Star test questions

[Stu]

I would wait for an answer from FLO before doing anything to the scope. Couple of questions though. If you go from inside focus to outside, to the direction of the oval change? This is a sign of astigmatism I believe.

I guess the primary cell is likely held on with three screws. It maybe possible to loosen these to reseat the cell, or even adjust it slightly to correct the problem.

[cwis]

1 hour ago, Stu said:

I would wait for an answer from FLO before doing anything to the scope. Couple of questions though. If you go from inside focus to outside, to the direction of the oval change? This is a sign of astigmatism I believe.

I guess the primary cell is likely held on with three screws. It maybe possible to loosen these to reseat the cell, or even adjust it slightly to correct the problem.

Will do - don't want to upset Auntie Flo!

I'll find out next clear night as to whether it's astigmatism. I do have an artificial star flashlight thingy, but to focus on it at the end of my garden the focusing tube will be in a hugely different position to how I use the scope it's not worth doing tests with, if we suspect the focuser.

[Peter Drew]

I agree with Stu's comments.  The depicted star images show collimation and astigmatism issues, either or both would significantly compromise the performance.  I have a smaller "fixed" mirror Newtonian and was able to adjust the collimation by loosening the three screws that hold the primary cell, twisting the cell until the collimation was better and then re-tightening the screws whilst being careful not to upset the new position.  I don't think the astigmatism can be collimated out and I suspect it is caused by whatever means is used to "fix" the mirror, it is quite common for manufacturers to attach the mirror too firmly that causes this issue.

[FLO]

16 hours ago, vlaiv said:

That should be easy then ...

@FLO

What is supposed to be done with SW Scope that has no collimation screws (by design) yet goes out of collimation?

Hi 🙂 

We asked ourselves the same back in 2011 when Synta released the Heritage 100p (a smaller telescope) with a non collimatable mirror cell. We used one for a while, the views were good out of the box and it appeared quite robust so we added it to our website. The Heritage 100p is now one of our bestsellers with a low return rate. We were admittedly more concerned when they later released the non-collimatable 130mm Newtonian discussed here but, considering the 100p's success, we decided to sell them for three months then review the situation... We did however add this cautionary note to our website product description: 

-------

NOTE: The 130PS telescope has a non-collimateable primary mirror. Please contact us for more info. 

-------

We were and are the only retailer to include that note in our product description 😇

We sold our first Explorer 130PS AZ GTi September 2017. Three years later, we have received only three customer returns (two because the mount wouldn't connect to the owners wifi and one unwanted Christmas gift). That is a remarkably low return rate! 

Owners are delighted with the views, which is remarkable when you consider the OTA features a 5" parabolic mirror yet costs only £74.

18 hours ago, cwis said:

I have to say though - when I got Mars in the sharp spot last night I was amazed. The optics are VERY good. All I need to do is get that spot in the middle!

🙂 

Some owners will of course outgrow the telescope and upgrade to a more advanced (more expensive) model. This is only natural. 

@cwis If you have concerns about your telescope's performance then you need only contact us. We will be pleased to help. 

Regards, 

Steve 

[FLO]

PS: Synta have also released a 6" Newtonian with a non-collimatable primary mirror. 

Sky-Watcher Star Discovery 150i WiFi

[andrew s]

In theory as long as it not too far out you should be able to bring it to collimation with just the secondary provided it has enough adjustment.

There is nothing optically that requires the optic axis to be exactly along the centre of the tube. Some collimation methods balance this with the secondary offset.

Regards Andrew 

[cwis]

23 minutes ago, FLO said:

Hi 🙂 

We asked ourselves the same back in 2011 when Synta released the Heritage 100p (a smaller telescope) with a non collimatable mirror cell. We used one for a while, the views were good out of the box and it appeared quite robust so we added it to our website. The Heritage 100p is now one of our bestsellers with a low return rate. We were admittedly more concerned when they later released the non-collimatable 130mm Newtonian discussed here but, considering the 100p's success, we decided to sell them for three months then review the situation... We did however add this cautionary note to our website product description: 

-------

NOTE: The 130PS telescope has a non-collimateable primary mirror. Please contact us for more info. 

-------

We were and are the only retailer to include that note in our product description 😇

We sold our first Explorer 130PS AZ GTi September 2017. Three years later, we have received only three customer returns (two because the mount wouldn't connect to the owners wifi and one unwanted Christmas gift). That is a remarkably low return rate! 

Owners are delighted with the views, which is remarkable when you consider the OTA features a 5" parabolic mirror yet costs only £74.

🙂 

Some owners will of course outgrow the telescope and upgrade to a more advanced (more expensive) model. This is only natural. 

@cwis If you have concerns about your telescope's performance then you need only contact us. We will be pleased to help. 

Regards, 

Steve 

Hi Steve, thanks for the reply!

What I will do is more investigation on the next clear night and find out exactly what is wrong.  If I still think there's an issue with the scope I'll contact you as requested.

As you  (very tactfully!) didn't say, the odds are it's my inexperience in something I am doing with secondary mirror alignment or similar showing rather than any issue with the scope - I totally accept that.  I've asked the good people of this forum for assistance on what I saw as a collimation issue and they have given me some pointers on how to be clear what exactly the issue (if any!) could be - I'll follow their guidance to a conclusion.

If we can get the sharp spot in the centre of the view (and therefore it will probably a lot bigger because it will be parallel to the focal plane) I will be very very happy with the performance of this scope. I thought it was "OK" before but the views of Mars the other night were, frankly amazing - I had no idea it could do that. Brilliant for a scope I effectively got "free" with the Az-gti and tripod!

 

[cwis]

Hi all,

I've done a little more investigation.... I attempted collimation from first principles - using the Astro-baby guide in so far as it applies to my 'scope. I'm still scratching my head.

Initially I wound the focusser down as far as it would go to close up the gap round the secondary mirror so I could centre the mirror as closely to it as I could. I put a folded plastic bag (green and white) behind the secondary, and a white bent card in between the secondary and the primary. Then I adjusted the centre screw in the secondary until it looked in the middle of the view down the focus tube (through the collimation cap) 

So here I am with the secondary mirror as centered as I can make it - looks like it's rotated a bit up in this photo (along the horizontal plane)

 

Next I rotated it to get it as circular as possible - turning it around the central screw. Then, fine adjustment with the tilt adjusters on the secondary to line up my doughnut on the primary with the c-cap, with my pupil - all in a line.

 So - doing this I encounter a problem, or question, or something that someone can point at and go "Nope, do it like this".

I've done this a few times now, and whether I have the secondary "too far out" or "too far in"  so to the left or the right of the centre of the focus tube (and then line up the primary) or rotate it up, or down (and then line up the primary) it always lines up with the view like the photo, with the edge of the primary vanishing off the side of the secondary. It doesn't matter where the focusser tube is - I can't detect any tilt in that until it's nearly all the way out, but obviously the more I wind it out, the more primary mirror vanishes. This is a photo though the c-cap by the way, so central as I could make it.

What am I doing wrong? My sharp spot, coincidentally, was at 45 degrees to the left of the view which lines up with this too so I'm pretty sure whatever I am doing here is the cause of that issue.

 

[Pixies]

1 hour ago, cwis said:

Then, fine adjustment with the tilt adjusters on the secondary to line up my doughnut on the primary with the c-cap

That's the problem. If you had a Cheshire, you would line up the doughnut with the cross-hairs. But with a collimation cap you can't do that. You need to centre the image of the primary under the focuser. Usually, you do this by making sure all 3 primary mirror clips are visible.

You see - your secondary is positioned pretty well:

 

But the secondary tilt is off:

You want the outside of the primary (with clips) concentric. The doughnut should be in the centre of the reticle above

It would be much easier with a Cheshire. But you are nearly there.

[Pixies]

I should make clear - you still need to adjust the secondary tilt to get the image of the primary concentric. Don't adjust the primary at this point.

[cwis]

2 minutes ago, Pixies said:

That's the problem. If you had a Cheshire, you would line up the doughnut with the cross-hairs. But with a collimation cap you can't do that. You need to centre the image of the primary under the focuser. Usually, you do this by making sure all 3 primary mirror clips are visible.

You see - your secondary is positioned pretty well:

 

But the secondary tilt is off:

You want the outside of the primary (with clips) concentric. The doughnut should be in the centre of the reticle above

It would be much easier with a Cheshire. But you are nearly there.

But that's my point:

I can get the  outside of the  primary concentric with  the secondary, OR I can get the doughnut lined up with the centre of  the c-cap. I can't do both at the  same time. 

If I could adjust the primary, I'd get the clips concentric as you say, and then work on the primary mirror to line up my doughnut. But I have no primary adjustment. 

[Pixies]

 

3 minutes ago, cwis said:

I can get the  outside of the  primary concentric with  the secondary, OR I can get the doughnut lined up with the centre of  the c-cap. I can't do both at the  same time. 

Correct. You don't want to line up the doughnut with the c-cap yet. That's the problem. 

Once the secondary is aligned with the primary (concentrically) using the secondary screws - then you align the primary back to the secondary by getting the c-cap eyehole inside the doughnut - by adjusting the primary screws.

Does that make sense?

[cwis]

2 minutes ago, Pixies said:

 

Correct. You don't want to line up the doughnut with the c-cap yet. That's the problem. 

Once the secondary is aligned with the primary (concentrically) using the secondary screws - then you align the primary back to the secondary by getting the c-cap eyehole inside the doughnut - by adjusting the primary screws.

Does that make sense?

It makes loads of sense and thanks to you and these last few messages I feel I completely understand collimation. Just one issue:

I have no primary screws!

Would you agree that it looks like the primary is out of collimation? That's the conclusion I am reluctantly coming to.

 

[Pixies]

2 minutes ago, cwis said:

It makes loads of sense and thanks to you and these last few messages I feel I completely understand collimation. Just one issue:

I have no primary screws!

Would you agree that it looks like the primary is out of collimation? That's the conclusion I am reluctantly coming to.

 

[removed word] - completely forgot that was the issue here. 

You might have a chance by slight twisting of the secondary and then trying to see whether you adjust the tilt to have the doughnut and eye-hole line-up as well as the primary image concentric. But it will be trial-and-error and if the primary is out, then probably impossible.

 

Perhaps worth a try if you fancy it. Otherwise, I'd contact FLO

 

[Pixies]

3 minutes ago, Pixies said:

[removed word] - completely forgot that was the issue here. 

Sorry mods. My Australian heritage means I forget that's seen as a rude word.

[cwis]

1 minute ago, Pixies said:

[removed word] - completely forgot that was the issue here. 

You might have a chance by slight twisting of the secondary and then trying to see whether you adjust the tilt to have the doughnut and eye-hole line-up as well as the primary image concentric. But it will be trial-and-error and if the primary is out, then probably impossible.

 

Perhaps worth a try if you fancy it. Otherwise, I'd contact FLO

 

Haha! I thought you'd remember eventually! Never mind - you've taught me how to collimate a Newtonian telescope! Many thanks!

I was expecting that when I got the primary concentric on the secondary, then the doughnut would be there or thereabouts so a few tweaks and I'd be home.

It looks like it's quite a way out (a good proportion of the secondary mirror's diameter) and I'd need to offset the secondary perpendicular to the primary to pick up the centre of focus - an adjustment it obviously doesn't have. Or put up with a tilted focal plane and an offset sharp spot (what I have currently). 

I'll contact FLO in the morning unless anyone else chimes in with any other thoughts. It could be the one that slipped though the net from the factory, or it had a right wallop during dispatch. The finish is flawless though, and the box didn't look particularly sorry for itself when it turned up, so I don't know.

[cwis]

For completeness, here's what it looks like with the secondary mirror concentric to the focusser:

If anyone can suggest how I get the doughnut and the c-cap to line up without using primary mirror adjusters, please pipe up now!

[Alan64]

If only I had it before me.  Whilst waiting, you can browse over what I had done to my 100mm f/4; same as the "Heritage" 100P, but branded "Zhumell", and with the fixed-primary just as your own...

https://stargazerslounge.com/topic/341568-zhumell-z100-100mm-f4-newtonian/

You might get something out of it, or perhaps not.

[Ricochet]

On 25/09/2020 at 11:00, cwis said:

For completeness, here's what it looks like with the secondary mirror concentric to the focusser:

If anyone can suggest how I get the doughnut and the c-cap to line up without using primary mirror adjusters, please pipe up now!

You would have to loosen the screws that hold the primary mirror cell to the telescope tube, and then shift the whole mirror cell to align the primary. This assumes that your doughnut is now in the centre. Personally, I would always use a cheshire/sight tube for the secondary so that you have the cross hairs as a reference. 

[markse68]

On 25/09/2020 at 11:00, cwis said:

For completeness, here's what it looks like with the secondary mirror concentric to the focusser:

If anyone can suggest how I get the doughnut and the c-cap to line up without using primary mirror adjusters, please pipe up now!

It doesn’t look too far off. Like Ricochet said maybe you have enough play loosening there screws that hold the primary cell in place and tilting it or if not, maybe try rotating the primary cell after removing the screws? Or failing that shimming the mirror in the cell though I have no idea what’s supporting the mirror back in the cell. If it’s spring loaded against the 3 front clips maybe you could use card or thin plastic to shim under the clips?

[cwis]

On 25/09/2020 at 11:33, Alan64 said:

If only I had it before me.  Whilst waiting, you can browse over what I had done to my 100mm f/4; same as the "Heritage" 100P, but branded "Zhumell", and with the fixed-primary just as your own...

https://stargazerslounge.com/topic/341568-zhumell-z100-100mm-f4-newtonian/

You might get something out of it, or perhaps not.

That's an amazing piece of work there - basically a different 'scope! Do the optics warrant that sort of attention or are all optics pretty good nowadays (at the entry level) and it's the engineering of the rest of the 'scope that makes up each price point?

I notice that it was perfectly collimated before you took it apart. Did you expect it to be or was this a pleasant surprise?

[cwis]

11 hours ago, markse68 said:

It doesn’t look too far off. Like Ricochet said maybe you have enough play loosening there screws that hold the primary cell in place and tilting it or if not, maybe try rotating the primary cell after removing the screws? Or failing that shimming the mirror in the cell though I have no idea what’s supporting the mirror back in the cell. If it’s spring loaded against the 3 front clips maybe you could use card or thin plastic to shim under the clips?

I am hugely tempted to get the screwdriver out and see what kind of adjustment I have available - it's not like I could make it much worse at the end of the day. I'd love to know whether there are springs or squidgy foam or something under those clips.

I'll wait for a reply from Auntie Flo first though - there may be a "procedure".

[Alan64]

8 minutes ago, cwis said:

That's an amazing piece of work there - basically a different 'scope! Do the optics warrant that sort of attention or are all optics pretty good nowadays (at the entry level) and it's the engineering of the rest of the 'scope that makes up each price point?

I notice that it was perfectly collimated before you took it apart. Did you expect it to be or was this a pleasant surprise?

Actually, it's quite similar to your own.  It's a short Newtonian, it once had three spider-vanes like your own, and it once had a fixed primary-mirror(which it still does in a way).  It is at f/4, whilst yours is at f/5, but it has a parabolic primary-mirror like your own.  But my mirror is not certified as being diffraction-limited, which means that the images may or may not be as good as one that is certified as being so.  Yours most likely is, so that is another difference.

I have purchased a few entry-level telescopes, and I feel that the user does receive a good, yea, a very good telescope, but usually everything else within a kit falls short, from the mount, to the eyepieces and accessories.  But the heart of any kit is the objective of the telescope, whether a primary-mirror(Newtonian) or a crown-and-flint doublet(refractor).  That's what really matters in the end, and what really counts.

I was impressed somewhat once the collimation was confirmed.  I'm still testing it however, and at the higher powers.  I will need to check its collimation, tweak it perhaps, and test it again in future.  It has to wait its turn, as I have quite a few telescopes.  All are not listed within my signature.

[cwis]

Update: 

With Auntie Flo's approval I loosened the screws holding on the primary cell to the tube of the OTA with a view to getting better alignment. The boltholes in the tube are slotted to assist in this, but FLO said this was the first one they had supplied that had required ANY adjustment at all - all the rest perfectly collimate when the secondary mirror is aligned correctly. The screws btw are surprisingly lightly torqued up - there are plastic threads in the mirror cell.

Once I had done this there was a surprisingly large amount of adjustment - I could shift the central doughnut about 1/2 of the secondary mirror diameter by wiggling the cell about.  Unfortunately this adjustment was not symmetrical about the position I first found the mirror cell in and there was not enough adjustment in the correct direction to line up the mirrors. I removed the screws completely and I could get it in the correct place, but then I couldn't get the screws back in. Rotating the cell was not possible as there is a cut out in it to accept the OTA tube seam.

I reassembled the 'scope - pushing the mirror cell all the way home put the mirror back into the same place I found it according to my collimation cap, and informed FLO.

It was picked up yesterday afternoon by a chap from DHL. 

Now I am without a 'scope I expect the weather to improve significantly....

 

[Pixies]

14 minutes ago, cwis said:

Update: 

With Auntie Flo's approval I loosened the screws holding on the primary cell to the tube of the OTA with a view to getting better alignment. The boltholes in the tube are slotted to assist in this, but FLO said this was the first one they had supplied that had required ANY adjustment at all - all the rest perfectly collimate when the secondary mirror is aligned correctly. The screws btw are surprisingly lightly torqued up - there are plastic threads in the mirror cell.

Once I had done this there was a surprisingly large amount of adjustment - I could shift the central doughnut about 1/2 of the secondary mirror diameter by wiggling the cell about.  Unfortunately this adjustment was not symmetrical about the position I first found the mirror cell in and there was not enough adjustment in the correct direction to line up the mirrors. I removed the screws completely and I could get it in the correct place, but then I couldn't get the screws back in. Rotating the cell was not possible as there is a cut out in it to accept the OTA tube seam.

I reassembled the 'scope - pushing the mirror cell all the way home put the mirror back into the same place I found it according to my collimation cap, and informed FLO.

It was picked up yesterday afternoon by a chap from DHL. 

Now I am without a 'scope I expect the weather to improve significantly....

 

Are you getting a direct replacement?