WO ZD66ED Petzval Coma!

[buddyarrows]

Hi, I hope I can get advice on this little scopes poor optics.

I bought this second hand on a bit of a whim with out much research if any, thinking, as it’s a WO with their reputation I couldn’t go wrong! Or so I thought.

The OTA was bought mainly for wide field imaging due to its short focal length. The problem is the sever coma on my images when using my Canon 450D at prime focus. The central area is fine with good sharp points but deteriorates toward the edges. My thinking is there an alignment or collimation problem or just inherent in the design. I had believed that the petzval design was to flatten the image or am I missing something?

My Skywatchers don't have coma like this, I have noticed slight coma using my 80ED and none with the 120ED.

Any advice would be welcome.

Thanks & Clear skies

David

[Ben Ritchie]

It's usual, the ZS66 isn't flat over the field of view of a DSLR - the 80ED has a somewhat flatter field, so it covers a DSLR better.

You need a field flattener for the '66. WO do a good one.

[buddyarrows]

Hi Ben, Ahh it all makes sense now, don't know why I haddn't thought of it as I have seen the WO Field Flatner when browsing for info on the ZS66.

Thanks for the quick reply

Regards

David

[beamish]

Warning !! What ,if any, filters are you using, ie clip-in or screw in ? I ask cos it could get expensive without you realising.

I found using the WO 0.8x FF/R that you can add a 2" screw in filter into the train without compromising focus-it's very tight indeed ! Without the filter, there's not a problem-rack it just about all th way in. But if you tend to use a LP filter of any sort, you'll find probs. I had to resort to buying a 2x part "sandwhich" system made by Borg ( pricey) to get a comfortable in/out focus.

I'm also aware that others have dealt with it in similar ways. There's a thread about it somewhere on here-I'll take a look.

Karlo

Here we go :-http://stargazerslounge.com/equipment-help/71036-cannot-reach-focus.html

[buddyarrows]

Hi Karlo, Thanks for the good advice. I normally use 2" filters and as most of my set-up is 2" based, so my first question to FLO or Ian King would be if it's 2" compatible. The OTA came with the 1.25" adapter fitted and that caught me out but luckily I had spare SCT connectors the bits to connect and then found out I couldn’t get prime focus with out an extension piece.

After reading the link I assume that it's the SCT-2" adapters built in extension to acheive prime focus adding to the length that causes the problem, and as far as I can see from FLOs website the FR is push fit, FLO closed untill Tuesday.

Can you confirm I've got this right

Thanks again

David

[beamish]

The included removable 2" adapter is a push fit , the FF/R has SCT threads to which the adapter attaches.

without the adapter, the FF/R screws into the SCT thread on the focuser.

Below are some images showing the FF/R and the Borg adapter(pair) for the 2" filter inclusion. Note the 2" push fit adapter is not shown.

HTH,

Karlo

[johninderby]

The ZS66 has a 1.6" focuser so you can't use 2" push fit accessories but have to use the SCT adaptor, and a 2" push fit adaptor adds a lot more length. The limited infocus this causes is this scopes Achilles heel, otherwise a nice little scope in every other respect.

The Megrez 72 uses a full size 2" focuser with decent infocus travel which is one reason I replaced my ZS66 with a Megrez 72.

John

[buddyarrows]

Thanks guys, I totaly understand now how everything fits to-gether. You have all been very helpfull.

Will have to consider my options before deciding. Migh sell the 66 and buy a 72 could work out cheaper in the long run.

Clear sky's to all

David

[ollypenrice]

Whoa, we are surely talking about two different telescopes here. The original post was about the ZD66 Petzval while all that follows is about the ZS66 doublet. Or am I wrong? I have never seen a ZD66 but use a ZS66 with dedicated flattener reducer regularly. I would have thought the Petzval should have a flat field but it seems it doesn't. We can't assume that the WO 0.8 reducer flattener will work on the Petzval.

A rather nice historical aside; the Petzval was originally a portrait lens and early astrophotographers like EE Barnard (wonderful man) discovered that they were superb widefield astro lenses. I like to call my early Genesis 'the portrait lens' in moments of whimsy...

Olly

[buddyarrows]

Has any one used the WO FF/R on a Skywatcher 80ED and or 120ED or similar F7.5 useing a DSLR with ALPS sensor?

Thanks

David

[Beyond_Vision]

The WO 66 FF/FR II doesnt totally correct the distortion and also you will get a lot of vignetting with a DSLR size chip. I wouldnt recommend the WO field flattner II for the ED80 or 120. The WO FF/FR III would be a better bet for the ED80/120 or the Televue TRF2008 0.8x FF/FR.

Regards

Kevin

[beamish]

The limited infocus this causes is this scopes Achilles heel, otherwise a nice little scope in every other respect.

The Megrez 72 uses a full size 2" focuser with decent infocus travel which is one reason I replaced my ZS66 with a Megrez 72.

John

Exactly ! and with the 66 that's why I went for the Borg adpters- get to use 2" filters and loads of infocus left over.

With the SCT-2" adapter and the FF/R+ 2" pushfit adapter, focus is Not achievable to a satisfactory point IMO and certainly no way of using the 2" filter, cos then Focus IS NOT achievable. I wasted alot time and some brass finding out !

[buddyarrows]

Thanks for the info Kevin, have learned a lot from this post and I think ollypenrice makes a valid point regarding the Petzval. Olly you say you use a dedicated field flattner, what one is it please?

David

[ollypenrice]

Hi David, I just used the one WO sell for the ZS66. It's on Ian King's website and elsewhere. I have had it about 18 months. I use a 285 chipped CCD, not a big DSLR chip, so I can't say how it works with those. I've an Atik 4000 coming soon so we will see how well it does on that. The ZS 66 still knocks me out every time though.

Oly

[Beyond_Vision]

The Williams Optics FF/FR II is designed for the 66SD and It's the one I have to go with my 66SD. As I have said it doesnt work that well with a DSLR size chip. The WO72FD would be a better bet as you can use 2" push fit FF/FR's. Such as the WO FF/FR version III.

Regards

Kevin

[buddyarrows]

Cheers guys, very informitive.

Thanks to all who posted, all info recieved and I'm very obliged.

David