SkyMax127 mounting plate - Please Help!

[OzDave]

Hi,

I have recently discovered that a major cause of much frustration with my scope and GOTO mount is due to a fairly major misalignment of the optical axis with that of the mount's RA axis. This has meant that when I polar align the mount accurately with the polarscope (my polarscope reticle is perfectly adjusted), I then cannot see Polaris through the scope no matter how I point it.

To combat this, I tried to apply the "standard" SkyWatcher procedure as documented in the manual. I then discover that I can't actually make the required adjustments because the scope mounting plate appears to be too big.

I seem to have a plate directly attached to the OTA, which is 18cm long and has various holes for attachment to a mounting plate. The plate supplied with the scope is a SkyWatcher 21cm medium dovetail.

When I first got the kit, there were no screws supplied to actually attach the dovetail to the scope, and on top of that, no two holes in the dovetail seem to lineup with the plate on the OTA, meaning that I could only attach the dovetail with one screw. I did that for a while, then drilled a second hole in the dovetail as I was getting a fair bit of flexure.

Now I discover that the 4 cone error adjustment bolts that go in the corners of the dovetail don't work. Or rather, only two of them do at the rear end of the scope. Looking at the SkyWatcher manual, it doesn't look like the dovetail I have is meant for connection to the OTA at all. It's all been a hodge podge to get it together at all and now I have serious misalignment which means the GOTO really doesn't work well.

So, bit of a long story, but can someone else with a SkyMax127 tell me how this is supposed to be connected to a dovetail? How big is the dovetail? Do you use the cone error adjusters? Any pics would be great.

Please help as this whole problem has been driving me nuts for getting on for a year now!

David

[OzDave]

bump!

[Astro_Baby]

Hmmm - what mount is the SkyMax on ? Can you provide any pictures ?

I am assuming (as you mention a polarscope) that the scope is mounted to some kind of EQ mount. A word here - you wont see POlaris in the main scope optics necessarily when the polarscope is aligned. Remeber that polar alignment is about aligning the mount to POlaris NOT the telescope.

I know that seems counter intuitive but its how its is. If I align my EQ mount to polaris I cant see Polaris in the main optics unless I use a very wide field eyepiece and even then POlaris is often only just about in view at the edge of the FOV. Thats how its supposed to be.

The cone erropr adjusters usually press against the tube rings of a scope with rings. I dont use them because a three star star alignment on a GoTo cancels out cone error (as far as I know). That may not be true for imaging but I only do observing so its never been a problem for me.

Its tough to know what the dovetail problem is without seeing a pic.

I have a SkyMax 180 and its dovetail has no cone arror adjusters on at all. the dovetail is mounted flush with the scope and the dovetail fits directly to the EQ mount.

[Cornelius Varley]

As Astro_baby says, the telescope is not aligned on Polaris so you wouldn't neccessarily see Polaris in the eyepiece. The dovetail bar on the base of the telescope should fit directly onto the mounting dovetail slot, the extra dovetail bar supplied with the mount is for use with telescopes without a built-in dovetail bar ie reflectors or refractors together with a pair of tube rings.

Peter

[Ant]

I have a skymax 127 (but the dovetail is fitted in the same way as Mel's 180.

The dovetail is attached to the scope tube (don't undo it as you could end up with loose stuff inside the telescope tube). The dovetail then fits into the "Puck" on the top of the EQ mount.

See picture below

Does your mount/scope look a little like this?

I've had two Mak's (180 and 127) and not had any issues (although seeing polaris through the main scope will be tricky to say the least with an EQ mount). In fact I have no isea what cone error is (off to find out).

Cheers

Ant

[OzDave]

Hi Astro_Baby,

My mount is an EQ3-2. I realise the polar alignment aligns the mount's RA axis to the polar axis. My problem is that once aligned, the scope's optical axis is clearly not aligned to that of the mount.

My scope is a SkyMax127 with 1500mm FL. If I fit a 40mm EP with x0.5 focal reducer, I should be getting a magnification of about 18.75 times...quite low. When I look though that combination I can see Polaris, because the FOV is wide enough. But when I rotate the mount 180 degrees about the RA axis (from one horizontal position to the other), Polaris seems to rotate about a point which is clearly not the NCP.

What I'm expecting to see in the EP, is the NCP approximately in the centre of FOV (not that you can see the NCP of course, but bear with me) and Polaris rotating in a nice circle about that point. Like it does in the Polarscope.

Unfortunately this does not happen and is not even close. The 3-star alignment seems to be affected by this as slews to targets "near" the NCP are wildly inaccurate. Also, I can notice that moving the mount in RA and in DEC does not move a target star in directions that are perpendicular to each other. This seems to indicate that the RA and DEC axes are not perpendicular somehow. Which would explain many problems I'm having.

I'll try to get some pictures of the OTA mounting plates.

[Astro_Baby]

I must admit I have never checked but I am pretty sure that POlaris wont in fact rotate in the eyepiece in a small circle when the mount is aligned to Polaris. I can be reasonably sure of this because when I centre Polaris in the eyepiece of a scope and then rotate the mount through RA the stars around polaris will rotate around it but Polaris stays still - thats as it should be. When the scope is centred on Polaris it has to move through RA and DEC a little to shift to Polaris from its 'parked position' ie when the mount is aimed at POlaris with the polarscope - if I selct POlaris from the GoTo the scope will move slightly. Thats (as far as I know) exactly what should happen.

You must remember that when you align the mount to Polaris the scope isnt in fact aligned to Polaris - the mount is. The scope will be offset by the distance between the mounts polarscope and the main optics. The scope is not aligned in anyway to the polarscope (like the finder scope is to the main optics) the scope is in fact more like parallel to the polarscope.

Imagine if you put two scopes parallel to each other and then pointed one at Polaris - its unlikley that the second scope would also see Polaris because of the separation between them - the same is true of the polarscope and the main tube.

Its more likley that the polarscope is not aligned properly or not aligned well to the RA axis. The scopes position on the mount is to some extent irrelevant for tracking purposes.

[johninderby]

Some of the SkyMax 127s have a mounting plate on the bottom of the OTA it isn't a dovetail plate. It's wide at the top and narrow at the bottom, a bit like an upside down dovetail. You do have to attach a proper dovetail to it.

The ADM dovetail plates are perfect as they have a flat top and a long mounting slot.

http://firstlightoptics.com/proddetail.php?prod=adm_vixen_dovetail&cat=62

A cheap and cheerful dovetail that would also do is this one from Scopes'n'Skies

http://www.scopesnskies.com/prod/ScopeTeknix-accessories/halfdovetail-bar/dovetailm.html

John

[OzDave]

I'm not sure I bought the kit from a reputable source. It has now ceased trading. In any case, I was only supplied with a 21cm dovetail bar. Nothing else.

This dovetail bar does not attach directly to the OTA. Instead, the OTA has its own metal plate directly attached. It was supplied like this. The plate on the OTA looks a bit like a dovetail attached upside down (that is, with the wide side toward the OTA). This plate seems to mate with the top surface of the dovetail bar, but no two of the bolt holes line up.

My scope does look like the picture posted, but in the picture the dovetail seems to be directly on the OTA. Whereas there is another intervening plate in my case.

Also, if you're not supposed to see Polaris through the scope when in the home position after polar aligning, what are you supposed to see? Surely the mount's polar axis should be aligned to the OTA's optical axis so that with a wide enough FOV, you should see polaris rotate about a fixed point that is in the middle of your FOV?

Also, shouldn't there be an RA/DEC setting of the mount that allows me to see Polaris through the scope? Because with my setup, there isn't for some reason. I can rotate the mount from one horizontal position to another and scan slowly in DEC, but cannot locate Polaris. Using a red-dot finder aligned to the scope shows me that the scope can only point to somewhere on a circle surrounding Polaris, but not with that circle. Not sure why this is, but I'm pretty sure it shouldn't be like this.

There are numerous polar alignment guides for people without polarscopes that involve sighting Polaris in your OTA. I'm not sure how that is supposed to work if you just can't!

Any ideas? (working on some pics)

[AJE]

But with a low power eyepeice, even a rough polar alignment should yield reasonable GOTO's.

Being an imager It's rare for me to even do a two star align (I normally do a one star align close to the object that I want to image, and then goto - never had the object NOT in the FOV of the camera).

Providing that the mount is roughly polar aligned, you should be able to do a 2 star align and slew to most things first time. As long as you align both the stars the starting position of the main scope doesn't matter.

HTH

[OzDave]

I can be reasonably sure of this because when I centre Polaris in the eyepiece of a scope and then rotate the mount through RA the stars around polaris will rotate around it but Polaris stays still - thats as it should be.

Ok, when I try this, whatever is on the cross hairs in the FOV, definitely doesn't stay put when I rotate the mount though RA. That's how I know there is a problem.

It's a bit like when you align the reticle in a Polarscope (as per your excellent tutorial on the subject) to avoid this problem. I've aligned my Polarscope so I can centre a star on the cross hairs and rotate in RA and have it stay put, but when I try this with my scope, the object moves.

So how can I properly align the scope so this doesn't happen? It's got to be a problem with the dovetail bar and the adjustment screws I think, but I can't use the screws because the dovetail is longer than the plate attached to the OTA and I'm not sure a smaller dovetail exists.

So I guess I need to know two things:

1. How to align the scope properly so rotation about RA keeps the object on the cross hairs.

2. If I need a new dovetail in order to perform (1).

[AJE]

I think you are getting too bogged down with the polar alignment - for visual it really isn't THAT important.

The polar scope often has to be aligned to the mount. There are little adjustments screws (allen key bolts) that adjust the "aim" of the polar scope.

It takes a little time (do it during daylight) to align - pick something distant and centre the polar scope on it. Then rotate the polar scope 180 degrees and adjust the polar scope HALF way back to it's previous position (using the little adjustment screws).

Then rotate the mount 90 degrees, re centre the polar scope on something distant, rotate 180 degrees and adjust half way back.

How many times you do this is up to you. But eventually you'll be able to spin the RA axis of the mount and the cross hairs will not move away from the distant object.

The polar scope is then aligned to the mount.

[AJE]

Forgot to say, when aligning the polar scope to the mount - it's easier to remove the scope and counter weights...

[Astro_Baby]

I think this dovetail issue is odd - most Maks (and I have to assume the SKymax 127 is the same) have the dovetail mounted directly to the scope. The dovetail on the scope plugs directly to the mount. No more needs to be done as far as I know.

On my newt and refractor I use dovetails which dont even have the cone error adjusters and if the trut were known I'd have no idea how to adjust them anyway - I could work it out but I doubt its very profitable for a GoTo set-up.

The one thing that jumps to mind - and its so obvious I scarcely dare make the suggestion is this - are you absolutely sure what you have aligned on is Polaris ?

When a scope is centred on Polaris and rotated through RA POlaris will stay still but other stars will cartwheel around it. That will only be true on Polaris. Other stars will tend to slip from the centre and describe arcs across the field of view. the RA is only used to keep them centred as they move.

I dont think there is a problem with the dovetail bar at all - if the dovetail is mounted to the scope (ie the bar that runs along the scopes own tube) is connected to the mount via the mount saddle then I cant see theres anything that can be wrong with that.

Where are you based ? Could anyone nearby help out ?

[OzDave]

The one thing that jumps to mind - and its so obvious I scarcely dare make the suggestion is this - are you absolutely sure what you have aligned on is Polaris ?

Yep, I checked that! :-)

When a scope is centred on Polaris and rotated through RA POlaris will stay still but other stars will cartwheel around it. That will only be true on Polaris. Other stars will tend to slip from the centre and describe arcs across the field of view. the RA is only used to keep them centred as they move.

Yes, I realised that it only works for Polaris after I submitted the last post. However, even when I put Polaris on my OTA's cross hairs, it moves from centre when I rotate in RA, and actually leaves the FOV at 18x magnification, which is really quite bad I think.

So forgetting about dovetails and polarscopes for now (sinc emy polarscope is aligned just fine), how can it be that I get Polaris leaving the FOV? I am certain it is Polaris btw.

Unfortunately, I live in Greece, and don't know anyone else into astronomy.

If the mount were not perfectly level, could that produce this problem?

What if the mounting plate combination on the OTA were subtly shaped like a wedge rather than being perfectly square? I imagine that could produce the problem.

I'm thinking this is so weird I'll just have to keep trying to analyse it until I figure it out. Usually I can figure this stuff out, but this one has me a bit stumped.

Dave

[Cornelius Varley]

The standard base for Skywatcher 90, 102 and 127 maks is flat with a number of 1/4"-20 tripod thread holes in it. The standard dovetail bar probably won't fit comfortably on the base due to the shape of top of the bar. I use a dovetail plate similar to this one on my ETX105 ota Dovetail Plate with 1/4" Camera Adapter (deluxe version) which has a tripod thread bolt and slotted hole to allow a second bolt to be used to align the bar with the ota.

Peter

[ringz]

German EQ mounts and ALT AZ mounts both have their Achilles heels. For the GEM it is pointing to objects near the pole

and for ALT AZ mounts it's difficult pointing to anything around the zenith.

A quick google came up with this link:

CN FORUMS ARCHIVE Telescope Reviews: Dobson's Hole?

Martyn.

[philming]

Hi !

I might have an idea about your problem. Do you know you're supposed to calibrate tje polar scope at first?

Best to do yhis by day, but the main thing is that if you rotate the RA axis while seeing something through the Polar scope (l'ets call it PS), it should NOT turn in crcle as you put it.

To do so, by day, aim at someting at a distance through the polar scope. Then rotate the RA axis. If the thing you're aiming seem to turn around a circle, you're supposed to adjust the PS. You have 3 little screws around the PS that you can adjust to push the reticle in different directions. Loosen one 1/4 of a turn, and tighten the 2 others, or reverse, just like you would do when you colimate a scope. When done, rotate the RA axis again, and keep adjusting till the aimed object stays exactly at its place, with the surrounding rotating around it.

Now your dovetail problel really is odd. I have a SW Mak 127 as well, at first with a goto AZ mount. I had an extra sort of dvetail provided with it, but have never user it. The allready screwed in dovetail plate is just fine by tself to attach to any mount that I know (AZ Goto or my present CG5-GT).

I think you should really post a icture of your setup to make thinks clear if you want us to understand your problem...

[OzDave]

I've already calibrated the polarscope quite well using AstroBaby's tutorial on the subject. I tried to calibrate the scope in a similar way during the day by swinging from horizontal east, to horizontal west and adjusting the dovetail plate by packing some stuff in it as required. I kind of got it ok-ish when moving from those two positions, but when the scope is vertical in the home position, the target moves from center. I can see Polaris in the scope now at least, but I can't figure out how to keep it in the cross hairs (or nearly) when I rotate about RA. I'll need to post a picture as I keep saying...