Esprit 100 Star Trailing in corners - why?

[PeterCPC]

@Adam J

Tried again with the artificial star last night and the air was much less turbulent. Temperature was 5C.

When I transformed these from FITS to jpeg they were much brighter than they appeared on the laptop screen so the first one showing the star in focus (by Bahtinov) is not much help I think. I managed to darken the others sufficiently I hope. 1-6 were taken from in focus going out. 7-10 were taken just past focus going the other way (if that makes sense)

 

[Adam J]

3 hours ago, PeterCPC said:

@Adam J

Tried again with the artificial star last night and the air was much less turbulent. Temperature was 5C.

When I transformed these from FITS to jpeg they were much brighter than they appeared on the laptop screen so the first one showing the star in focus (by Bahtinov) is not much help I think. I managed to darken the others sufficiently I hope. 1-6 were taken from in focus going out. 7-10 were taken just past focus going the other way (if that makes sense)

 

Great work with the star test this is exactly what I needed to see.

So what I am seeing is two problems probably linked.

1) You have a turned edge indicated by the fuzzy outer difration ring here. But that is probably largely being caused by the second issue.

2) In my opinion you do have pinched optics which simply should not be occuring at +5c in any scope, you can make an argument that in mass produced scopes some amount of pinching has to be accepted at sub zero temperatures. But +5c is right in the heart of the temperature range that you are going to be doing most of your imaging in the UK during the autum and winter months and the problem is that on the nights when it is -5c its going to get much worse. 

The second thing that tells me its pinching is that it is not visible in the room temperature testing you sent in earlier.

Here is the best image showing the pinching:

I have pointed out the 6 x small dark notches in the outer diffration ring here. As noted above the Esprit doesnt have lens clips, it uses a continual ring gasket to space the lens elements and so the only explanation for such a perfectly symetrical pattern is pinching by the 6 evenly spaced lens centering grub screws used on the esprit. This is so caractoristic of the Esprit (so few manufacturers using 6 centering screws) that if you showed me that star test without knowing what scope it was I would guess its a Esprit and that its pinched.

Its normally harder to see the effect on the other side of focus. But in this case I can just make out the 6 bumps in the outer defration ring (opposite side of focus opposite effect).

It could use more exposure but the in focus image is starting to show the fluring that is associated with the pinching.

On the bright side colimation is ok.

So my bottom line opinion remains the same as it was following my assessment of the jelly fish nebula image you posted and that is that the lens is pinched. The pinching is visible in the star test and in focus star at temperatures of +5c and (potentially higher), the effect is not confined to the halo of bright stars, extends to background stars and this is sufficient to cause amounnts of aberation so as to detract from the image.

So thats my assessment, but I dont want to tell you what to do, as thats your choice. 

The corner star shape issue is caused by incorrect back focus unrelated to the pinching.

Hope that helps.

Adam J

 

Edited January 25, 2023 by Adam J

[newbie alert]

3 hours ago, PeterCPC said:

@Adam J

Tried again with the artificial star last night and the air was much less turbulent. Temperature was 5C.

When I transformed these from FITS to jpeg they were much brighter than they appeared on the laptop screen so the first one showing the star in focus (by Bahtinov) is not much help I think. I managed to darken the others sufficiently I hope. 1-6 were taken from in focus going out. 7-10 were taken just past focus going the other way (if that makes sense)

 

Are these bites out from the sides from collimation clips, does anyone know?

[PeterCPC]

@Adam J Thank you for your efforts, really appreciated. I have referred the issue back to FLO to get their comments. I'm not worried about the corner stars - I am making changes to the spacing.

[Adam J]

2 hours ago, newbie alert said:

Are these bites out from the sides from collimation clips, does anyone know?

Not clips, clips are for Newtonian, they are caused by the lens centring screws pushing on the edge of the lens elements. Its annoying really because the Esprit lens cell has design elements to reduce the chance of this cork buffers between the grub screw and the lens and so this is realistically most likely being caused by pure human error in over tightening the adjustment screws or forgetting to back them off once the lens retaining ring has been tightened. Very frustrating. 

Edited January 25, 2023 by Adam J

[Adam J]

2 hours ago, PeterCPC said:

@Adam J Thank you for your efforts, really appreciated. I have referred the issue back to FLO to get their comments. I'm not worried about the corner stars - I am making changes to the spacing.

What I would say to you is that as the problem is not present at room temperature the solution may be to just to slacken each of the centring screws a tiny bit. 

Edited January 25, 2023 by Adam J

[PeterCPC]

14 minutes ago, Adam J said:

What I would say to you is that as the problem is not present at room temperature the solution may be to just to slacken each of the centring screws a tiny bit. 

I would worry about this in case I do it too much. Are they around the lens cell under the dew shield?

[Adam J]

4 minutes ago, PeterCPC said:

I would worry about this in case I do it too much. Are they around the lens cell under the dew shield?

Sorry NO i don't mean for you to do it yourself at all! I mean that your retailer might be able to do it for you or SW. Never mess with those yourself. If you accidentally de-centre a lens element you will not get it reentered without specialist equipment. 

For that matter if you turn it the wrong way and tighten it when its already tight worse case you can crack a lens element. 

Adam 

Edited January 25, 2023 by Adam J

[PeterCPC]

@FLO Could I have your response please.

[Adam J]

1 hour ago, PeterCPC said:

@FLO Could I have your response please.

Peter, just want to politely note that although I am quite happy to give my opinion as a scientist, I don't want to be drawn into a debate with the supplier. 

Adam

[PeterCPC]

46 minutes ago, Adam J said:

Peter, just want to politely note that although I am quite happy to give my opinion as a scientist, I don't want to be drawn into a debate with the supplier. 

Adam

No of course not. Thank you for all your help. FLO has agreed that there are pinched optics so I am trying to sort out a resolution.

[Adam J]

5 minutes ago, PeterCPC said:

No of course not. Thank you for all your help. FLO has agreed that there are pinched optics so I am trying to sort out a resolution.

Good news, I am sure they will sort it for you. 

Adam

[PeterCPC]

2 hours ago, Adam J said:

Good news, I am sure they will sort it for you. 

Adam

I am not that confident as it is out of warranty.

[Adam J]

6 minutes ago, PeterCPC said:

I am not that confident as it is out of warranty.

Ah, that is unfortunate, I thought that you had just purchased it. The trouble with this sort of thing is that often people only see issues with optics as their experience increases and so by the time they have the experience to see that there is a issue its out of warranty. 

There are people who will fix it for you at a cost. But most of those are in the EU so involve postage. 

Interested to hear how its resolved and I can give you some contacts if you FLO cant help. 

Adam 

[FLO]

8 hours ago, PeterCPC said:

@FLO Could I have your response, please.

I understand you are already talking with my colleague, Ian. 

6 hours ago, PeterCPC said:

FLO has agreed that there are pinched optics so I am trying to sort out a resolution.

Kinda. 

3 hours ago, PeterCPC said:

I am not that confident as it is out of warranty.

It is after hours so I am not at my desk but my colleague, Ian, did mention this earlier today. 

He said (after looking at the images you emailed him) the pinching was only very minor, and not present at all until recently. 

I don't know if he communicated this to you, but when speaking with me, his opinion was it isn't worth your spending money trying to correct such a minor aberration. He was concerned chasing it down, to perfection, would only introduce another. 

HTH, 

Steve 

[PeterCPC]

15 hours ago, FLO said:

I understand you are already talking with my colleague, Ian. 

Kinda. 

It is after hours so I am not at my desk but my colleague, Ian, did mention this earlier today. 

He said (after looking at the images you emailed him) the pinching was only very minor, and not present at all until recently. 

I don't know if he communicated this to you, but when speaking with me, his opinion was it isn't worth your spending money trying to correct such a minor aberration. He was concerned chasing it down, to perfection, would only introduce another. 

HTH, 

Steve 

Noted but how, in good conscience, could I sell the scope on at some point without declaring this issue. If I do who would buy it? I wouldn't.

I feel that I am left with no choice but to see if it can be improved. If not then I feel it could be sold on knowing that I tried.

It's odd that the telescope was fine during it's warranty period and then develops this fault but that's just my bad luck I suppose.

I expected better from a scope at this cost.

[FLO]

15 hours ago, FLO said:

It is after hours so I am not at my desk 

The morning after, so back at my desk.

I see yesterday afternoon, Ian emailed you saying we would arrange for Es' to check your telescope if you wish. For your peace of mind. 

You replied that you wanted this to happen, so Ian arranged collection for Monday. All this before I replied to you here at SGL. 

I know from experience how two conversations, one here and another with a colleague via email, can confuse, so I'll bow out and leave you in Ian's capable hands 🙂 

HTH, 

Steve 

[PeterCPC]

4 minutes ago, FLO said:

The morning after, so back at my desk.

I see yesterday afternoon, Ian emailed you saying we would arrange for Es' to check your telescope if you wish. For your peace of mind. 

You replied that you wanted this to happen, so Ian arranged collection for Monday. All this before I replied to you here at SGL. 

I know from experience how two conversations, one here and another with a colleague via email, can confuse, so I'll bow out and leave you in Ian's capable hands 🙂 

HTH, 

Steve 

Thank you Steve.

[PeterCPC]

Tried the scope in earnest last night (first real chance).

Whilst there is a bit of an issue in the corners, it's not much and easily cropped out. The stars in general look much better I think.

[Ouroboros]

On 26/02/2023 at 11:34, PeterCPC said:

Tried the scope in earnest last night (first real chance).

Whilst there is a bit of an issue in the corners, it's not much and easily cropped out. The stars in general look much better I think.

I have read through this thread with interest. Perhaps I missed it, but what happened between the posts in January and your last post. Was the scope changed/tweaked or did it turn out to be a spacing issue after all? 

[PeterCPC]

The optics were pinched. Scope went off to be tweaked. Spacing was not the issue.

 

[Ouroboros]

56 minutes ago, PeterCPC said:

The optics were pinched. Scope went off to be tweaked. Spacing was not the issue.

 

…. And you’ve had good results since hopefully? 

[PeterCPC]

I haven't actually done any imaging since Feb for various reasons. I am happy with the result obtained back in Feb though.

[Stuart1971]

On 14/07/2023 at 10:08, PeterCPC said:

The optics were pinched. Scope went off to be tweaked. Spacing was not the issue.

 

An old thread I know, but what actual backspacing did you end up with from sensor to the flattener as you say you had to alter from the original 55mm..just curious…