Alkaid 12 - a few questions :)

[BGazing]

Yes, the thread title is a knock-off of @Piero's Tak topic.

I've read many good things about Sumerian scopes (especially the latest generations). There were some hiccups but they appear to have been sorted out and the production is reduced and streamlined. TS takes European orders and imposes uniform mirror choices. Unlike @FLO, TS is not fun to deal with, slow to answer and the answers are terse and sometimes cryptic. Michael (who makes the scopes) does not appear to answer the mails (at least not yet to me) so I am left with the collective wisdom of the forum to pry some more information.

I understand that the 12 inch case weighs somewhere around 14kgs and the rest are truss poles. I'd be grateful if anyone comes up with the exact figure. Total weight is 16.5 as per both websites.

F RATIO AND BALANCE - Most reviews suggest that 10 inches version are bottom light and that they are not suitable for heavier eyepieces, and I presume Paracorr. 12 inches should have a better balance due to heavier mirror. Going F4 would make it more stable, sticking to F5 would make it easier to collimate, be easier on the EPs and (who knows) dissuade me from buying a Paracorr...straight away. I do notice coma at F5...

QUARTZ or PYREX - TS streamlined mirror offers to GSO Pyrex and TS branded quartz mirrors. At F5, both are available, at F4, only Pyrex. Quartz glass adds about 400eur to the bill, but the mirror seems to be 39mm thick just like Pyrex, presumably to avoid astigmatism because of the design of the cell. Lighter mirror would also be problematic because of the balance. So Quartz or not? There are fans behind, hopefully helping the cooldown and dealing with the boundary layer. Pyrex is slower to cool, but how much does that matter when you have the fans? Does Quartz offer much benefit when they are of the same thickness? GSO is Pyrex, Quartz is TS branded, anyone had any experience comparing them? 

So here are the options:

F5 and Quartz - more expensive, easier on the EPs (although mine are all TV in 7-24mm range). Might still itch for Paracorr on top of it, adding to the bill. I know I will, sigh. Less stable than F4 balance-wise, although T6 plus Paracorr is almost like an Ethos.

F4 - more stable, more demanding on EPs, Paracorr a must (there goes the balance advantage, although still less strain than F5 with Paracorr). Pyrex only.

F5 in Pyrex, easier on EPs than F4, easier collimation, slower cooldown, again Paracorr dilemma.

EQ Platform or not? Sumerian platforms are listed at 2 kgs weight...I find that hard to believe.

What are your thought and suggestions? 

 

 

[Piero]

54 minutes ago, BGazing said:

F RATIO AND BALANCE - Most reviews suggest that 10 inches version are bottom light and that they are not suitable for heavier eyepieces, and I presume Paracorr. 12 inches should have a better balance due to heavier mirror. Going F4 would make it more stable, sticking to F5 would make it easier to collimate, be easier on the EPs and (who knows) dissuade me from buying a Paracorr...straight away. I do notice coma at F5...

QUARTZ or PYREX - TS streamlined mirror offers to GSO Pyrex and TS branded quartz mirrors. At F5, both are available, at F4, only Pyrex. Quartz glass adds about 400eur to the bill, but the mirror seems to be 39mm thick just like Pyrex, presumably to avoid astigmatism because of the design of the cell. Lighter mirror would also be problematic because of the balance. So Quartz or not? There are fans behind, hopefully helping the cooldown and dealing with the boundary layer. Pyrex is slower to cool, but how much does that matter when you have the fans? Does Quartz offer much benefit when they are of the same thickness? GSO is Pyrex, Quartz is TS branded, anyone had any experience comparing them? 

Hi,

There are many factors determining the balancing of a dobson, not just short f-ratio or heavy mirror. The size of the altitude bearings as well as the number and how teflon squares are installed, are also very important. Being a truss design, the telescope is very portable at both F4 and F5. Between the two I would opt for an F5 (you did not mention, but you will also need an adjustable chair). 12" F4 is as tall as a 8" F6 or 10" F5. For that size, a solid tube offers more advantages in my opinion. 

F4 dobsons are nice but many factors become critical aside from eyepiece quality and coma. Mirror cell, misalignments, mechanics, etc are all more demanding.

A pyrex mirror takes more time to cool down, but if the outside temperature continues decreasing, it can easily be that the mirror will never cool down enough. A decrease of 1C per 45min is more than sufficient to cause this. When the mirror is cooling, the optics look overcorrected and can show some astigmatism (which dominates coma). Sure, you can use the telescope at low power during these situations, but at some point you will also want to use it at high power. A cooling fan will help decrease the mirror temperature faster and also minimise temperature differentials. A Quartz mirror expands less, which can be beneficial particularly if the temperature in your location changes a lot during the night. 

Regarding a coma corrector, the general advice is that a coma corrector is needed at F4, whereas one should wait and see before buying one at F5. The region where the optics are meant to have a Strehl value >= 0.80, namely the coma "free" region, is very limited. An overestimation of the angular coma free region is about 16 deg AFOV for an F4 and 25 deg AFOV for a F5. Therefore, although it is possible to live with coma, the previous sentence means that the views through an Ethos eyepiece and F5 mirror with 0.99 Strehl (excellent), will be <0.80 Strehl (average and less than average) from 25 deg AFOV to the edge. In my opinion, if one decide to go for fast optics and UWA eyepieces, a coma corrector should be included. Why would you get a Ferrari, and decide to get the tyres from a Fiat Punto for saving money? 

I would also check that the secondary mirror is well supported. This is often the main cause of misalignment when the telescope is moved in altitude as the weight of the secondary moves from 4 supports to just 2. A good focuser is also important in order to maintain this axial alignment. If you don't have one, I would suggest the purchase of a good collimator - a one-off purchase -. The latter can help you identify many potential weaknesses of your telescope and therefore how your can make improvements.

Piero

Edited September 12, 2019 by Piero

[garryblueboy]

I to had A Sumerian 14” ultra light with Zambuto Quartz optics the build was very good and it was very light weight it did struggle with two things large heavy eyepieces like Ethos and the mirror sling made collimating very hit and miss also the flex in the truss poles made the issue problematic the Quartz mirror did cool down slightly quicker than previous Pyrex I had but that’s really due to the thickness As Piero says about the coma and use of Paracorr I have one for my now F4 but found it not necessary on the Zambuto  

[BGazing]

@Piero @garryblueboy thank you both.

Quartz should flex less while cooling so less astygmatism. But cooldown might be just as long, as @Piero noted. It appears that the older models had more problem with flex and slop, I have read better reviews recently, and that might be due to streamlining of production. Prior runs were often custom-made with many little tweaks.

@garryblueboy was that 14 inches f5? trusses look looong...and the focuser is FTF, I believe a bit heavier than Moonlight CR2 they use now. Did it struggle with Ethos WITH Paracorr or without...and which Ethos...17 and 21 are true beasts. Did it go out of collimation when changing altitude? I've not read similar experiences with recent 12 f/5s...although have no idea whether it was Paracorr or not. Paracorr plus T6 is, I believe, below 700grams...

@Piero any particular suggestions for collimation tools?

[garryblueboy]

Hi it was without Paracorr F5 and yes when moving up and down the collimation would drop out  it was a 21 and 17 Ethos 

[estwing]

Problem is.....

These lightweight scopes are made with lightweight ep's in mind. You really can't expect this design not to flex when putting a 21 ethos in .

[Stu]

I've got a 14" Alkaid, and previously had a 16" Canopus. I know the dob boys thought the Canopus was floppier than a boneless chicken 🤣🤣 but as my first experience with a truss dob I was quite happy with it.

These scopes are always a balance between portability and rigidity obviously, but in my case portability wins out.

The Alkaid is clearly another step along (down?) the portability road, and it does show in a few areas. Rigidity is less than the Canopus I would say from memory, but then portability is on another level. It is the difference between being able to take a large aperture scope away with me and not.

I assume all Alkaids are the same, and in mine you have to remove the secondary every time in order to pack it down into the box. That means secondary collimation needs doing from scratch pretty much each time. With the right tools that is not too much of a challenge, particularly as one of the secondary adjusters is fixed so that gives a good reference.

The upper assembly is just the single layer, and as standard just has an RDF attached to it which is functional but not great obviously if looking for more challenging targets. I've thought about adding either a TelRad or even an optical finder but it is not necessarily that easy to find somewhere to mount it. I have tended to stick to more familiar objects as a result, whereas, for example, I had Nexus Push-to on the Canopus which really helped when trawling around the Virgo Cluster for instance.

I think in terms of mirrors I would go with whatever is heaviest! That would tend to help with balance. Assuming quartz and pyrex were similar thickness and rigidity then I guess it's just a cost exercise? I would avoid anything which may require a very advanced primary cell in order to avoid astigmatism because that is not what the Alkaid has.

I have had similar problems as Garry says with collimation, particularly when observing objects at lower altitudes. The mirror clips are a long way above the mirror surface and the mirror has a habit of flopping forward on the sling when down at the horizon. That is one reason why I avoided the low down objects when observing in Wales recently; the collimation got noticeably worst plus it was just pretty awkward. I need to see if I can adjust the clips closer but it is not a five minute job.

Hope that is of some use.

[BGazing]

Interesting @Stu, your Alkaid is 14, so larger than the production offered now. Have not read about mirror slip in 12s. How low was your low...I guess many nice objects are tantalizingly low up there, especially in summer. I tend not to observe DSOs under 20 degrees even at my dark site.

Mirror weight seems to be the same no matter the option and the thickness is indeed to prevent astygmatism on a simple cell.

Paracorr plus T6 is about the same weight as Ethos 17. Ethos 21 is on another level, a true beast. @GuLinux wrote that he experimented with 1kg and it was fine.

Has anyone tried Sumerian's EQ platform. It looks unbelievably light at 2.5 kgs...it appears not to be a big issue unless one is sketching as Alkaids seem to track smoothly.

Edited September 13, 2019 by BGazing

[Stu]

4 minutes ago, BGazing said:

Interesting @Stu, your Alkaid is 14, so larger than the production offered now. Have not read about mirror slip in 12s. How low was your low...I guess many nice objects are tantalizingly low up there, especially in summer. I tend not to observe DSOs under 20 degrees even at my dark site.

Mirror weight seems to be the same no matter the option and the thickness is indeed to prevent astygmatism on a simple cell.

Paracorr plus T6 is about the same weight as Ethos 17. Ethos 21 is on another level, a true beast. @GuLinux wrote that he experimented with 1kg and it was fine.

Has anyone tried Sumerian's EQ platform. It looks unbelievably light at 2.5 kgs...it appears not to be a big issue unless one is sketching as Alkaids seem to track smoothly.

Yes, I think the 14" was a custom build, not available generally.

I was wanting to observe M8 and M22 which are down at 13 degrees or so, pretty low and towards the limits of the alt bearings.

I didn't mention that I was able to use my 30mm ES 82 degree eyepiece, although it does create some flex but within acceptable limits for me, although I needed to use both elastic tensioner to stop the scope sliding down with the weight.

I haven't yet used it with my Watchhouse EQ platform but have found it very beneficial at high powers for planetary and lunar observing with my 8" f8. I think that at low to mid powers with a smoothly moving scope it is less of a requirement.

[BGazing]

30mm is a proper 1kg godzilla, no wonder it flexed 14 inches at F5.

[Stu]

Just now, BGazing said:

30mm is a proper 1kg godzilla, no wonder it flexed 14 inches at F5.

It's f4.5, but yes it's a beast.

It can be a little frustrating having to adjust the tension using the elastic bungies when switching between eyepieces of very different weights.

[Piero]

Regarding your question about collimators, I use the 2" Glatter laser with 1mm aperture stop and 2" Glatter tuBlug for adjusting the focuser and primary axial alignments. I prefer the Catseye Telecat XL for centering the secondary mirror under the focuser, a once in a while adjustment.

[BGazing]

1 hour ago, Stu said:

It's f4.5, but yes it's a beast.

It can be a little frustrating having to adjust the tension using the elastic bungies when switching between eyepieces of very different weights.

That was with or without Paracorr?

[BGazing]

28 minutes ago, Piero said:

Regarding your question about collimators, I use the 2" Glatter laser with 1mm aperture stop and 2" Glatter tuBlug for adjusting the focuser and primary axial alignments. I prefer the Catseye Telecat XL for centering the secondary mirror under the focuser, a once in a while adjustment.

How different is that combo from say...this https://www.teleskop-express.de/shop/product_info.php/info/p754_Baader-Justierlaser-fuer-Newton-Teleskope-fuer-genaue-Justage.html

[Stu]

1 hour ago, BGazing said:

That was with or without Paracorr?

Without. I don't find the coma objectionable and prefer to keep the weight down.

[Piero]

5 hours ago, BGazing said:

How different is that combo from say...this https://www.teleskop-express.de/shop/product_info.php/info/p754_Baader-Justierlaser-fuer-Newton-Teleskope-fuer-genaue-Justage.html

I have never used that collimator so, cannot comment.

It's a 1.25” model, so it's fine if you use 1.25" eyepieces. If you use 2" eyepieces, you should make sure that your 2-to-1.25 reducer does not affect the focuser axial alignment. Examples of reducers that do not affect the alignment are the Baader clicklock and the Glatter's reducer.

Edited September 13, 2019 by Piero

[BGazing]

Does anyone have any ideas where TS sources its quartz mirrors? GSO quartz mirrors (one, to be precise) did really well in a double-pass test of Tommy Nawratil and Wolfgang Rohr.