Do I need a 5” ED doublet?

[Deadlake]

Going back to 4 vs 5 " the mount is the main factor. 

I suspect if the mount was the same weight for both the 4 and 5" then the 5" would get most use if the cool down time is similar.

I actually find my 5" LZOS perfectly balanced (the extra 1.4 kg of the FT3545 focuser versus the APM stock focuser) and the 4" has poor balance, the FT3545 tips it over and the only way of mounting is using the Y-axis adapter or a Harmonic mount.

Has anyone used the AZ75 with the Y-axis adapter?

Edited December 5, 2023 by Deadlake

[Cjg]

12 hours ago, IB20 said:

The AZ75 holds 15kg on each saddle depending on length of OTA. It should handle it relatively comfortably, I’d imagine? 🤷🏼

What do other members mount their 5” scopes on?

A no longer made Altair Sabre Mount.

Once balanced, it really is a joy to use; the only thing lacking are the 'slow-mo' cables.

[StevieDvd]

48 minutes ago, Cjg said:

A no longer made Altair Sabre Mount.

Once balanced, it really is a joy to use; the only thing lacking are the 'slow-mo' cables.

 

These are still around there's even a 2023 Sabre model.

 

[Stu]

2 hours ago, Cjg said:

the only thing lacking are the 'slow-mo' cables.

Sssssssshhhhh! 🤐🤐🤐

[DirkSteele]

Probably not adding much having read all the replies but I would be big advocate for a 5” scope when you already own a 4”. The increase in light grasp is as much as 70% which is very noticeable on DSOs at the eyepiece and the extra resolution on planets and double stars is also obvious. Given average seeing conditions, 5” will often show all that can be seen on the planets.

It is however, amazing how quickly the scope seems to scale up with that extra 25-30mm of aperture. Even a lighter weight doublet will be that much longer such that moment of inertia becomes a factor meaning a more robust mounting solution is desirable.

The below image exaggerates things a little as the 105/650 has a long sliding draw tube to keep transportation length down but compare it to my 130/1200 (both set to transportation length).

On to the mount. My Tele Optic Ercole is more than up to the task of 2 4” class refactors, both triplets which with finders, diagonals and eyepieces was probably c.16kg.

 

But it started to struggle (above 200x magnification) with my 130/1200 which is around 12kg with the same accessories.

 

[F15Rules]

On 05/12/2023 at 20:04, IB20 said:

What do other members mount their 5” scopes on?

My 5" sits on a 25 year old Tak EM2s Equatorial mount on a hardwood custom fixed height tripod (I have a Geoptik Nadira adjustable height astro chair to compensate for OTA tube altitude). The mount is quite similar to a Vixen GPDX in size and heft, and I have successfully mounted this scope on a GPDX before. I would say that the EM2 does have a slight engineering edge though.

The mount has a nominal capacity of c 10kg, however, in typical Japanese fashion their load tolerances do seem very conservative. I would say that this mount could handle 12 kg or more, especially for a shorter scope like a Mak or SCT.

My FS128 5" is light for its size.. just 7.5kg. Add finder, binoviewers, diagonal, eps and its up to 9-9.5kg. Balancing of the OTA in both axes is crucial!

When properly balanced, the whole rig is very stable, with no more than 1 second settling time. A solid wooden tripod I believe, makes a big difference.

I do only need to lift the set up a maximum of 5-6 metres to my observing position.

HTH..😊

Dave

 

 

 

Edited December 6, 2023 by F15Rules
Text Correction

[John]

On 05/12/2023 at 20:04, IB20 said:

.....What do other members mount their 5” scopes on?

My 5.1 inch F/9.2 is mounted on an alt-azimuth mount like the rest of my scopes. The Skytee II and the Giro Ercole will carry the scope but it really needed something more solid. I use the T-Rex alt-azimuth mount which is now, sadly, long out of production since the Japanese engineer who designed and built the mounts passed away in 2016. It has about the same capacity as the Rowan AZ100.

 

[Cjg]

14 hours ago, StevieDvd said:

These are still around there's even a 2023 Sabre model.

 

Yay! Can highly recommend the mount, it’s great news that it’s available! 

[Mr Spock]

I've just noticed the StellaMira 125mm has dropped in price to £1359. That's so tempting.

[ScouseSpaceCadet]

On 05/12/2023 at 09:52, IB20 said:

Having 76mm and 102mm “APOs”, how much benefit will I see if I acquire a 5” ED doublet?

How often do users take their 5” scopes out? How long do they take to acclimate and how affected are they by local or atmospheric seeing?

The scope in question is the Stellamira 125mm and so far the reports seem really good. It’s quite long at 1m extended but it seems extremely light compared to most other 5” OTAs. My longest scope is 820mm so I don’t think the length would surprise me. 

The thought of sitting in my garden looking at summer doubles with this scope is an incredibly enticing one!

 

 

 

I've not used my 127mm Mak or 102mm f7 refractor since buying the Stellamira 125mm. Being visual only I'd describe the image through the eyepiece as tack sharp, CA free and brighter than the 4".  Bear in mind the ota being carbon fibre makes it around 3 kg lighter than an equivalent aluminium scope. The tube seems to cool down at about the same speed as the 4" and is affected by seeing conditions much the same.  Maybe this is pure luck or something to do with the carbon tube but the objective lens is yet to dew despite observing without heated dew bands. Even at damp by default Kielder when the SM125 was pointed toward the sky for about 5 hours each night the objective was dry. 

I'm really looking forward to lunar observing with the Stellamira. Weather and work just haven't aligned. I can imagine I'll be very happy when the time comes!

 

Edited December 7, 2023 by ScouseSpaceCadet

[IB20]

6 hours ago, Mr Spock said:

I've just noticed the StellaMira 125mm has dropped in price to £1359. That's so tempting.

Yes, sorry! 😅😈

[Deadlake]

14 hours ago, DirkSteele said:

Probably not adding much having read all the replies but I would be big advocate for a 5” scope when you already own a 4”. The increase in light grasp is as much as 70% which is very noticeable on DSOs at the eyepiece and the extra resolution on planets and double stars is also obvious. Given average seeing conditions, 5” will often show all that can be seen on the planets.

It is however, amazing how quickly the scope seems to scale up with that extra 25-30mm of aperture. Even a lighter weight doublet will be that much longer such that moment of inertia becomes a factor meaning a more robust mounting solution is desirable.

The below image exaggerates things a little as the 105/650 has a long sliding draw tube to keep transportation length down but compare it to my 130/1200 (both set to transportation length).

On to the mount. My Tele Optic Ercole is more than up to the task of 2 4” class refactors, both triplets which with finders, diagonals and eyepieces was probably c.16kg.

 

But it started to struggle (above 200x magnification) with my 130/1200 which is around 12kg with the same accessories.

 

Do you find the 105 /f6.25 a little tail heavy, I was thinking of swapping the FT3545 for the 2.5" flavour but the price....

 

[Deadlake]

Astrograph have a similar 125 mm, slightly heavier however it has a minimum Strehl on the green line of 0.95.

https://astrograph.net/epages/www_astrograph_net.sf/en_GB/?ObjectPath=/Shops/www_astrograph_net/Products/AGTEC125F78

I suspect to beat this it is TSA 120 etc territory... However for most nights would you tell the difference between a 0.95 or 0.98 Strehl scope, Thomas Black stated no.

[DirkSteele]

29 minutes ago, Deadlake said:

Do you find the 105 /f6.25 a little tail heavy, I was thinking of swapping the FT3545 for the 2.5" flavour but the price....

 

Opposite actually. Only has the 2” Feathertouch on it so the heavy lens pulls it forwards so need the scope far back in the tube rings to balance it. But balance it does when I do that.

[Deadlake]

2 hours ago, DirkSteele said:

Opposite actually. Only has the 2” Feathertouch on it so the heavy lens pulls it forwards so need the scope far back in the tube rings to balance it. But balance it does when I do that.

Going with a lighter focuser will just move the balance point and not necessary to make the scope balanced. 
Harmonic mounts here I come...

[IB20]

7 hours ago, Deadlake said:

Astrograph have a similar 125 mm, slightly heavier however it has a minimum Strehl on the green line of 0.95.

https://astrograph.net/epages/www_astrograph_net.sf/en_GB/?ObjectPath=/Shops/www_astrograph_net/Products/AGTEC125F78

I suspect to beat this it is TSA 120 etc territory... However for most nights would you tell the difference between a 0.95 or 0.98 Strehl scope, Thomas Black stated no.

What is on OWL? Optical white light? OTA with lickables?🦉

[globular]

1 hour ago, IB20 said:

What is on OWL? Optical white light? OTA with lickables?🦉

I'm guessing, but I think it's Ohara With Lanthanum.
i.e. it is a doublet with Ohara FPL-53 glass and a lanthanum element.

[Deadlake]

If you didn’t go for the 5” you could go for one of these 😁

Edited December 8, 2023 by Deadlake

[IB20]

The VAT alone is making me wince. 😅

[Mr Spock]

If I win that £200 million tonight I might be setting up an obsy with one of those 300mm f9s... 

[Space Hopper]

3 hours ago, Deadlake said:

If you didn’t go for the 5” you could go for one of these 😁

That makes my scope seem like great value. Especially at one quarter of the price of this (ok it was 5 years ago, but still)

[DirkSteele]

3 hours ago, Deadlake said:

If you didn’t go for the 5” you could go for one of these 😁

I think it is fantastic that someone is trying to push what is possible in optical technology forward (though I think we are probably spending the $1mn to get 1/10th of second lap time improvement like in F1 here) but I fear for the prospects of the company here as even idiots like me who spend way too much money on scopes won't spend 20k on a 4" scope.

 

Still, if they want me to review one, I am more than happy to put it through its paces.

[Mr Spock]

I think we should stick to the 125mm ED as being a more realistic scope 

[Elp]

I would also consider the Askar 120 triplet (or dare I suggest the 140?). But one inch difference, you'd have to have fantastic darkish skies to notice the difference from my experience trying to see things from a bortle 7. I can just about make out the faintest smudge of Andromeda with the Starfield 102.

Edited December 8, 2023 by Elp

[Cjg]

4 hours ago, DirkSteele said:

I think it is fantastic that someone is trying to push what is possible in optical technology forward (though I think we are probably spending the $1mn to get 1/10th of second lap time improvement like in F1 here) but I fear for the prospects of the company here as even idiots like me who spend way too much money on scopes won't spend 20k on a 4" scope.

 

Still, if they want me to review one, I am more than happy to put it through its paces.

If you are going to, please, do it well before next April's Cwmdu.🤣  and there'll be a huge queue at 'frac alley!