulled it apart last night to see what's under the hood and fix a few issues I picked up at astrofest. It's a very similar design to an SCT. I flocked the tube as it was pretty bright in there and I had issues with alnitak off field while trying to image the HH. Also pinched optics on the secondary. Collimation screws were tight as and I had three spiked stars close to focus at high mag. Other than that it's built very nicely. The primary cell is a solid design with very minimal flop on the baffle tube. The secondary cell is well designed and I've had no dew issues to date on either. The primary does get dirty like any other open scope but nothing out of the ordinary. I don't know if it has an overcoat. The advantage of the Mewlon is that it's light weight compared to a similar FL SCT and significantly sharper as well. I attached a few shots showing the mechanics.
Diffraction spikes are not a concern to me.
Coma is not a concern either. Heck, I don't bother with a coma corrector in my f/4.5 Newts. Plus as this Cassegrain will be pretty much just for high power, coma is even less problematic. Any photo applications with me will be confined to the Moon and planets with this scope.
I'm sure any mirror flop will be significantly less than the SCT's I've had to deal with...
Thanks for the focusing tip too!
Where I store my scopes it is a cool location to begin with - I try not to let my scopes get warm.
Hi Alex,
I notice a bit of mirror flop on my old u-210, so added a FTF. A great addition, especially with a focus motor.
I occasionally use a dew shield when there's a lot of dew about - adds a bit of weight to front, but is counter-balanced by FTF.
As others have stated, primary mirror does attract dust, but can be blown off easily & gently washed occasionally.
The finder/carrier is awesome at both roles, but I also like to use a RDF. So I bought a slightly longer screw that attaches the primary cell to the tube, and thread it through a RDF base first and then back into the original hole - works fine. Alternatively, you can purchase a K-Astec adapter to piggyback RDF on the mewlon finder: https://www.kyoei-osaka.jp/SHOP/k-astec-fb53.html
Also, I was uncomfortable with the two small screws that attach the Tak dovetail to the foot of the u-210, so I tapped a couple of extra holes in it and added a bigger dovetail bar. I've never heard of the original Tak screws failing, & the u-210 is light, but I feel better knowing the extra screws are there.
Also, I was uncomfortable with the two small screws that attach the Tak dovetail to the foot of the u-210, so I tapped a couple of extra holes in it and added a bigger dovetail bar. I've never heard of the original Tak screws failing, & the u-210 is light, but I feel better knowing the extra screws are there.
Yeah I was wondering about that too. Good idea. I'm going to get rid of the dovetail altogether and tap 4 holes directly through the Losmandy bar. I hate these profiles with one angled side and one straight side. A recipe for disaster.
With my Mewlon 180 and 210 I have never experienced any dewing despite rivulets of condensation running down the outside of the OTA. However, I do use a home-made closed cell foam dew shield to prevent general suburban lights from affecting my viewing.
Mine came fitted with a Vixen style mounting plate held on by M5 bolts which seemed to be adequate, but I ended up replacing both plates with a Losmandy style plate.
I really value their light weight and no-dew performance compared to my C9.25, where I always have to use a dew heater.
Mine have held collimation very well although the x3 secondary collimation bolts are very, very stiff. I did fit Bob’s knobs to the M180 but some time later, I broke off one of the knob heads so re-fitted the Tak socket head cap screws. When I collimate I have the Allen Key tethered to my wrist with fishing line in case I accidentally drop it down the OTA.
I collimate using a ZWO ASI120MM and x2 TeleVue Powermate, so I can inspect the live image on screen in real time. This obviates the need to flit between the front and back of the ‘scope between minor tweaks.
How close to the coast are you? How do you find the mirrors deal with suburbia and coast breezes?
Alex.
Hi Alex
We live in Brisbane, some 22km inland from the coast. The coast is essentially the closed region of Moreton Bay (protected by Moreton Island) rather than a surf beach.
So, I cannot comment of the effects of salt laden breezes although we do live under the flight path of Brisbane Airport and near busy roads so there is a fair bit of airborne pollution - cars, furniture, etc. left outdoors collect grime and dust quite quickly.
I have only washed the M180 mirror once in some 6 or 7 years and it was a straightforward operation.
I like the Mewlons because they are light weight and do not require the fitting of dew heater strips. The views through them are stunning (seeing dependant) although I have not viewed through a modern SCT (my C9.25 is from c2006).
Generally, I would only set up the C9.25 now if I required an F6.3 capability for imaging, as the M210 has become my 'scope of choice as it simply does everything I expect of it with no fuss.
It can be a little frustrating (and expensive) accessorising these 'scopes with adapters etc. compared to the ubiquitous SCT's, but this is something I have gotten used to.
Both my M180 and M210 tubes "fold over" inwards at the open, (i.e. they are lipped) and I wonder if this helps manage any tube currents and/or prevents the 2ndary from dewing up?
Both my M180 and M210 tubes "fold over" inwards at the open, (i.e. they are lipped) and I wonder if this helps manage any tube currents and/or prevents the 2ndary from dewing up?
Cheers
Dennis
It might be that it masks the spider vanes anchors from the aperture, so no added diffraction.
