Need help on best solution for William Optics ZS66 focussing...

[ibraidwo]

Hi all,

Can someone provide a recommendation for the best way forward with using 1.25” EPs on my recently purchased William Optical Petzval ZS66, which I plan to use as a travel astronomy scope.

Apparently the WO 1.25" 45 degree erecting prism (with 2" barrel) it came with should only really be used for camera work. When I tried it to look at the heavens with 1.25” EPs the focuser has to be at the furthest inward stop to get focus, and is not focusable when applying a 1.25” barlow.

At this stage I have a number of 1.25” EPs so don’t yet want to move onto 2” ones.

The manual suggests a number of alternatives:

1) Add a ‘Violet Reducer Filter” to the WO 2-1.25” prism (am not sure where this would sit – does it sit on the 1.25” thread where a standard filter would sit?)

2) Purchase a 1.25” 45 degree erecting prism

3) Purchase a SCT Diagonal and use a 2” to 1.25” adapter

I have searched/browsed for soluitions, but would like to try and understand the optimum solution.

What have you used? what’s best optically? and what’s best pricewise?

Any particular recommendations on manufacturers?

Any help greatly appreciated, as I need to buy something to fix this quickly before my travels….

Best regards,

Ian

[garethmob]

i dont have the zs 66 i have the megrez 72 but i use a 90 degree diagonal and that seems fine, focus is somewhere in the middle for me and im using the cheap skywatcher diagonal i had with a older scope (102st)

[ibraidwo]

Thanks Gaz, I may have to buy a cheapie 1.25 90 degree diagonal from ebay (or go to the local astronomy shop and try one), before investing in a quality/expensive one.

[Stu]

I use an SCT 2" diagonal with mine, it came with the scope so it's a WO.

I like this setup because it is very solid in the attachment to the scope, the quality is great, and it gives me the option of using my 2" ep's. Probably not the cheapest option of the three though

Cheers

Stu

[ibraidwo]

I use an SCT 2" diagonal with mine, it came with the scope so it's a WO.

Thanks Stu,

Does your set up work with a 1.25" barlow?

I am surprised that a 2" SCT diagonal plus 2-1.25" adapter would be that much different to the WO 2-1.25" diagonal on its own. Surely the optical path would be quite similar?

Or does the fact the it's an SCT mean that it brings the mirror in that much closer to the focuser, making a difference i the focussing point.

Confused

Cheers,

Ian

[Stu]

I believe it will work with a barlow but can't remember, don't often use them. I will try next time I am using it.

I suspect the fact that it is an SCT fitting does shorten the light path a little but I don't know that for certain.

Stu

[ibraidwo]

Having done a bit more digging I have discovered some later versions of the manuals for ZS66, where the 1.25" 90 Degree Dielectric Diagonal is included as a suggested accessory (my original manual didn't have this included). In fact it is recommended in the ZS66 Ferrari Anniversary Ltd edition manual.

They also suggest a SCT 2" one however these are more than double the price of the 1.25" which I can't justify as the scope cost me about the same and I currently don't plan to move to 2" EPs.

I already have a SCT to 1.25" adaptor that came with the scope plus the slight saving on weight makes this a no brainer.

To complete this thread, I'll post my experiences with it after it arrives.

Best,

Ian

[ibraidwo]

My conclusion is that the ZS66 works well as follows...

For use as a spotting scope:

- WO SCT to 2" adapter (approx 40mm - mine came with the scope)

- WO 2" to 1.25" 45 deg diagonal

- EPs without barlow only - I mainly use a Celestron 8-24mm zoom

For use as an astronomy telescope:

- WO SCT to 2" adapter (approx 40mm that came with the scope)

- ScopeTeknix ultra low profile 2" to 1.25" adapter

- WO 1.25" dielectric 90 deg diagonal

- EPs with and without 2x barlow, again I mainly use a Celestron 8-24mm zoom, however the 2x barlow makes the image very faint at 8mm.

Other options may work, however these configurations work well for me giving sufficient backwards and forwards travel on the 2" focuser (mine isn't the one with the 1.6" focuser).

I believe (from other posts) that it also works with a WO 2" 90 deg diagonal which comes with its own short SCT adapter - however I can't validate this and this diagonal costs a lot more then the 1.25" 90 deg diagonal I bought.

It doesn't work with the included SCT to 1.25" adapter (with or without diag) as it is far too short. (I just use this to close off the focuser as it comes with a 1.25 end cap).

Best,

Ian

[Chris]

"It doesn't work with the included SCT to 1.25" adapter (with or without diag) as it is far too short"

Nice review Ian, it might be worth double checking the above though because I achieve focus fine with my 1.25" 90 degree diagonal inserted into the 1.25" to SCT adapter.

[ibraidwo]

Ok starfox - ! Will try again as I have my scope all set up and have clear skies where I am.

When I tried, I had to physically hold my WO 1.25" 90 deg diag away from the SCT to 1.25" adapter to get focus. The 40mm depth of the WO SCT to 2" adapter made all the difference...

How deep is your adapter and what diagonal do you use?

Thanks,

Ian

[ibraidwo]

Hi again starfox,

I just read your signature and your ZS66 is a full APO by the looks of things. Mine is a Petzval semi-APO and they have different focal lengths.

I definitely have an issue using the 1.25" SCT adapter.

The Petzval version is known for having focusing issues when used for astronomy.

Best,

Ian

[Chris]

Hi Ian, I've just measured my SCT to 1.25 adapter and its 28mm from the start of the threaded nut to where the diagonal sits, I've attached a couple of pics below, my diagonal is just a bog standard 1.25 dielectric 90 degree.

[Chris]

Hey Ian:) ah I see, what focal length do you have mine is 388mm.

[ibraidwo]

This makes yours the SD Doublet APO. Mine is 400mm.

From the early reviews on these scopes, I believe this issue only applied to the Petzval version.