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Star shapes and guiding


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@vlaiv noticed the other day that some of my star shapes were weird in my post of M51. I have the same I think last night. I've attached the guidelog and the relevant subs for that log to show what I mean.

My average guiding is very good I think. I suppose I'm wondering if, however short, the peak deviations of 2" and -2.67" (according to PHD2 Log viewer) could be causing this ? And if so - what on earth I can do about that? And if not, what else it can be ? Some seem to show 'blurring' or stars in a direction. Some seem to show a doubling - both of these seem possibly caused by the spikes. But.. I mean.. what can be done about the spikes ?

 

Archive.zip PHD2_GuideLog_2022-01-17_173148.txt

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The RA and Dec figures were very similar on most of your sessions, so should result in round stars.

The exceptions were on session 10 at 23:50, RA = 0.33, Dec = 0.53.

And session 12 at 03:46, RA = 0.33, Dec = 0.62.

The stars in those sessions could be elongated in the Dec direction.

If the other sessions don't have round stars then you have Differential Flexture - the guidecam is being well guided, but the imaging camera isn't.

Oh and guidecam focus is poor HFD = 5 to 6 pixels.

Michael

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I took a look at that guide graph - and it really does not look that poor as is.

You have very decent RMS values of around 0.5-0.7 and even P2P error is not that bad really.

Problem is - I'm convinced that poor star shapes are due to guiding, and I think that log does not match what I see in the subs

Look at this animation of your subs

guiding.gif.8ae7ac1124ec17de691e632fb3171297.gif

Guiding shows that most of your errors are below 1" and that is reflected in RMS (you can't have total RMS of ~0.5" and have lots of errors higher than 1".

image.png.819f1258aeed2ec56610c174ed09d2cd.png

However, stars in your images are distorted by much larger amount. You are imaging above 1"/px (ASI1600 + 750mm of focal length or less) - so single pixel is 1".

Look at this star profile:

image.png.d191b02c85bfc496c65f47de4e4ee8f8.png

Two centers are separated by 4px - that is at least 4". Guide log shows no such error.

I think there are few possible explanations:

1. Wrong guide scope / guide camera parameters entered (like pixel size and focal length) - which results in wrong arc second reading from PHD2 (but that is not very likely)

2. some sort of differential flexure.

How is your guide scope mounted? Is it on OTA or attached separately to rings or somewhere else?

Could it be that mirror is loose in the cell and moves around (although unlikely - as wind would have to be strong to move it and it would be seen in guide log)?

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it's mounted to the OTA directly.  And when I saw I was still getting them last night, I went back out around 10pm and made sure everthing was solidly mounted with no wobbling - ota to mount, guide camera to finder, finder to mount, camera to ota, etc.

The FL of the guide camera is set to 175mm and is a svbony 50mm. This is the one I mentioned a few weeks ago says in docs is 190mm, but asiair plate solved to 175mm so I'm using that. Perhaps that IS the screw up and I should have left that as was ? (https://www.svbony.com/sv106-finder/)

and yeh, mirror wise, it was calm, and so no reason for that i can think.

I did think at 10pm or so I had found the reason though - as the screws holding the focus onto the ota were very loose and you could move the focus tube connection on the ota. it wasn't wobbling, but you could move it.. so I tightened all that thinking id sorted it, but it made no difference at all.

As you say, the subs looks exactly like they do when guiding isn't right. which is what is confusing me.

Maybe I set FL back to 190 ?

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I think some confusion is creeping into this Svbony F4 50mm guidescope Focal Length.

Total length is quoted as   240mm total length 

That's not the same as the Focal Length.

Further on it's :

Focal Length: 190mm 

F4 x 50mm = 200mm Focal Length.

So if you have Plate Solved it to only 175mm, then somebody has got their sums wrong :-<

Within this range of FLs you won't see much difference in the quality of guiding, as to an extent PHD2 will accommodate the error when Calibrating.

There will be a miscalculation of the Guide Error figures though.

PHD2 won't accommodate entering 50mm as the FL :-<

The guidescope rings appear to have soft-tipped adjustment screws.

Diff Flex is measured in microns, so there's a source straight off.

Michael

Edited by michael8554
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it's set to 175mm as it says above. yes, it has soft tipped screws, but there are not external forces on it - the cables are tied to the ota so the scope has no forces on it during guiding. I can't see how they would make the slightest difference. And it's the same setup I used on my C925 and I had no issues with that at all.

No, there must be something loose somewhere. I need to double check everyhing. secondary mirror, etc. It must be something simple. 🤔

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14 minutes ago, powerlord said:

must be something simple

Hi

Is the primary mirror sealed to the cell or sitting therein under gravity?

Send shots so we can see...

-how the guide camera is attached to the guide telescope

-the latter is attached to the imaging telescope

-the cabling.

Remove the guide telescope and shake it. Is the lens secure?

Cheers

Edited by alacant
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So here's some pics. Not mounted at present. But I've triple checked everything for any movement, tightened all screws. Checked mirrors, etc. All tight. Collimation is out so need to fix that though. But other than that I'm stumped. IMG_20220118_212118.thumb.jpg.be54cac1cb9cbea4ae16edb96178ddda.jpgIMG_20220118_211849.thumb.jpg.b1905879895b25cf1d6998c77095e5f8.jpgIMG_20220118_211311.thumb.jpg.a87b966c3e225a2cfd101cfb60c9ba10.jpgIMG_20220118_211301.thumb.jpg.315fa0ca35af6e8bd30124251fa6abe8.jpgIMG_20220118_211256.thumb.jpg.46cf9bbba2237afec3c83a3d05d7dbcd.jpgIMG_20220118_211241.thumb.jpg.ac42b3b84d03c0b398e2f142c29ca90e.jpgIMG_20220118_211238.thumb.jpg.f45f82c6126d16cc1c13624289b05afa.jpg

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@powerlord

Can you stack your images without alignment using max as stacking operator?

That will create "star trails".

You can then see if there is actual drift between first and last exposure. If guiding is working properly - stars should be roughly in the same place even if you dither - as dithers are random.

If you see stars streaking - that is sign of differential flex (which is, by the way, very hard diagnose / pin point what part of setup is causing it).

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17 hours ago, powerlord said:

other than that I'm stumped

Hi

AH, OK. It's the GSO. Lots of opportunity for components moving;)

In particular, but not limited to...

The mirror cell (stronger springs and fixing the floating mirror), the mounting of the guide telescope (on a separate rigid top rail but with hard nylon retaining screws under compression) and the distance between the tube rings need to be further apart and with a wide dovetail. Another GSO speciality: the metal secondary holder is too heavy for the spider causing droop and tilt away from the vertical.

The only way we could get our Newtonians guiding reliably using a separate guide telescope was just that. 

Of course the easiest way to guide an off the shelf Netwonian is to simply use an OAG.

Here is one of our bashed around 150mm teaching reflectors. As well as that which you can see, the mirror springs are 6 off 1.6mm wire and the secondary holder of light plastic. The main mirror is held by neutral silicone sealant to prevent it floating as the tube angle changes.

IMG_20220119_114254_1.thumb.jpg.8d62d8a26dd793fd015960c73c0ff20a.jpgIMG_20220119_114346_1.thumb.jpg.e511e36b61fcbc66cfb70f88588263bd.jpgIMG_20220119_114400_1.thumb.jpg.b39083ca874673151ba4563a0a8ddf43.jpg

 

 

 

 

 

Edited by alacant
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Is GSO same as TS-Photon then ? just rebrand ?

Good advice @alacant thanks. easiest is to do top bar and attach guidescope there. tbh that's how my 200p is, I was jjust being lazy using the provided attachment, but its just thin sheet steel - its the likeliest problem point and the easiest to fix. stay tuned.

stu

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25 minutes ago, powerlord said:

GSO same as TS-Photon

Yes. nice optics, shame about... 

The Photon series are the economy versions and many corners have been cut. But hey, an hour or so's investment -mainly in time- will make it just as good as as the ones TS knock out at twice the price. And still don't work out of the box!


Here is an example. The supplied GSO 200 springs alongside supplied SW 200 springs. In fact, to stand a chance even with the 150, you need the next size up.

Yes... we speak from bitter experience!

resorte.jpg.a37808faee100e71bf7b9e352cd33e69.jpg

 

 

 

Edited by alacant
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19 hours ago, powerlord said:

it has soft tipped screws, but there are not external forces on it - the cables are tied to the ota so the scope has no forces on it during guiding. I can't see how they would make the slightest difference.

Don't take my word for it, read about guidescope attachment and Diff Flex in the PHD2 Instructions.

But you haven't said if the stars are trailing in only session 10 at 23:50, and session 12 at 03:46 ???

Or in the rest of the sessions, which had good guiding.

If the trailing is in all sessions then it's Diff Flex.

Michael

Michael

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So update from last night:

1. tried with new top plate and finder mounted to it. still same.

2. changed finder and camera. still same.

3. changed ota to my altair 80ed. issue gone. round stars. So, thanks for headsup @michael8554, but that eliminates any issues with finders.

So.. definately the newt. So that at least has narrowed it down.

I imagine prime culprit is now the primary mirror moving. and that does make sense with it being a bit out of collimation, since it's new and I imagine it was collimated when I got it. And tbh I remeber I sort of dropped it over xmas - not heavily, but it slipped out of mount slowly from a height of about 1 foot onto concrete - so the primary would have got a bit of a shock/bash.

So I think I need to get some new springs hopefully from a uk source that's a bit quicker delivery @alacant but thanks for the link, and dismantle the primary and fit em. Whether it's a spring broken, or something just got loose, if I'm checking I might as well fit stronger springs when I'm in there.

thanks all - and IF/WHEN I fix I will update this will remedy.

stu

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1 hour ago, powerlord said:

new springs

To cover other sources of flex and movement, you'll also need a stron(er) and longer dovetail and top bar the finder rings bolted directly to it (or a heavy clamp), silicone sealant and a light plastic secondary holder. 

If you don't want to fiddle around, it may be better to simply fit the springs and go with an OAG. The problem then is holding collimation during and between sessions; still no free lunch. Then there's the focuser...

Anyway, well done for having narrowed it down.

EDIT: AliExpress take under a week. To Spain at least. 

Edited by alacant
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thanks Rob./alacant - I've taken back mirror assemble off. 6 screws - 3 push, 3 pull I assume ? with the pulls having springs. but surely, if you have the push all tight, the springs are irrelevant ? or have I got the wrong end of the stick here ?

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Hi Stu, The mirror cell is effectively 'floating' on the 3 springs / pull screws. The push screws are just for 'locking' after collimation. I see what you're saying, but without the spings it would be very difficult to make adjustments. To be honest I don't make much use of the locking screws. With good stiff springs they are not really necessary IMHO, but I'm still very much a beginner & don't always get imaging results I'm happy with (i.e. take everything I say with a pinch of salt!).

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