Diagnosing "eggy" stars

[Zakalwe]

Righto, I've not done any DSO imaging in months, due to various reasons. Now I want to get back into it, but am having problems with star shapes.

 

Rig:

• Mesu 200 mount
• Altair 115mm triplet
• TS Photoline 0.79 flattener/reducer
• SX Filterwheel and Baader filters
• Atik 428 camera.
• Piggybacked ST-80 guidescope
• QHY5L-II guide camera.
• PHD2 guiding.

The guidescop seems rigid enough- certainly I can't detect and flex. Polar alignment is good- I've got it close with PHD drift aligning and verified it with a polemaster.

 

Here's the problem (5 minute subs, though 10 or 20 minutes look exactly the same):

 

And the same image taken after rotating the camera/FW/Flattener through 90 degrees:

Rotated 90 degrees_002

 

The fact that it remains after rotating the kit suggests tilt within the optical train?

Guiding was OK- not perfect, which I think is another problem. The RA line seems to stay consistantly below the middle mark on the PHD graph and all the guiding commands are pushing one way. Here's the guiding log:

PHD2_GuideLog_2016-05-18_235520.txt

 

Also note the three massive spikes in the RA graph. I get these occasionally with the Mesu where one or both axis spike momentarily- thats a whole other problem. The images were taken after these anomalies.

 

Any ideas or advice?

 

[The Admiral]

Sorry to divert this thread with a question, but which reducer/flattener are you using, the 2" or 3" screw on, please? I'm planning on getting one or other for my 102mm Altair triplet, so I shall be interested in the outcome of this.

Ian

[Zakalwe]

10 minutes ago, The Admiral said:

Sorry to divert this thread with a question, but which reducer/flattener are you using, the 2" or 3" screw on, please? I'm planning on getting one or other for my 102mm Altair triplet, so I shall be interested in the outcome of this.

Ian

 

The 3" one, Ian:

https://www.teleskop-express.de/shop/product_info.php/info/p6085_TS-PHOTOLINE-3--0-79x-Reducer-4-element-for-Astrophotography.html

 

My optical train is completely screwed together, so I will be peeved if the star shapes are caused by the sensor not being orthogonal. I don't want to throw money at it by replacing the focuser until I can track down the problem.

[The Admiral]

1 hour ago, Zakalwe said:

 

The 3" one, Ian:

https://www.teleskop-express.de/shop/product_info.php/info/p6085_TS-PHOTOLINE-3--0-79x-Reducer-4-element-for-Astrophotography.html

 

My optical train is completely screwed together, so I will be peeved if the star shapes are caused by the sensor not being orthogonal. I don't want to throw money at it by replacing the focuser until I can track down the problem.

Is this the first time you've used it? Otherwise, are you happy with its performance?

Just thinking aloud, would you need to replace the focuser; wouldn't something like this work, or perhaps an M48 version?

 

Ian

[Zakalwe]

40 minutes ago, The Admiral said:

Is this the first time you've used it? Otherwise, are you happy with its performance?

Ian

Ive had it a while and it seems to work fine. But I need to find out why I am getting these eggy stars.  I'm going to try it without the corrector in place and see how it performs.

It was good enough to get me this image:

NGC6888 narrowband

So something has changed as I was getting good star shapes last year. It might be that I am swapping the CCD in and out when I use my Quark and disturbing something?

[ollypenrice]

I don't think the rotator itself is of the best on these scopes. A single lockscrew pushes down on a conical surface so far as I can see. I wonder if turning the rotator to a new position, settling it manually and retightening it might have any effect? It seems that you are looking for a distortion which is sometimes there and sometimes not. The rotator might cause it. Does the SX wheel have a rotator. The Atik does. Basically anything likely to vary is what you might be looking for.

Olly

 

[Zakalwe]

25 minutes ago, ollypenrice said:

I don't think the rotator itself is of the best on these scopes. A single lockscrew pushes down on a conical surface so far as I can see. I wonder if turning the rotator to a new position, settling it manually and retightening it might have any effect? It seems that you are looking for a distortion which is sometimes there and sometimes not. The rotator might cause it. Does the SX wheel have a rotator. The Atik does. Basically anything likely to vary is what you might be looking for.

Olly

 

I'm beginning to suspect the focuser Olly (and thanks for replying). If it was a guiding issue then the egginess wouldn't have kept the same orientation (to the stars) when the camera/FW/flattener was rotated 90 degrees?

I could do with a copy of CCD Inspector to check the tilt.

There's no rotator on the SX filterwheel...everything back from the focuser drawtube is screwed together and tight (the flattener screws into the focuser drawtube).

[gnomus]

Stephen

Sorry to hear about these issues.  I think you read my Tak 85 thread where I also had eggy stars.  One of the things that really helped me clear my mind was Sara Wager's suggestion of shooting some very short exposure subs (5 secs) with the telescope pointing straight up.  

If you still get eggs, then the short exposure rules out guiding issues or polar alignment error.  Furthermore, the 'straight up' shooting eliminates any problems with the focuser tilting as a result of gravity (although your 90 degree rotation test has already done that I think).  

You could also test sensor tilt in your CCD by attaching another camera - a DSLR maybe - and seeing if you get the same issue with that.  After that I reckon it must be something amiss with the scope.

I hope these suggestions are some use - you'd probably already thought of them.

Steve

[Zakalwe]

2 hours ago, gnomus said:

Stephen

Sorry to hear about these issues.  I think you read my Tak 85 thread where I also had eggy stars.  One of the things that really helped me clear my mind was Sara Wager's suggestion of shooting some very short exposure subs (5 secs) with the telescope pointing straight up.  

If you still get eggs, then the short exposure rules out guiding issues or polar alignment error.  Furthermore, the 'straight up' shooting eliminates any problems with the focuser tilting as a result of gravity (although your 90 degree rotation test has already done that I think).  

You could also test sensor tilt in your CCD by attaching another camera - a DSLR maybe - and seeing if you get the same issue with that.  After that I reckon it must be something amiss with the scope.

I hope these suggestions are some use - you'd probably already thought of them.

Steve

Cheers Steve,

It's a bit frustrating...I haven't had the opportunity to do any DSO stuff in months, and then when I get a wee chance the blooming stars are manky.

 

The images above were taken near the zenith...I was trying to rule out focuser sag to try and isolate the problem. Good idea on the short subs, though I have a sneaking suspicion that they'll be the same. I was shooting some 20 minutes subs and they were the same as these 5 minutes ones. Did you manage to sort your eggy stars out?

I must be a masochist for trying this hobby, never mind trying it when I live in Lancashire!

[gnomus]

Yes - the fix turned out to be simple.  Return the Tak, get a refund and buy a WO Star 71.  I'm now getting round stars.

[Zakalwe]

8 minutes ago, gnomus said:

Yes - the fix turned out to be simple.  Return the Tak, get a refund and buy a WO Star 71.  I'm now getting round stars.

I've just had a re-read of the thread, Steve...I had forgotten that I had posted in there (I haven't been round these parts much). I would have expected better from a Tak.

I'll have to experiment some more, but I am suspicious of the focuser. I'm away for a few days, so it will have to wait for a wee bit longer.

[ollypenrice]

I agree that the 90 deg rotation test seems to eliminate guiding but the short subs test is worth doing. Small stars show distortion better than large ones, which is something to remember.

How to test the orthogonality of the focuser? You could put a laser collimator in the EP holder and a circle of paper cut with a compass over the objective. The laser beam should strike the centre of the paper. If the laser is miscollimated or tilted in the EP holder you could rotate it one full circle and see where the circle beam descibes is located relative to the centre.

These scopes have the objective cell adjustable by means of three pairs of push-pull bolts at the front. If the cell were tilted I'd expect this to show in the star test as an imperfectly concentric set of diffraction rings. I know that the scope can give stunning star tests because the one I tested did so. It's an easy test with an eyepice. Whether or not it would produce the elongation you're getting I don't know, though.

Olly

Edit: BTW, spikes from a Mesu are well out of order. Have you tried changing the power supply? Maybe try a power supply independent of any other devices, too. I quite simply never get them in mine.

 

[ian_bird]

I had a similar problem recently. I had been using my rig (WO 98) down at Ollie's - and it was perfect.

Got back home - eggy stars. Someone suggested checking that the lens cell was tight - but the suggestion went straight over my head. I had never touched the cell - so why shouldn't it be tight.

Wind forwards quite a few months without using it. Finally get a chance to use it and notice the objective is a bit dirty. (OK - very dirty). Decided to clean it whilst it was still on the mount. To do that I had to move the built in dew shield back. There I was gently twisting it this way and that when I noticed the front of the OTA was rotating too! Which was odd - as the back of the OTA was firmly clamped in tube rings. ;-)

Turned out the front lens cell had been slowly unscrewing itself. At least 5mm of thread was showing! Could have been expensive.

TL;DR - check absolutely everything. Even the bits you *know* are prefect.

BTW - No eggy starts now!

[ollypenrice]

2 hours ago, ian_bird said:

I had a similar problem recently. I had been using my rig (WO 98) down at Ollie's - and it was perfect.

Got back home - eggy stars. Someone suggested checking that the lens cell was tight - but the suggestion went straight over my head. I had never touched the cell - so why shouldn't it be tight.

Wind forwards quite a few months without using it. Finally get a chance to use it and notice the objective is a bit dirty. (OK - very dirty). Decided to clean it whilst it was still on the mount. To do that I had to move the built in dew shield back. There I was gently twisting it this way and that when I noticed the front of the OTA was rotating too! Which was odd - as the back of the OTA was firmly clamped in tube rings. ;-)

Turned out the front lens cell had been slowly unscrewing itself. At least 5mm of thread was showing! Could have been expensive.

TL;DR - check absolutely everything. Even the bits you *know* are prefect.

BTW - No eggy starts now!

Yes, this is the kind of thing to look for. Doing AP is like being a spy hero in a John le Carré novel. Everyone, everything, is out to get you and on the double cross. Double? Sextuple!*

Olly

* Can you say 'sextuple' on SGL? Let's find out... 

[Zakalwe]

Never a truer word spoken....sometimes we dive in at the deep end without checking the basics. I'm sure that this will end up being something or nothing as the stars were perfect previously.

 

Failing that, it might be time to throw money at the problem and buy that nice Moonlite that I've been mulling over......

[gnomus]

1 minute ago, Zakalwe said:

.......  Failing that, it might be time to throw money at the problem and buy that nice Moonlite that I've been mulling over......

It really is most unfair of you to be trying to goad Mr Penrice in this way - he has been trying to help, you know ....  

[ollypenrice]

3 hours ago, Zakalwe said:

 

 

Failing that, it might be time to throw money at the problem and buy that nice Moonlite that I've been mulling over......

I used to like you, you know....

May your anodizing be pretty and red!!!!

lly

[Zakalwe]

Right- Ive been away sunning myself in Andalucia, so tonight is the first chance to poke and prod this thing.

I rechecked my polar alignment using the PoleMaster and it appears to be bang on.

Guide Log:

PHD2_DebugLog_2016-06-02_220604.txt

 

This is a 30 second exposure:

And a 60 second exposure:

Finally, a 300 second exposure:

 

So it looks like the egginess is caused by something not being orthogonal in the optical train (phew- at least it's not the mount!). Time to bring out the big hammer and start thumping things.....

[Zakalwe]

Moonlite incoming......

 

 

I did find that the stock focuser appeared to be out of collimation and appeared to have the drawtube tilted. I have adjusted it, but it's been cloudy ever since (typical). More money has been thrown down the bottomless pit that is this hobby.

In other news, my damn Mesu is not tracking correctly and is running slow on the RA axis. I think that i need a new hobby.....

[Zakalwe]

 

New focuser fitted. Just need time and some clear skies to test it.

[gnomus]

Hope you get this fixed Stephen.  The forecast looks good for tonight.

[nightster]

That is surely a sexy setup.  It is funny how a chunk of billet can stir amorous feelings in me.  Looks like you went with the blond.  I prefered violet, always had a thing for punk rock chics.  All kidding aside. Congrats, that is truly a monster rig. Hope is produces perfect circles. Oh and that stepper is black magic, enjoy it.

[Zakalwe]

Holy thread resurrection Batman!

A mixture of bad weather, work commitments and ennui has meant that I've not been able to update this thread.  So here's where I am at the moment.

I've been having problems with me Mesu as well...as mentioned in the original post the guiding trace was always one side of the line. I've been working with Lucas to try and diagnose that issue along with another problem (the mount intermittently "jumps" and loses the guide star). I've also followed Singlin's thread where he had a similar problem. last night was clear, so I had a determined effort to get to the root of the problem. I used PHD2's Guiding Assistant to measure the drift, which showed it to be about 3 arc-seconds per minute. I built a simple spreadhseet using the calculations in the thread above which gave me a correction. That was plugged into the SiTech RA Tick field and I then re-measured. As luck would have it I had subtracted the calculated error and it needed to be added. Another measure and I could see that the error was reducing, so I recalculated the error. PHD2 now shows the drift to be well under 0.5 arc-seconds per minute which the guiding should handle.

I ran some test subs- the guiding was a lot better. I still had the mount "jump" a couple of times, but that could be down to a lost guide star as I had some drifting cloud. The mount performance is a lot better now, and happily, it seems to have helped to sort the star shapes. I am guessing that the constant drift was not within PHD's correction abilities and this was causing elongated stars. The attached image shows the improvement. The stars are not 100% perfect, but there might be a bad sub or two in the stack. They are certainly a lot better now though!

So, I finally feel like I am making some progress here. It's not perfect and I need more time with it to see what has caused the few spikes that I had last night. I'm going to power the mount off a separate PSU to isolate any power issues. During one run of PHD Guiding Assistant the mount jumped, but at least this time it was in RA only.

 

On a separate note, I am blown away by the ASI1600 mono camera. The attached image is 30 x 3 minute hydrogen-alpha. Its giving better detail in a 3 minute narrowband sub than my old Atik 428 was giving in a 10 minute sub. There's no darks or bias in this image, just some flats (I need to redo these as there is a bit of ampglow in the top right corner). There's very little processing done on this image...just some basic levels and a tweak of curves.

 

 

[Thalestris24]

Hi there!

The 1600 does look quite good! It's a pity, though, that it's 12-bit - that must limit it's dynamic range? I guess you have the cooled version? I had eggy stars on my heq5 quite a while ago. I couldn't figure it at first as my PHD2 target showed a nice, tight clump of stars - but to one side of the centre. In the end it turned out that I was slightly unbalanced. My heq5 has a belt mod and it likes to be nicely balanced in all 3 axes i.e. no 'east heavy'. Dunno whether there might be similar issues with a Mesu? Your RA trace in the graph above has fallen off a cliff! I wouldn't expect that behaviour unguided - it's as though something has given it a yank! However, as DEC has continued ok, I'd say there may have been a communication loss which has affected only RA.

Anyway, hope you can sort it.

Louise

[kirkster501]

Steve (gnomus) you sent your FSQ85 back? Did I read that correctly?