First Light with Tal 100 RS

[RobertI]

I had a chance last night to briefly test my new Tal 100RS, recently acquired in person from @Chris Lock (thanks Chris!).  I decided to do a side by side comparison with the Megrez 72 (also acquired from Chris a few years ago) which is a scope I know well. I equipped each scope with a Baader Hyperion eyepiece to give magnification of around 50 in each.

Given that I bought the scope for clusters and doubles, and given the fact that I was in the full glare of the streetlights with little chance for dark adaption, I decided stellar objects were the best test.

So first test was the beautiful binary of Almach in Andromeda.  It was immediately obvious that the pair was much brighter through the Tal and the orange colour of the primary was much richer and more obvious. At 10” separation the pair was easily resolved in both scopes as would be expected.

Next was Miram (Eta Persii), another beautiful binary with an orange primary separated from the secondary by 28”. No problems resolving both but again the colour was more vibrant in the Tal and brighter.

By this point I had concluded that the focused star in the Tal was not as pinpoint as in the Megrez, and also the focus point was more difficult to find in the Tal (the single speed focuser did not help). Also the defocussed star was not as circular as the Megrez, but not sure what this means, or what effect it may have. But these shortcomings were more than made up for by the increased brightness and colour of the stars, which only highlighted the wonderful contrast of the stars against the jet back sky.

On to a couple of clusters.

The double cluster in Perseus was really lovely in the Tal; bright, glistening and fitting perfectly into the field of view with lots to explore. The view in the Megrez was much less spectacular, in fact I had to look up to see whether a cloud had floated across the view! The relative star colours also seemed more pronounced in the Tal.

A quick look at M34 confirmed the Tal’s superior capability to show open clusters in all their glory.

M13 was unremarkable in both scopes, with just a hint of resolving the outer stars in the Tal, but a million miles away from the C8’s ability to resolve hundreds if not thousands of stars in the cluster.

I am not sure what CA looks like in a scope (I have only had semi-apos) but when looking at Vega I could not detect anything different to the Megrez. I guess I need to try it on something brighter like the moon to see anything like that, but I was very pleased at the absence of any obvious ‘purple halos’.

So overall I am extremely pleased with the scope so far, especially given the price! It’s a bit unwieldy like any scope of this size and needs a solid mount like the Skytee to hold it steady. It could also do with a better, two speed focusser. However the additional 28mm over the Megrez makes a big difference on star brightness and colour.

When the conditions are right I shall try some higher magnifications and have a go at testing the real world resolving power on some tight doubles.

[iPeace]

Nice review! 

[Chris]

Hi Rob, I don't know if having an invested interest in the review made it more enjoyable to read but I really did enjoy that, and by the sounds of it so did you  I couldn't see any sign of CA on bright stars either, I think I mentioned I thought it behaved a bit like an ED scope  

I quite liked the smoothness of the focuser on the Tal, but then again that Meg of yours is a very good scope, so maybe it's just too tough a competition for the Tal in that respect. Still, it sounds like the Tal got it's own back regarding resolving power and star colour, amazing what an extra inch will show you   

Not sure why a defocused star wouldn't be round though, can you say a bit more about this Rob? 

[RobertI]

29 minutes ago, iPeace said:

Nice review! 

Thanks!

 

5 minutes ago, Chris Lock said:

Hi Rob, I don't know if having an invested interest in the review made it more enjoyable to read but I really did enjoy that, and by the sounds of it so did you  I couldn't see any sign of CA on bright stars either, I think I mentioned I thought it behaved a bit like an ED scope  

I quite liked the smoothness of the focuser on the Tal, but then again that Meg of yours is a very good scope, so maybe it's just too tough a competition for the Tal in that respect. Still, it sounds like the Tal got it's own back regarding resolving power and star colour, amazing what an extra inch will show you   

Not sure why a defocused star wouldn't be round though, can you say a bit more about this Rob? 

Thanks Chris, glad you enjoyed.   Regarding the non circular defocussed star, what I saw was a slightly irregular circle, with slightly flattened edges, but the defocussed disc remained concentric and circular. Hmm, not explaining myself very well. To be honest, I think I will treat that with a pinch of salt and try some higher mags next time to see what the diffraction rings look like, that may give a better idea. I probably over emphasised the flaws of the Tal in my effort to do a thorough review; at the end of the day it's all about what the scope can do for me, and basically I loved the views through the Tal and it will be the first scope I grab for some star action this autumn and winter.  

[Chris]

Hi Rob, lol I think I just got a bit worried in case I'd sold you a scope with pinched optics or astigmatism or something not good like that, but from your description I think it might be seeing related, but please let me know how it goes  On a 'brighter' note, I'm really pleased you loved the views through the Tal, that and saying it will be the first scope you grab for star action says it all   

[RobertI]

Worry not Chris, I love the scope.  

[laudropb]

Very nice review. I have always fancied getting one of these scopes. I will keep an eye out for when one becomes available.

[Roy Challen]

Yep, can't go wrong with a Tal RS!

Changing the focuser bearings and roughing up the contact area for the spindle improves the focuser no end. As does ensuring they are correctly adjusted- a trial and error task.

[RobertI]

Well, I have had a couple of clear nights recently and took the opportunity to play with the Tal 100RS some more. In my first outing above I alluded to the fact that there appeared to be star spikes and the defocussed star was not round. Well to cut a very long story short it turned out that I had overtightened the three grub screws which attached the dewshield to the scope, with the result that the lens was deformed. Loosening them slightly completely resolved the problem and now I have lovely tight stars, right to the edge of the FOV with no spikes and perfect defocussed circular stars. In my latest two sessions I have confirmed the scope's excellent abilities, viewing a variety of open clusters and doubles. On coloured doubles and coloured stars, the more magnification applied, the more colour was visible. The Garnet star in Cepheus was superb.  The highest mag I could get was x200 with the BST starguider, which definitely made the tighter doubles much easier to resolve, but stars were not so tight at this mag, probably due to the less than perfect seeing. I actually ended up using the Seben zoom 8-24mm eyepiece and it worked really well to view a range of easy doubles at the best magnification to make them shine. I did find the length of the scope meant I had to contort into all kinds of positions, I think a viewing chair might be needed. I'm really looking forward to more observing with the Tal over the coming months.

[Chris]

I know we've talked about this via PM but I'm really pleased you've got it all sorted  Yes, get yourself sitting down chap else you'll put your back out

[RobertI]

15 minutes ago, Chris Lock said:

I know we've talked about this via PM but I'm really pleased you've got it all sorted  Yes, get yourself sitting down chap else you'll put your back out

Yes I've already got a crick in my neck! 

[Roy Challen]

A pillar extension for your mount would help with that, but a chair would also be good - and probably cheaper!

[Fozzie]

16 hours ago, RobertI said:

Well, I have had a couple of clear nights recently and took the opportunity to play with the Tal 100RS some more. In my first outing above I alluded to the fact that there appeared to be star spikes and the defocussed star was not round. Well to cut a very long story short it turned out that I had overtightened the three grub screws which attached the dewshield to the scope, with the result that the lens was deformed. Loosening them slightly completely resolved the problem and now I have lovely tight stars, right to the edge of the FOV with no spikes and perfect defocussed circular stars.

I knew Tal's have a reputation for "robustness" but what.. really? the grub screws deformed the lens...?  How big are the grub screws?

 

Glad your enjoying the long achro and 72mm ED combo.. I find that the 200p I have doesn't get a look in..

I'd say pier extension is a better option, camping chair is all you need then.

Ta

Fozzie

[Roy Challen]

41 minutes ago, Fozzie said:

I knew Tal's have a reputation for "robustness" but what.. really? the grub screws deformed the lens...?  How big are the grub screws?

 

Good point, my 100RS has a push on dewshield, and my previous 100R had grub screws that were almost microscopic.

OP, are you sure the lens cell wasn't overtightened?

[Chris]

17 minutes ago, Roy Challen said:

Good point, my 100RS has a push on dewshield, and my previous 100R had grub screws that were almost microscopic.

OP, are you sure the lens cell wasn't overtightened?

Hi Roy, I had the Tal before Rob and that particular scope had 3 small grub screws that attached the dew shield to milled out flats on the outside of the lens cell. The flats are probably close to where the doublet lens is so it could well be that over tightening could apply a bit of pressure to the glass.... I mean I'm surprised! but I now believe it.

[Roy Challen]

Hmm, interesting. I've heard that some of these dew shields don't fit amazingly well (mine's ok though), so I guess this could be a case of someone trying to cure that problem. And creating a new one.

[RobertI]

 

35 minutes ago, Roy Challen said:

so I guess this could be a case of someone trying to cure that problem. And creating a new one.

I agree! It was me that created the problem, it was fine when I bought it off Chris!  

The grub screws are right over the lens cell when the dewshield is at its furthest extension, so perhaps not a clever design. I could move the dewshield back but the grub screws would be against the tube and potentially dent it. The grub screws are pretty robust, with a hex socket so one can accidentally apply too much pressure with an allen key. The reason I tightened them was that the dewshield could be moved with some pressure applied and I thought that the dewshield should be solid as a rock, but I see now that its part of the design that the dew shield should be 'lightly' fixed to the ota to prevent issues.