GT81 - Now it's the Star shapes - Is this the final straw?

[Sp@ce_d]

I could do with some experienced eyes on this one... I think the scopes not right but I may be wrong of course...

(Rant on, warning)

To set the scene I have to say it's been a nightmare in the Obsy this past couple of months or so. One thing after the other, Laptop woes, Maxim 6 instability and things generally just deciding to intermittently not work anymore. Argghh It's doing my head. I've stripped the rig down several times and started again. So I guess I was asking for trouble introducing another scope into the mix in the midst of it!

I really wanted to like this scope but I'm beginning to regret my choice having gone by my previous happy WO experiences. First off the focuser (with CCD/FW attached.. yikes) almost fell out after a few slews..

http://stargazerslounge.com/topic/224838-gt81-focuser-they-all-like-this/

I put it down to my stupid fault for not checking tightness. Once I'd realised what was going on I was happy with it as I then understood whats required to adjust & tighten. However, it seems it's not going to stay put for too long as it's worked loose again since. Not ideal for a remote Obsy installation. Then the foot (to which I've attached an ADM Vixen bar) came loose from the scope leaving the scope wobbling side to side. So I've had to go over the whole thing checking everything was tightened.. again I thought silly me for not doing it when I took it out of the box... Foolishly thinking they do that when its made.

After that hurdle then there's the problem of not enough back focus without a reducer (I'm still waiting for the reducer to come in) but I have a flattener designed for the original GT81 that I used on the ZS66, so I popped that on. It focusses just about an inch short of full extension.. not great but at least I could use it. Now finally the clouds give me a break and I get some subs, but the stars aren't what I was expecting. Hmm maybe it's the flattener, so off I go in search of an extension tube.... another week passes.

So, now tonight I've finally got the thing tightened up stable enough and I've got an extension tube so I can focus without any other glass (apart from filters) in between sensor & optics...

I was expecting round stars like I have on my other scopes but this doesn't look right.  I'm still happy to adjust, if that's possible... I still really want to like this scope.. I really do.. but if it is the scope that's at fault then right now I'm kicking myself for not waiting a couple of weeks to hear about the Borg discount and I'd sling it as far as I could for a Baby Q!

Crops.. in & out of focus.. these are through a Lum filter but it looks the same with the others.

[Sp@ce_d]

Here's a comparison... hot off the scopes, although the seeing is lousy..

I've got the GT81 in a side by side config with the Esprit80 so both subs captured under same conditions same time.

Both 900s with 314L+ & Baader Ha 7nm

Just a quick STF stretch in PI

GT81

Esprit80

[perfrej]

Going "nuclear", are we... I had the same problem with a Megrez 90 that I eventually returned. It is either pinched optics or three screws intruding the light path. The latter is easily checked; just take off the objective and see if there are three evenly spaced screws somewhere. The former is move difficult as it involves the collimation screws of the objective.

/per

[ollypenrice]

Since there are three of the artefacts, equally spaced, I'd go for lens clip intrusion, essentially Per's three screw hypothesis. Just make a small aperture mask using a compass cutter to stop down below the diameter of the intrusion to confirm this.

How would we characterize WO? I do have a two-word epithet in mind but I'll spare you that!

Olly

[Sp@ce_d]

Thanks Per & Olly

I thought it looked like clips but also wondered about the 2 thumbscrews & recessed screw on the compression ring at the focusser end. Having swapped the flattener to an extension tube & carefully checking they're not tightened too much I think they can be eliminated.

last night was colder than the last couple of chances I had to check it. It was a lot more pronounced & easier to see in last nights subs.

I don't have experience of playing with lens cells so im not sure if I can do anything with it myself? Always willing to learn tho.

Sent from my iPhone using Tapatalk

[FLO]

Since there are three of the artefacts, equally spaced, I'd go for lens clip intrusion, essentially Per's three screw hypothesis. Just make a small aperture mask using a compass cutter to stop down below the diameter of the intrusion to confirm this.

How would we characterize WO? I do have a two-word epithet in mind but I'll spare you that!

Olly

Hi Graham,

Olly is partly right, they are diffractions caused by lens spacer tabs but it is not a characteristic of William Optics telescopes because the same three spacer tab arrangement can be found in a wide range of telescopes, including some that cost even more than his and Per's Takahashis. In most instances the diffractions are so slight they are insignificant but they are rather pronounced in your example. 

I understand you are not pleased so please email us for a free collection and refund 

HTH, 

Steve

[Ruud]

What a pity that the telescope is defective.

Thank you, Steve, for solving the problem for the OP.

[Skyline]

I take it this optical issue arose from the Gold Versions not the Red coloured GT81's.

Al.

[FLO]

I take it this optical issue arose from the Gold Versions not the Red coloured GT81's.

Metal spacers are not an optical issue, they are a method used by several manufacturers to set the correct lens spacing. Most people reading this if they look carefully into their refractor telescopes will find metal spacers between the lens elements. Unfortunately in this instance they are protruding far enough into the light path to be a distraction.

The same is true with a Newtonian telescope's mirror retaining clips. They also cause a diffraction pattern, particularly with smaller apertures where the clips are relatively large in relation to the mirror diameter. Like refractors it isn't considered a problem (more a characteristic of the design) unless the diffractions are excessive. 

Sp@ce_d purchased his telescope from us only a few weeks ago so has the option to return it but If anyone else has a telescope with the same excessive diffraction pattern they can remove it completely using an aperture-mask (a circular ring cut from card or plastic placed in front of the lens objective or Newtonian mirror) to cover the spacers/clips. You will of course lose a few millimetres of aperture but you might consider that worthwhile if the diffractions are excessive. 

HTH, 

Steve

[DaveS]

Going "nuclear", are we... I had the same problem with a Megrez 90 that I eventually returned. It is either pinched optics or three screws intruding the light path. The latter is easily checked; just take off the objective and see if there are three evenly spaced screws somewhere. The former is move difficult as it involves the collimation screws of the objective.

/per

Interesting, as I've never seen this in my Meg 90. Manufacturing tolerences?

But either way I won't be buing another WO 'scope.

[FLO]

It isn't pinched optics or screws protruding into the light path. The diffraction pattern caused from pinched optics produces triangular shaped stars. I have never seen screws protruding into the light path so can't comment on that  

HTH, 

Steve

[Skyline]

Alright Spacers then, I saw this effect once on a FLT132, I wonder if the spacers slipped due to any moisture in the lens cell ?

[FLO]

Alright Spacers then, I saw this effect once on a FLT132, I wonder if the spacers slipped due to any moisture in the lens cell ?

No, they haven't slipped. It is probably only they are a little wider than usual or have been positioned further in. It happens occasionally. 

You are probably thinking of the FLT132 sent to an astro mag for review, it was a particularly good (or should that be bad) example of aberrations caused by spacers. WO later updated the lens cell, though interestingly the new cell was introduced not to reduce this aberration (though it did) but to reduce pinching, which is an entirely different aberration. The new cell has a bearing mechanism that better handles changes in temperature (pinching mostly happens at cold temperatures). 

HTH, 

Steve

[dmahon]

Good on FLO sorting it so quickly for you. My question:

Why buy the GT81 when you have an Esprit 80? Aren't you happy with the Esprit? I ask as someone who was considering one (and the GT81).

[Sp@ce_d]

Good on FLO sorting it so quickly for you. My question:

Why buy the GT81 when you have an Esprit 80? Aren't you happy with the Esprit? I ask as someone who was considering one (and the GT81).

Yes.. As always Flo's excellent customer support shines through.

I am happy with the Esprit. In fact I've decided to replace the GT81 with another Esprit for the dual shooter rig. I have been concerned in the past of the variation in SW's QC however, Flo tell me they check them before they go out

The reason I got the GT81 was because its more compact & almost half the weight of the Esprit. My intention was to also use it with a portable setup for weekends/Hols/Dark sites etc.