Ovoids with Mesu and Tak FSQ 85

[gnomus]

If the guide star were well off-axis relative to the Tak then wouldn't polar misalignment also produce this effect?

Which connector is that, Steve? Since mine departed some years ago I can't remember it in every detail but the only connector to give me gyp was the tilt-adjuster (if that's what it was meant to be) on the reducer. Steve (Gnomus), are you using the reducer here? If you are then the 'three radial screw horror' is designed to test your tilt adjusting skills! I just loosened the screws, put the flattener on the table, pressed the top part down onto the bottom and tightened them. That worked in my case.

Steve. (Just going with the flow - or should that be FLO?  )

I have one of these SkyWatcher Guidescope Mounts http://www.firstlightoptics.com/guide-cameras/skywatcher-guidescope-mount.html and I have ensured that the guidescope and the Tak are pointing at exactly the same point of the sky.

I do not have the reducer - was thinking of getting one but the stories relating to that adapter and its screws do not leave me brimming with optimism.

Can I ask again about orthogonality of my unmounted 36mm filters.  Could this be an issue or would I just be wasting my time checking this? 

[ollypenrice]

I have one of these SkyWatcher Guidescope Mounts http://www.firstlightoptics.com/guide-cameras/skywatcher-guidescope-mount.html and I have ensured that the guidescope and the Tak are pointing at exactly the same point of the sky.

I do not have the reducer - was thinking of getting one but the stories relating to that adapter and its screws do not leave me brimming with optimism.

Can I ask again about orthogonality of my unmounted 36mm filters.  Could this be an issue or would I just be wasting my time checking this? 

OK so bin my suggestion about PA if the guide scope is on axis. I have to say that I never like adjustable guide scope holders or rings* and always prefer a bolted-down guide scope even if it isn't perfectly on-axis. With good PA this won't matter. That adjuster has to be a suspect, though it may be perfectly innocent.

I guess the orthogonality of your filters could play a role but does changing filter change the distortion? If not it's highly unlikely to be that. (You'd have done well to replicate the same tilt in every filter...)

Olly

*Except in the case of dual rigs where, on two occasions, we've found the system prefers an on-axis guide star. I can't account for this. It could be experimental error but it happend twice so I just accepted the fact.

PS, I found the Tak and reducer to be a stunning combination and Sara always runs the reducer, I believe, so I wouldn't be put off it too easily.

Edited October 30, 2015 by ollypenrice

[Astroscot2]

If I were you I would remove the Skywatcher Guidescope mount and secure your guidescope by some fixed means . I suspect younever need to move the guidescope to find a guidestar anyway. Im think Mark is right pointing towards Fexure as the issue. At that focal length I would want pinpoint stars and you right to expect them.

Mark

[gnomus]

OK so bin my suggestion about PA if the guide scope is on axis. I have to say that I never like adjustable guide scope holders or rings* and always prefer a bolted-down guide scope even if it isn't perfectly on-axis. With good PA this won't matter. That adjuster has to be a suspect, though it may be perfectly innocent.

I guess the orthogonality of your filters could play a role but does changing filter change the distortion? If not it's highly unlikely to be that. (You'd have done well to replicate the same tilt in every filter...)

Olly

*Except in the case of dual rigs where, on two occasions, we've found the system prefers an on-axis guide star. I can't account for this. It could be experimental error but it happend twice so I just accepted the fact.

Thanks again Olly.  I have spent some time going through various subs.  At first I had convinced myself that the Ha filters all had the same issue - worse in the top left and top right corners - and that my Lums were worse in the top right and bottom right corners.  But the more I looked at this, the less certain I was that I was seeing a consistent error.  I am increasingly coming to the view that the issue may be one of a cable dragging.  If that were the case then I might get different directions on my elongations depending on where the scope was pointing - isn't that right?.  When setting up, I was a little unsure where I should attach the cable bundle.  In the end I ran a piece of velcro around the join between the top of the Tak clamshell and the mounting plate for the guide scope bracket - I hope this image shows what I mean (ignore the velcro on the Tak lens hood: this was early on when I had temporarily attached a Telrad to the system to help set up):

 

I did think about velcroing the bundle to the join between bottom of clamshell and the dovetail bar.  I am planning to moving to a dual rig with Tak and C8 side by side over the next few days and I will try to find somewhere else to attach my cable bundle. 

I had intended to fix the guide scope in a set position with a Baader clamp until I read something on this site about how important it was that the guide scope and imaging scope be aligned!  'You say poh tah toh, and I say ......'

Edited October 30, 2015 by gnomus

[Zakalwe]

If I were you I would remove the Skywatcher Guidescope mount and secure your guidescope by some fixed means . I suspect younever need to move the guidescope to find a guidestar anyway. Im think Mark is right pointing towards Fexure as the issue. At that focal length I would want pinpoint stars and you right to expect them.

Mark

I'd agree with this. I personally never liked the look of those guidescope mounts and have always treated them with some suspicion. Too many pivot points and potential areas of movement. With a short focal length guidescope is there any need to move the guidescope at all?

[steppenwolf]

Ouch.  However, I have not found this to be the case with astronomical or indeed any other type of gear, and I too have suffered from the 'hifi sickness'.     

What I am trying to do with my question is ascertain whether or not I do have a significant issue and, if I do, seek advice as to how to fix it (thereby getting the best out of my equipment).  I may well be a nincompoop, but I was getting rounder stars with my ED80.

That word 'Ouch' and your subsequent paragraph has given me a terrible feeling that you may have thought I was in some way criticising you or your methods - please be absolutely assured that that was not my intent and please accept my apologies if that it how you read it! On the other hand, if the 'Ouch' was because you have spent more money but not found this to be a panacea then you have read me correctly!

Nincompoop - no, you may recall that I have sought your advice on a Mesu related issue and I wrote the flippin' manual!!

Which connector is that, Steve?

The tilt adjuster module.

Your cable management looks pretty good to me but I would rather see the loom attached to the base of the clamshell as it could pull on the Sky-Watcher adjustable guide scope mounting. I have one of these mountings and found that it was very good but I also made the following comment when I reviewed it although it should be borne in mind that I was already suffering from differential flexure before I installed this adaptor:-

The  incremental  flexure-induced  movement  of  the  star  over  50 minutes of autoguided tracking (10 x 5 minutes) was 9.438 pixels in RA and 2.267 pixels in Dec. At first  glance,  this  seems  a  disappointing  result  but   it  needs  to  be  read  in  the  context  of  the  results from  the  fixed  attachment  method  that  was  used  as  the  control  and  this  yielded  results  of  8.927 pixels in RA and 1.949 pixels in Dec. The differential flexure contributed by the guidescope mount was, therefore, 0.511 of a pixel in RA and 0.318 of  a pixel in Dec

You could try removing this to see if it makes any difference to your results - without doubt, the guiding system should be as rigid as possible -  did you use this guide scope mounting with your ED 80 as well?

Having both guide telescope and imaging telescope perfectly aligned is not a pre-requisite for accurate guiding.

[ollypenrice]

That word 'Ouch' and your subsequent paragraph has given me a terrible feeling that you may have thought I was in some way criticising you or your methods - please be absolutely assured that that was not my intent and please accept my apologies if that it how you read it! On the other hand, if the 'Ouch' was because you have spent more money but not found this to be a panacea then you have read me correctly!

Nincompoop - no, you may recall that I have sought your advice on a Mesu related issue and I wrote the flippin' manual!!

The tilt adjuster module.

Your cable management looks pretty good to me but I would rather see the loom attached to the base of the clamshell as it could pull on the Sky-Watcher adjustable guide scope mounting. I have one of these mountings and found that it was very good but I also made the following comment when I reviewed it although it should be borne in mind that I was already suffering from differential flexure before I installed this adaptor:-

You could try removing this to see if it makes any difference to your results - without doubt, the guiding system should be as rigid as possible -  did you use this guide scope mounting with your ED 80 as well?

Having both guide telescope and imaging telescope perfectly aligned is not a pre-requisite for accurate guiding.

I honestly don't remember a tilt adjuster module on mine. I'm not at all sure that I had one, but Tak do change things around with their hardware. I certainly never used it since I didn't know about it.

Well, it looks as if nobody likes that guidescope carrier and, on single scope setups, I've never had any issues arise from approximate alignment of guidescope and imaging scope.

While I like and use (two) ST80s for guiding there are a couple of things to watch, though I doubt you've missed them. The entire rear end of the scope is held into the main tube by three screws. These need to be very tight. I wouldn't baulk at a drop of epoxy to hold the back end in firmly, either. I did have the back end cause flexure once. The drawtube is also very vague so I tighten the lockscrew rather pitilessly to keep it still. Any extensions carrying the guide camera also need careful attention.

Olly

[gnomus]

That word 'Ouch' and your subsequent paragraph has given me a terrible feeling that you may have thought I was in some way criticising you or your methods - please be absolutely assured that that was not my intent and please accept my apologies if that it how you read it! On the other hand, if the 'Ouch' was because you have spent more money but not found this to be a panacea then you have read me correctly!

Nincompoop - no, you may recall that I have sought your advice on a Mesu related issue and I wrote the flippin' manual!!

The tilt adjuster module.

Your cable management looks pretty good to me but I would rather see the loom attached to the base of the clamshell as it could pull on the Sky-Watcher adjustable guide scope mounting. I have one of these mountings and found that it was very good but I also made the following comment when I reviewed it although it should be borne in mind that I was already suffering from differential flexure before I installed this adaptor:-

You could try removing this to see if it makes any difference to your results - without doubt, the guiding system should be as rigid as possible -  did you use this guide scope mounting with your ED 80 as well?

Having both guide telescope and imaging telescope perfectly aligned is not a pre-requisite for accurate guiding.

Ah the perils of non-verbal communication.  Please be assured Steve that I took no offence whatsoever from your comment - you are trying to help me after all.  My 'Ouch' was meant humorously - perhaps "Touché" would have been a better response.  I am however a little upset that you so lightly dismiss my credentials as a nincompoop.  I am nincompoop-and-proud.  (As this is the Internet, I should perhaps add a ).

I used the ST80 when guiding the ED80, but it was not attached with the Skywatcher thingamajig.  I had everything on the CGEM back then, and I was probably trying to keep the weight down.  The Skywatcher does add quite a bit of height to the set up.

I have this arrangement of holes on the top of my Tak clamshell:

I could try bolting the ST80 rings direct to this (although those rings will only end up an inch and a bit apart).  Alternatively I have a Baader mini dovetail that I could affix to the Tak clamshell.

I will also try moving the point where I attach the cables to lower down - between the Tak and the Dovetail plate.

[gnomus]

I honestly don't remember a tilt adjuster module on mine. I'm not at all sure that I had one, but Tak do change things around with their hardware. I certainly never used it since I didn't know about it.

Well, it looks as if nobody likes that guidescope carrier and, on single scope setups, I've never had any issues arise from approximate alignment of guidescope and imaging scope.

While I like and use (two) ST80s for guiding there are a couple of things to watch, though I doubt you've missed them. The entire rear end of the scope is held into the main tube by three screws. These need to be very tight. I wouldn't baulk at a drop of epoxy to hold the back end in firmly, either. I did have the back end cause flexure once. The drawtube is also very vague so I tighten the lockscrew rather pitilessly to keep it still. Any extensions carrying the guide camera also need careful attention.

Olly

I do super- tighten the focus lock screw. I hadn't cottoned on to the three screws holding the focuser in place. I will check these later today.

[steppenwolf]

I honestly don't remember a tilt adjuster module on mine. I'm not at all sure that I had one, but Tak do change things around with their hardware. I certainly never used it since I didn't know about it.

It was a couple of years ago, Olly so it could well be that the tilt adjuster that I am referring to is only used with the reducer (that I also bought).

Alternatively I have a Baader mini dovetail that I could affix to the Tak clamshell.

I'd prefer that choice as the load will be spread over a wider contact area.

[Davey-T]

I hesitate to add my 2d to the debate in view of all the expert advice offered but on the ST80 guider subject, I used one as a guidescope for a while but after the usual endless fault finding it did turn out to be the weakest link so I said goodbye to it, after that I initially used my 100mm refractor for guiding which despite the increased weight was a bIg improvement over the ST80, mainly because of the much better focuser I think.

In for a penny in for a pound Steve, more money to spend 

Dave

[gnomus]

I hesitate to add my 2d to the debate in view of all the expert advice offered but on the ST80 guider subject, I used one as a guidescope for a while but after the usual endless fault finding it did turn out to be the weakest link so I said goodbye to it, after that I initially used my 100mm refractor for guiding which despite the increased weight was a bIg improvement over the ST80, mainly because of the much better focuser I think.

In for a penny in for a pound Steve, more money to spend 

Dave

You enjoyed that last sentence didn't you Dave? I do still have the ED80 that I put a Moonlite focusser on, but that doesn't have the same focal ration as the ST80. I assume that trying to guide with my C8 (once I get dual mounting up and running) would be complete madness?

[Zakalwe]

 I assume that trying to guide with my C8 (once I get dual mounting up and running) would be complete madness?

Hmm..you have the potential for mirror flop to contend with.

Rigidity is everything in guiding...and a moving mirror does not give rigidity.

<edit> I'd be tempted to try the ED80. The slightly longer focal length won't cause an issue. The ED80 is a lot heavier than the ST, but you've got the Mesu now. You'll not be running out of capacity just yet!

Edited October 30, 2015 by Zakalwe

[gnomus]

Hmm..you have the potential for mirror flop to contend with.

Rigidity is everything in guiding...and a moving mirror does not give rigidity.

<edit> I'd be tempted to try the ED80. The slightly longer focal length won't cause an issue. The ED80 is a lot heavier than the ST, but you've got the Mesu now. You'll not be running out of capacity just yet!

I could try the ED80 but what about th f/5 vs f/7.5 (from memory) issue?

[swag72]

I could try the ED80 but what about th f/5 vs f/7.5 (from memory) issue?

I used to use an OAG with a 2.5m SCT at f10 ............. never failed to get a guide star.... although they could be a little dim!! I'm a great one for experimenting and not following published theories etc ............. Give it a go. I bet you get a guide star at f7.5 every time.

[Zakalwe]

I could try the ED80 but what about th f/5 vs f/7.5 (from memory) issue?

What issue?

Even with a small sensor like an original Lodestar you will have a 35.33' x 27.25' FoV. I'll eat my metaphorical hat if you can't find a guidestar in that!

[gnomus]

What issue?

Even with a small sensor like an original Lodestar you will have a 35.33' x 27.25' FoV. I'll eat my metaphorical hat if you can't find a guidestar in that!

I'll give it a go, but, before I do, can I see a photo of the hat?

[gnomus]

I used to use an OAG with a 2.5m SCT at f10 ............. never failed to get a guide star.... although they could be a little dim!! I'm a great one for experimenting and not following published theories etc ............. Give it a go. I bet you get a guide star at f7.5 every time.

Thank you Sara. My wife has repeatedly told me that she'd like me to experiment more. Now I know what she has been on about.

[Zakalwe]

I'll give it a go, but, before I do, can I see a photo of the hat?

Thank you Sara. My wife has repeatedly told me that she'd like me to experiment more. Now I know what she has been on about.

:grin:

[sharkmelley]

Thanks for looking and confirmng my suspicions Mark. The elongation is all in the same direction - it is worse in some corners but it is present in the centre. I am dithering, so it is just possible that the 'blink' test is coincidental.

No problem.  It was a good test for a PixInsight script I'm writing.  It takes the centre, corners and edges of an image, analyses the shape of stars it detects and then overlays the detected stars with a symbol showing the elongation direction.  It then averages the stars in each section of the image and displays a much larger version showing the elongation of the average star, together with statistics:  FHWM, Eccentricity, Angle of elongation.

I forgot to say that the length of the line in the large symbol is a function of the eccentricity.  The more misshapen the star, the longer the line.

Mark

Edited October 30, 2015 by sharkmelley

[gnomus]

That is interesting to see. Thanks for posting it. So, much more likley to be flexure than any optical issue then?

Edit: Poor PA wouldn't look like this would it?

Edited October 30, 2015 by gnomus

[sharkmelley]

That is interesting to see. Thanks for posting it. So, much more likley to be flexure than any optical issue then?

Edit: Poor PA wouldn't look like this would it?

Yes, flexure is the most obvious explanation.

Mark

[Steve 1962]

So, much more likley to be flexure than any optical issue then?

Edit: Poor PA wouldn't look like this would it?

I would have thought poor PA would give rise to field rotation rather than a "tilt" effect.

Bear in mind that we're looking at micron scale pixels here. A human hair is about 100microns across - or 20 pixels to put this into perspective.

Looking at the problem from that perspective makes it obvious (to me at least) that the slightest flex or slop can show up - so I only use big, solid, bolted or threaded fittings and an OAG. The OAG takes a whole raft of "risks" out of the equation - and in my opinion doesn't have a down side - providing you've got the spacing available.

[gnomus]

OK chaps. I'll do some serious reconfiguring tomorrow and then wait for a clear night to test. I am not sure I am quite ready for off-axis guiding - do folks like the Atik OAG? (I have an EFW2.). I would have enough spacing, even if I did eventually get the Tak reducer.

Thanks to everyone who has contributed to this thread - all very helpful.

[ollypenrice]

It was a couple of years ago, Olly so it could well be that the tilt adjuster that I am referring to is only used with the reducer (that I also bought).

I'd prefer that choice as the load will be spread over a wider contact area.

Yes. I used a Baader saddle bolted onto the clamshell. Then both my solarscope and guidescope, on Vixen rails, could be swapped easily for each other. The system also allows easy fine tuning of Dec balance by sliding the guidescope fore and aft.

Olly