Re-mounting and 80ED and ST80

[IanL]

I have a Skywatcher Evostar 80ED and an Orion ST80 mounted piggy-back on my NEQ6. The current arrangement is below:

I have used the standard Skywatcher rings and short/solid vixen-style dovetail for the 80ED, and attached the long thin vixen-style dovetail that came in the box with the NEQ6 to the camera-piggy-back holes on top of the Skywatcher rings. The ST80 is then mounted to the (upside down) dovetail using the standard Orion rings, and finally I have used the four cone error screws on the long dovetail back down on to the 80ED OTA to stabilise the dovetail/ST80. (Neither the camera holes nor the mated surface of the long dovetail are flat!)

It is a bit Heath-Robinson but it works. I have managed to get 15 min guided subs with no flexure problems or other issues, and could probably go longer if I wanted to. The problem I have (I think) is cone error on the 80ED. Despite doing a good polar alignment (using the polarscope and EQMOD) my first goto slew is usually off by a good two or three degrees and the target will not make it anywhere near the DSLR chip, leading to a lot of hunting around (sometimes with no success).

- I have tried aligning initially on stars near to the pole but there isn't much near enough that is bright enough to be seen in DSLR liveview with an LP filter as well. I could do it the hard way and take lots of 5sec exposures I suppose, but it takes about 10x longer to get aligned that way.

- I don't want to remove the imaging and guiding cameras and put an eyepiece in to do the initial goto alignments. The cameras are screw fitted, nicely orthogonal to the OTA/mount axes, etc. and it would probably take me 10-15 minutes to get it all sorted again if I demount a camera. (Focussers are non-rotatable so the OTAs would need rotating, etc.)

- I don't have a finder on this setup, though I do have a spare Meade 6 x 30 finder in a set of Meade finder rings. Trouble is I can't find any off-the-shelf way of mounting it in to the Skywatcher finder shoe (either on the 80ED or the ST80). The feet are incompatible and I haven't yet found a set of six-point finder rings with a Skywatcher shoe (do not want one of the o-ring types, cannot see that staying aligned between sessions). Definitely do not want to drop a load of cash on yet another finder either.

- The ST80 and guider have a wide enough FOV that the initial mis-alignment would still put the target on-chip, but the ST80 is not aligned with the 80ED and there is no easy way to get them aligned in this set up. I do have some cheap guidescope rings for the ST80 but I want to avoid them due to potential flexure if possible.

- I suppose I could try cone-error adjustment on the 80ED's short dovetail, but due to the hole arrangement between the upper and lower bars, I can only use the cone error holes at either the front or the back of the bar, so would be a hassle if I have to strip and re-assemble it all if the error is the wrong way. I'm not convinced I have got the rings that well seated/aligned with the dovetail anyway, since the clamshell design appears to allow the rings to be mis-aligned by a fair amount and still close.

Given my limited observing time since the summer (rain, work, kids, etc.) I don't want to waste too much time on fixing what was always a bit of a bodge in the first place. (Have managed three sessions in December, two of which were a total waste due to the alignment problems taking so long to fix that I got clouded out by the time I was good to go). So my initial thought is to move to a better mounting set-up:

- Either get some CNC-type tube rings with flat surfaces and a better (long/solid) vixen dovetail for the 80ED (should be easy enough to adjust cone error with multiple screws and some shims?). I could then get/make a bar to go on top and mount the ST80 in its own rings or even go mad and get some CNC rings for it too. That would allow me to deal with the cone error on the 80ED and ensure the rings were aligned with the dovetail due to multiple holes in the base rather than just one hole in the current rings. With a bit of luck the ST80 would also be better aligned with the 80ED without needing to resort to guidescope rings (if I can get the 80ED aligned, guider mis-alingment is not a problem for me to be honest).

I can easily find 90mm CNC rings (metric) for the ST80 (e.g. http://www.amazon.co...d=IR8QHN7QCDYS6) but so far the only place I can find 100mm CNC rings is in Italy (http://www.geoptik.c...421/30a401.html). They're about double the price of anyone else's there is not much information on hole spacing, etc. on the site plus they are a hideous orange colour .

The only other suggestion is to go for 105mm CNC rings, which are a comparable price to the 90mm for the ST80. Another SGL poster has used these with some really thick felt pads on and 80ED, but I am reluctant on two grounds; firstly all that felt would be a nice source of flexure, and secondly you are basically relying on whatever tape/glue you have used on the felt to hold your scope in the rings. (I know that is true of any lining to some extent, but a few 10ths to a mm of felt is not the same as 2.5mm of felt in my mind).

Any other sources of 100mm CNC rings (the OD of the 80ED measures a bit over 98mm as best I can tell with my calipers; it isn't 105mm which is the dew shield, and it isn't 90mm which some other 80ED clones seem to have been)?

- The second option is to go for a side-by-side setup, e.g. (http://www.teleskop-...---cameras.html). I can use the existing dovetails (I have a short/thin vixen for the ST80 as well) and would be able to use the cone screws on the 80ED to fix any error. (Still have to jig around with getting the rings straight but easier in a non-piggy-back arrangement). This seems like it would be more cost effective and the quickest route to a proper mounting solution.

I know balancing of a side-by-side is a bit harder but were' talking lightweight scopes on a big mount here so not as hard as it would be if I had a big newt and a small guidescope for example.

Any thoughts on this or other ideas welcome before I get the credit card out!

[steppenwolf]

If you are getting 15 minute exposures with no differential flexure then I'd be very loathe to change the existing mounting and I'd certainly avoid adjustable tube rings at all costs. Balancing a side-by-side arrangement is very straightforward in fact if you follow a simple workflow but again, you don't really appear to need to go down this route.

Your main issue seems to be one of achieving that all important first star alignment and this may or may not be due to cone error. It is more likely that you are simply not getting the 'Home' position smack on and this is vital to a successful first star alignment.

I'd be tempted to try the following:-

Set up with the best 'Home' position you can determine and carry out a one star alignment BUT instead of centring the alignment star on the sensor using the movement controls of the hand controller, release the RA and DEC clutches and manually push and pull the telescope until you get the alignment star as closely centred on the sensor as possible (a continuous series of 6 second exposures should be fine for most alignment stars). Once centred, tighten the clutches and accept the alignment. Now Park the mount. Using masking tape or suitable sticky labels, place markers on both the RA and DEC axes indicating this park position very accurately.

In future sessions, use these alignment marks to get your 'Home' position correct.

Why does this work? You have fooled the system into believing that it had a perfect start position first time round as no input was required from the hand controller and when you immediately parked the mount you returned to this 'false' start position!

Hope that helps - it certainly works very well for me.

[IanL]

Steve,

Thanks for that; I am attacking the problem on several fronts and spending another £150-200 on a dual mount setup is not my first choice, just one I am considering since I cannot bear the thought of losing more of the few decent winter nights to such a simple problem.

I am really loathe to do anything to the set-up for fear of breaking it! I just cobbled something together out of the parts I had to hand when I got the 80ED so I could try it out. I fully expected I would have to spend some cash later on a proper mounting setup so I was completely gobsmacked when I was getting 15 min subs on the first night from a collection of bent metal and random bolts. I do fear that buying a new mounting system will (as usual) bring on yet another round of complications to be solved which is why I am looking for opinions and experiences before I even go near the idea.

Is it cone error or home position? I'm pretty certain it is cone error but I am going to perform some daylight tests to be 100% certain and maybe have a go at further bodging to cure it before I spend any money. I use my 2,000mm focal length LX10 OTA on the same mount with the same home position markings (which are pretty much spot on). I can get a first alignment star (far from the pole) on the DSLR chip first time every time with that setup, but not with the 80ED at a quarter of the focal length. I am off chip by at least a degree and maybe a fair bit more. I can't think of any other variables than the ED80's optical axis not being aligned with the rotational axes of the mount since my PA routine is the same for each. Maybe I will try) aligning using the LX10, then swapping the 80ED in place without powering down or changing the alignment model to see how far out it is on the same alignment stars.

I am using EQMOD with a gamepad to hunt for the alignment star rather than a handcontroller. If I can't get a bright star on chip by slewing around gently with the motors, I am going to have even less chance of doing it manually with the clutches unlocked. (I could if I had a finder that I had aligned with the 80ED during the day, but don't have one as noted above). So one of my other lines of attack was to do a polar alignment, park the mount to home, take an image and plate solve it. That would tell me exactly how far off I am in RA and dec.

I could then use the setting circles to offset from my true home position marks to the 'home' position needed by the 80ED. I figured this out on Tuesday night standing in the dark and cold, so was reasonably impressed with myself. Problem was that I had just fitted my lovely new IDAS LP filter which had thrown out the focus position. I had roughly focussed by doing repeated short exposures, but it wasn't good enough for the astrometry.net plate solver to handle, and had been LiveViewing for so long that the camera chip was throwing out a sea of noise anyway which wouldn't have helped.

I needed to get on to a reasonably bright object to get good focus using LiveView through the DSLR, but of course I couldn't get on anything due to the bad alignment. Eventually I did get on to Jupiter, tweaked the focus enough that I could finally get on to Aldebaran and fine focus. I was doing all this through breaks in the cloud, and just as I got the focus nailed, the clouds rolled in for good that night!

My second line of attack (and ideal solution to be honest) is that I am playing with AstroTortilla. Tuesday was my first chance to play with it in the field. The mechanics of it all seemed to work fine, talked to the mount, software is functioning, etc. The main issue was that it wasn't plate solving anything; poor focus being one issue and frankly I haven't got the parameters nailed down well enough yet to get plate solves in a few tens of seconds reliably. I have hopes that this might be the eventual solution, since I wouldn't have to care about cone error or anything else if using automated plate solving to get on target, could just go inside and make a cup of tea whilst the mount sorted it out

Anyway, in the interests of moving things forward on multiple fronts, still interested in people's views on the various dual mount systems out there (just found another thread that said the Teleskop Service Vixen dual bar didn't fit an ADM losmandy/vixen dovetail saddle (presumably an upgrade) but did fit the standard NEQ6 dovetail saddle). Seems rather odd, also looks like it is a single screw locking mechanism for the saddles at the ends of the bars, rather than a full length clamp, which makes me think I might have to go down the ADM route instead if this is what decide.

Most of all I am looking for a bit of encouragement; at the start of December with the new scope I thought I was really starting to get a good setup that would make me more productive, so a string of failures since then and no end in sight is a bit discouraging!

[jnc71106]

Why not just get a RDF attach it to the ST80 then line the RDF up with the ED80 when you setup. Once you goto that first star as long as you centre it in the RDF it should also be on the imaging chip

It's how I do it

[IanL]

Why not just get a RDF attach it to the ST80 then line the RDF up with the ED80 when you setup. Once you goto that first star as long as you centre it in the RDF it should also be on the imaging chip

It's how I do it

Which RDF do you use and how did you attach it to the ST80, how do you adjust the alignment? It is definitely an option I would consider as it is simple and relatively cheap.

I used to manually sight along the tube of my LX10 when it was on forks/wedge before using its finder to adjust, but the GEM arrangement makes that much harder in general and the longer tube of the ED80 means I'd be on my knees half the time, as I would if the RDF was at the focuser end. Can they be mounted at the objective end and used successfully.

(Maybe dumb questions but it is one piece of kit I have never owned or seen in use personally).

[jnc71106]

I've got a baader skysurfer III which slots into the foot on the st80 focuser

It has 2 knobs that you adjust to move the dot. So pop an eyepiece in the ED80 get a distant object centred i.e tree or pylon that sort of thing, then adjust the knobs on the RDF until the dot is also on the same object

I only use my RDF once in the night for aligning the first star after that all the other stars are on the chip first time so no need to be on your knees all the time

Easy peesy

[steppenwolf]

I use my 2,000mm focal length LX10 OTA on the same mount with the same home position markings (which are pretty much spot on). I can get a first alignment star (far from the pole) on the DSLR chip first time every time with that setup, but not with the 80ED at a quarter of the focal length.

Don't forget that the pointing of the telescope is dependent not only on the mount axes but also on the mounting of the tube rings to the dovetail bar so one telescope may well work from your known 'Home' position but that certainly doesn't mean that a second telescope will also be a good match positionally - my suggestion above is a 'one telescope, one mount solution' so a change of telescope will required the process to be repeated to get a new start point calibration.

[IanL]

Don't forget that the pointing of the telescope is dependent not only on the mount axes but also on the mounting of the tube rings to the dovetail bar so one telescope may well work from your known 'Home' position but that certainly doesn't mean that a second telescope will also be a good match positionally - my suggestion above is a 'one telescope, one mount solution' so a change of telescope will required the process to be repeated to get a new start point calibration.

Yep, that's my point and why I think it is cone error rather than something else that I have done wrong (such as a bad PA or having JNOW in one software component and J2000 in another). If the LX10 is bang on with its small FOV, and the 80ED is off with its much bigger one, then either the LX10 or the ED80 is misaligned.

I found the home position by using a bubble level and took great care over it, plus the LX10 is bolted to an ADM Losmandy dovetail with radius blocks; mechanically there is no room for adjustment in that setup and the SCT isn't suffering from mis-alignment of screw holes between the corrector cell and the rear cell, otherwise the dovetail wouldn't have fitted (it is tight, no slack or wiggle room in the screw holes and it either fits or it doesn't, end of!)

The 80ED is dangling off a narrow /short Vixen Dovetail in cheapo Skywatcher clamshell rings (single screw at the base of each ring). Would be pretty easy for the rings not to be properly aligned with the dovetail in all three axes, plus the clamshell design with the ST80 hanging off the top half means I can't even guarantee the OTA is sitting square in the rings itself. Plus of course the optical axis of the OTA may not be aligned with the mechanical axis of the OTA, but there is nothing I can do about that.

All in all, I am pretty certain that the LX10 is spot on and the home position I have found is the mechanically correct one for the mount.

Either I mess around with the current 80ED mounting to try to reduce the cone error (risking losing yet more nights), get a better mount (ditto and costs money too), find a better home position for the ED80's optical axis (free but tricky without a finder), or get a finder of some sort - looked for the Baader and they are out of stock everywhere at the moment The alternatives are pricier and mainly seem to involve fixing them on with sticky pads :(

Auto-plate solving is my long term goal, but it is not proving as simple as I would like.

[kirkster501]

Maybe a daft question from a numpty like me but why is this as issue? I go to home position, select Kochab as first alignment star and it is not in the field of view of ED80. So I use controller or EQmod to nudge it so it is, select align and then the next star I use to refine an alignment, say Capella or Alperatz is much better and in the field of view. Third star is almost crack on...

Why this procedure you [very expert] guys are suggesting above? The standard procedure with three stars only takes five minutes.... Perhaps I am missing something!!! Very interested.....

[Jessun]

there is no easy way to get them aligned in this set up

I don't have anything really similar in terms of setup but always wondered if there was any potential gain in simply rotating two scopes in their rings to see if their FOV got any closer. How 'straight shooting' is a SW refractor? No idea myself...

For my side-by-side setup I use Altair Astro stuff and find their system to work very well. I'm not too picky about where the scopes point though...

[The Sailor]

I have the OTAs side by side, the ST80 on a guide mount on the right hand side and if I use any of my other OTAs on the left I can adjust the ST80 accordingly, works quite well.

Jim

[IanL]

Maybe a daft question from a numpty like me but why is this as issue? I go to home position, select Kochab as first alignment star and it is not in the field of view of ED80. So I use controller or EQmod to nudge it so it is, select align and then the next star I use to refine an alignment, say Capella or Alperatz is much better and in the field of view. Third star is almost crack on...

Why this procedure you [very expert] guys are suggesting above? The standard procedure with three stars only takes five minutes.... Perhaps I am missing something!!! Very interested.....

Well the issue for me is that I am not using an eyepiece but my DSLR. The FOV is pretty small, about 2 degrees by 1.5 degrees, plus with the LP filter fitted I can only see down to about Mag 1 in liveview mode. So Capella or Aldebaran would be fine for alignment, but Kochab just isn't visible. By the time I move that far off the pole with no alignment I am miles off.

Whilst I can hunt around (and do) with the directional controls, it is a real pain in the backside and I have literally wasted up to an hour just getting a basic alignment on two occasions. Given the limited imaging time I have, not time I want to lose.

Nor do I want to be switching cameras for eyepieces or adding/removing the LP filter, just want to drop the ready-assembled/tested OTA/Cameras/Guider on to the mount. Taking it all apart, putting it back together, getting the cameras orthogonal, re-focussing, etc. all takes time, usually a lot more than 5 mins and usually results in some kind of software, mechanical or usually operator error.

[IanL]

Anyway, I took an exposure in the home position last time out with a view to plate solving it. I struggled with that for a while as it looks like I might have some issues with the corrector/filter/camera not being properly aligned/spaced. The stars on the left third of the image are showing serious coma (elongated towards the image centre) but the right two-thirds of the image the stars are round.

I cropped out the middle of the image (ensuring the centre point remained central) and solved that. I then used Stellarium to find the calculated image centre and overlay the FOV of the DSLR. See below:

As you can see, the 80ED is off the pole by about 2 degrees. I don't think this can be fixed by the cone error screws on the dovetail as these would adjust the pointing up and down relative to the image. I think I would have to work on rings/dovetail alignment or maybe the scope/rings being off axis.

[The Sailor]

I have the OTAs side by side, the ST80 on a guide mount on the right hand side and if I use any of my other OTAs on the left I can adjust the ST80 accordingly, works quite well.

Sorry, I should have said the ED80 with QHY5 is on the guide mount and that it is adjusted to the ST80 or whatever OTA is used at that time

Jim

[IanL]

Had a bit more of a chance to experiment, and what I have found is:

- I can now get AstroTortilla to reliably solve the snapshots I am taking with the camera. I have also tested the sync/refine process with the EQMOD simulator and it works. My problem was that if I didn't give the software a min/max image scale it would take forever to (not) solve a plate. If I gave it what I thought was the right image scale it would fail to solve. Turns out I was setting the max image scale parameter to 2 arcseconds per pixel, which was too low as it is nearly 3.9 arcseconds per pixel (think I was using values from an online solve of an image without the LP filter which has clearly changed the pixel scale more than I thought it would).

So now I should be able to plate solve my way to alignment (or actually not bother, just tell AT to sort things out and get me on target whilst I have a cup of tea).

- Failing that I know roughly where I am now in terms of being off target and it will make manual correction much easier (see above). I'm loathe to re-work the mounting rings, etc. when they have done so well in terms of flexure.

- I may have some horrible coma problems. Each of the snapshots I took to help figure out the misalignment appears to have a slightly different pattern of uneven coma (but all with the same tendencies). Not sure why, but suspect the introduction of the LP filter has thrown things off (or maybe I never really noticed before). I will start a different thread on that one though