Skywatcher ST 102 would 2in diagonal and EP’s be better than standard 1.25?

[Andrew stuart]

As the headline suggests I have a ST 102 refractor and using it more now as grab and go on a AZ GTI mount. So question is would I get better observations with a decent 2in diagonal to go with a couple of 2in EP’s I already use in my 10in Dobsonian. Thanks in advance 

[Knighty2112]

2 inch EP’s are generally best for wide field views, so not sure if views would be any better than what you already see in your 10  inch dob with your 2 inch EP’s as both focul lengths of both scopes are pretty much the same at about f4.7/4.9. The views might be wider than what 1.25” EP can show, but its down to you if the cost of a 2” diagonal to get these wider views. If you find the 10” dob a chore to lug out and set up then maybe it is worth it with the quicker setup time of the ST102 ‘frac to get these views with the 2” EP’s, although the extra weight of the 2” diagonal does add more weight to balance out on you mount you use.

Edited August 14, 2022 by Knighty2112

[Paz]

If you have a number of large dsos you are interested in observing then the larger true field of view that is possible in the st102 is an advantage.

I have had a st120 for many years and for most of that time I only used 1.25" eyepieces to keep the set up simple amd lighter. In the last year or so I got a 2" diagonal so that I could use my 2" eyepieces with my refractors (I previously only used them in my relector). In practice  I find I dont use 2" eyepieces much in my refractors as the light pollution is enough to make the background quite bright and bright enough to wash out many of the large dsos i might be trying to see. 

You would get similar exot pupils with the st102 as the dob if they have similar focal ratios, and if that is the case then if you can use 2" eyepieces to your satisfaction on the dob with your skies then theres a good chance of them working ok on the st102.

[Cosmic Geoff]

With the ST102 a 25mm 1.25" eyepiece gives x20 magnification.  The exit pupil will be 5mm.  I would question whether lower magnifications possible with a 2 dia eyepiece would deliver much benefit without making the exit pupil too large and wasting light.

[Andrew stuart]

Thank you for the informative replies, all good. I am fortunate to live in the countryside with no light pollution and my Dob serves me very well for star hopping, bright DSO’s, clusters etc but lugging it around gets a bit much. I love my AZ GTI with the ST102 and want to try my 26mm and 38mm 2in Panaview. However contrary to these I have a 1.25 16mm OVL Nirvana 82’ which I love. so now need a decent 2in diagonal with 1.25 adapter. 

[catburglar]

I had great views of the entire Veil nebula and the North America nebula with an ST120, 30mm 80 deg AFoV eyepiece and OIII filter. 
 

The slightly shorter focal length of the ST102 means that you could get similar FoV with a 25mm 68deg AFoV eyepiece.

It’s true that at exit pupils larger than 5-6mm lead to bright background sky, but with filters this is reduced and and contrast is enhanced, and a 30mm 70-80deg AFoV eyepiece will give a bit more area around these large objects that can make it easier to determine where they start and end…

I’d be tempted to give it a go TBH…

[Zeta Reticulan]

I predominantly use my ST102 with 2" eyepieces. 

EP's vary a bit: 36 & 31mm Baader Aspherics, 27mm Panoptic and 18mm ES (19mm Luminos shown below). 

If I want higher magnification I usually use a zoom. 

[Moonshane]

In your position I'd certainly get a 2" diagonal and use the eyepieces you have already. If possible buy a dielectric model but it's not critical.

[Louis D]

I'd recommend checking the classifieds for a GSO 2" dielectric.  I've got three of them under various brands that I keep with various scopes.  They are not light, though.  I don't think I paid more the $70 for any of them.  Admittedly, used prices and availability may be different in the UK/Europe.

[Zeta Reticulan]

These are probably the best value for money. At 390g it's the lightest 2" diagonal I own.

Edited August 18, 2022 by Zeta Reticulan

[Louis D]

9 hours ago, Zeta Reticulan said:

These are probably the best value for money. At 390g it's the lightest 2" diagonal I own.

Does it have the restriction at the bottom of the eyepiece holder like the 1.25" version?

[Mr Spock]

2 minutes ago, Louis D said:

Does it have the restriction at the bottom of the eyepiece holder like the 1.25" version?

For this reason I'd recommend a Revelation 2" Quartz Dielectric. Unlike other 2" diagonals it doesn't restrict. Works great on my 102mm with my 42mm LVW. No vignetting and a wide, 3.85° field of view. Great for star fields.

 

[Louis D]

Yep, that's a GSO dielectric.  I've got them under the TPO and ScopeStuff labels as well.

[Zeta Reticulan]

1 hour ago, Louis D said:

Does it have the restriction at the bottom of the eyepiece holder like the 1.25" version?

Probably. Is it that much of a problem? The only 2" EP I use in it is a 30mm SuperView. In an ST80. Although the diagonal itself was originally bundled with my 80ED DS Pro. 

Edit: It gives a 6mm exit pupil (13.3x) which is about right for rich field IME. I don't recall any vignetting.

My other 2" dielectrics are an Altair twistlock, Baader Maxbright and a TV Everbrite. The Baader has an oversize mirror.

Edited August 18, 2022 by Zeta Reticulan

[ollypenrice]

On 14/08/2022 at 17:37, Knighty2112 said:

2 inch EP’s are generally best for wide field views, so not sure if views would be any better than what you already see in your 10  inch dob with your 2 inch EP’s as both focul lengths of both scopes are pretty much the same at about f4.7/4.9. 

The focal length of the ST102 is 500mm and that of the Dob is 1270mm. That's an enormous difference.

I used to have a TV Genesis F5, also with 500mm FL, and absolutely loved using it with widefield 2 inch EPs. The ST102 will have much more distortion at the edge of field but, still, you're getting a binocular field in a telescope, really, and there's so much to enjoy.

Olly