Guiding MN190

[Gina]

Unless something goes wrong, I'm expecting to get a SkyWatcher MN190 next month and I'm thinking now about guiding options. ATM I have an ED80 guided with an ST80, QHY5 and PHD and I'm using an Atik 314L+ mono CCD camera with EFW2 and filters. I also have a widefield rig where I attach a lens to the EFW2 and this is currently mounted on top of the 2 80mm scopes which are mounted side-by-side.

Now I am considering my options for using the MN190 and welcome thoughts on ST80 v OAG guiding. Is the ST80 still suitable for guiding the MN190 or would OAG be better? I know guidescope v OAG has been discussed before but I can't find anything that relates to my setup.

Two ways of using the new scope come to mind - making a yet more complicated mounting system and incorporating into my present setup (I can hear my NEQ6 groan) or swap the present rig for the MN190 with OAG. I have a hoist system to help with moving heavy weights to and from the mount.

Your thoughts would be most welcome - thank you

[Sp@ce_d]

The MN190 is on my shortlist too. From what I've gleamed so far.. it's heavy, could do with uprated rings & dovetail.. maybe one on top to help rigidity and the focuser may need attention. I think I'd be looking down the OAG route primarily.

[Gina]

The MN190 is on my shortlist too. From what I've gleamed so far.. it's heavy, could do with uprated rings & dovetail.. maybe one on top to help rigidity and the focuser may need attention. I think I'd be looking down the OAG route primarily.

Thank you I shall probably be getting a MoonLite focuser for it as the one I have been offered is quite old and has the older single speed focuser. But I'll see what it all looks like and whether I could get away with the original focuser and a stepper motor remote focussing system. I do like the look of the MoonLite, I have to say.

[Komet]

I own an a MN190 and an NEQ6Pro.

Initially I set it up using my ZS66SD as the guide scope on a side by side setup. I had to use 4 counterweights to balance this and never really got it anywhere near balanced. Guiding was poor suffering from (probable) differential flexure and just general overloading of the mount. I already used a SX filter wheel and a Lodestar so I opted to add the SX OAG to these ending up with a cluster containing both cameras and the filter wheel which is swappable between my MN190 and ZS66SD depending which one is on the mount at the time. Handy.

I digress .....

Getting rid of the side by side mounting bar and the ZS66SD meant I could also lose one of the counterweights. Balancing the scope became easy and the guiding improved no end. From a monster it turned into a kitty cat. Without doubt go the OAG way - it will pay off (more in a moment).

One thing that is poor with the MN190 is the dovetail which is made from Victoria Sponge I think. It does however have the adjusting bolts on it to enable you to get rid of cone error. Now this is quite important which I discovered when I fitted a solid non flex dovetail which did not come with these adjusting bolts. I happened to have a William Optics dovetail lying round which I bolted the original SW dovetail too enabling me to adjust out the cone error.

Pointing is now very good either side of the meridian. so if you are aiming to stiffen the mounting (and you should) then make sure you have the adjusting bolts on whatever dovetail you end up using.

As for guiding - when on the side by side mount it was rubbish, pure and simple. Using the OAG the guiding is very good plus as I have no differential flexure and no drift I can stack subs without aligning them if I wanted to. Its that good. OAG all the way - you will not regret it.

The MN190 is a fabulous scope (albeit a pain to collimate). My one gripe is the fact that the focuser uses just one screw to hold the camera in place. I am sure this causes tilt in the imaging train. The focuser is very good but I would love to be able to screw the imaging cluster onto it rather than using a compression fitting.

Images of my setup is in my gallery (click the link in my sig .... then Gallery and Equipment)

PS - I have an as new NEQ6 counterweight (in white) for sale if anyone wants one!

[nightvision]

If you don't go OAG and given the weight of the MN190 it might help if the ST80 was fixed under the 190 close to the mount and your wide-field camera on the other side.

What is cone error?

[Uranium235]

Are you sure you want to go down this road? Great scope, but its heavy and needs a longer cooling down period.

Plus you have to deal with the dreaded C word..... collimation. (noooooo!)

But, if the price is right I probably wouldnt say no either!

[Gina]

Thank you all Please excuse my not replying in chronological order.

[Gina]

Are you sure you want to go down this road? Great scope, but its heavy and needs a longer cooling down period.

I know how heavy it is and how long it is. Cooling down is not a problem as it will reside in my obsy scope room all the time so will be at ambient temperature when I image.

Plus you have to deal with the dreaded C word..... collimation. (noooooo!)

I know it's basically a Newt and will need collimation but OTOH I gather it holds collimation pretty well - that remains to be seen.

But, if the price is right I probably wouldnt say no either!

Though it's a few years old I think I am getting it at a pretty good price

[Uranium235]

Good luck with it then

It will be a killer on galaxies and planetary nebulas, especially at f5. But dont forget the usual gubbins that you will need for it (b-mask and dew shield), taking flats will be an interesting experience for you!

[Gina]

I own an a MN190 and an NEQ6Pro.

Initially I set it up using my ZS66SD as the guide scope on a side by side setup. I had to use 4 counterweights to balance this and never really got it anywhere near balanced. Guiding was poor suffering from (probable) differential flexure and just general overloading of the mount. I already used a SX filter wheel and a Lodestar so I opted to add the SX OAG to these ending up with a cluster containing both cameras and the filter wheel which is swappable between my MN190 and ZS66SD depending which one is on the mount at the time. Handy.

Yes, I think this will probably be what I end up doing.

One thing that is poor with the MN190 is the dovetail which is made from Victoria Sponge I think. It does however have the adjusting bolts on it to enable you to get rid of cone error. Now this is quite important which I discovered when I fitted a solid non flex dovetail which did not come with these adjusting bolts. I happened to have a William Optics dovetail lying round which I bolted the original SW dovetail too enabling me to adjust out the cone error.

Pointing is now very good either side of the meridian. so if you are aiming to stiffen the mounting (and you should) then make sure you have the adjusting bolts on whatever dovetail you end up using.

That's interesting - I'll certainly check up on that.

As for guiding - when on the side by side mount it was rubbish, pure and simple. Using the OAG the guiding is very good plus as I have no differential flexure and no drift I can stack subs without aligning them if I wanted to. Its that good. OAG all the way - you will not regret it.

That sounds pretty definite - thanks

The MN190 is a fabulous scope (albeit a pain to collimate). My one gripe is the fact that the focuser uses just one screw to hold the camera in place. I am sure this causes tilt in the imaging train. The focuser is very good but I would love to be able to screw the imaging cluster onto it rather than using a compression fitting.

I'll probably replace the stock focuser with the CR2 MoonLite. SW seem to fall down on their focusers.

Images of my setup is in my gallery (click the link in my sig .... then Gallery and Equipment)

Thanks - I'll take a look

PS - I have an as new NEQ6 counterweight (in white) for sale if anyone wants one!

I'll bear that in mind and PM you if I find I need another - thanks

[Gina]

If you don't go OAG and given the weight of the MN190 it might help if the ST80 was fixed under the 190 close to the mount and your wide-field camera on the other side.

Thank you It's certainly looking like OAG is the way to go - as I suspected.

What is cone error?

I have a rough idea but will need to bone up on it.

[Gina]

Good luck with it then

Thank you It'll definitely be interesting, that's for sure and after all - nothing ventured, nothing gained

It will be a killer on galaxies and planetary nebulas, especially at f5.

I'm hoping so

But dont forget the usual gubbins that you will need for it (b-mask and dew shield), taking flats will be an interesting experience for you!

Oh, I'm used to taking flats and I know it'll need a dew shield (maybe dew heater too - no problem). I use FWHM rather than a b-mask nowadays - much easier especially for NB.

[Freddie]

As an example, this is a single 10 min sub.

This was with a DMK21 on a ST80 using PHD.

As has been said, replace the bendy standard dovetail with a Losmandy. I then have the std dovetail across the top of the rings and the ST80 direct onto that. Never had a problem finding a guide star yet so no need for any adjustment and reduces chances of flex.

[ollypenrice]

I'm sure an OAG is the way to go. Loses weight, uses the same light cone, lots of reasons... At F5 it should find stars.

Cone error arises from the scope not being truly orthogonal with the mount's RA and Dec axes.

Olly

[Gina]

As an example, this is a single 10 min sub.

This was with a DMK21 on a ST80 using PHD.

As has been said, replace the bendy standard dovetail with a Losmandy. I then have the std dovetail across the top of the rings and the ST80 direct onto that. Never had a problem finding a guide star yet so no need for any adjustment and reduces chances of flex.

That is probably what I would do if using the ST80 - thank you

[Gina]

I'm sure an OAG is the way to go. Loses weight, uses the same light cone, lots of reasons... At F5 it should find stars.

Thank you Olly Certainly seem like it.

Cone error arises from the scope not being truly orthogonal with the mount's RA and Dec axes.

Ah yes, I thought it was something like that. Must do some reading

[Gina]

Well, the consensus of opinion certainly seems to be OAG and that was my feeling too. Thank you everyone

Next question is which OAG? (of course). Choices as I see it are :-

1. Atik - matches up well with the EFW2 and seems to have a good guide camera focussing system but it has a large optical depth
   
2. TS - only 9mm optical depth but I can't see how the guide camera is focused
   
3. FLO have a new one on sale which looks just like the TS one but no details yet.
   

A short one like the TS would let me attach the OAG to the ED80 focal reducer - I needed 11mm of T2 extension tubes to make the spacing right so 9mm OAG + 2mm extender. Without the FR any OAG would work. OTOH no OAG will work with my lenses as the extra spacing required is only 1mm. So no OAG would work with lenses and the OAG would need to be taken off the filter wheel for usng the widefield rig. This would seem to indicate use of a T2 OAG fitting which could simply be unscrewed (though I guess the Atik's 3 screws wouldn't be too bad).

Again, I'm after all your thoughts please

[alan potts]

It would be nice for someone like me to able to fully understand what you are saying, what is an OAG and PHD? It would be nice once in a while if the full words were written so people like me could try to follow the thread, or is it me just being an Old Aged Grumpy Person Hopelessly Deranged.

Don't under estimate the weight of the M/N 190mm, I did. I was foolish and believed a photo on SW 2010 catalogue with it on a HEQ 5 Pro. Olly very kindly stopped me buying an Ioptron mount for it too. I can use it but only for visual and that is only if well balanced.

Alan

[Zakalwe]

OAG = Off Axis Guider

PHD= Push Here Dummy, a commonly used free guiding program by Stark Labs

http://www.stark-lab...phdguiding.html

[ollypenrice]

Yes, the 190 is a big scope with powerful moments from the glass at both ends. No shortcuts on mount or guiding!

I still would like one, I must say, but I like to be significantly over mounted and wouldn't be with the EM200.

Alan, there's an abbreviations translator available here;

HTH ( :grin: )

Olly

[alan potts]

Olly,

Many thanks most I know but these two I just couldn't work out, the OAG looks a good way of doing it. I have a Lacerta stand alone device, I bought it just over 18 months back and I have never used it, it's meant to be very good. One day I will try it. I must get a EQ 6 for the M/N 190mm. The problem I have is there is little choice above the EQ6, I wanted a G11 but the wait was almost a year.

Cheers,

alan.

[Gina]

I have just received a reply from Martin at FLO and he has confirmed that their OAG model off-axis-guider-ovl will attach directly to the SW 0.85x FR that I use with the ED80 so I'm going for that one

[JamesF]

Cone error arises from the scope not being truly orthogonal with the mount's RA and Dec axes.

To expand on this (I had to look it up myself a while back), I believe it's when the optical axis of the telescope is not aligned with the RA axis of the mount. What doesn't seem to be so clear is under what circumstances it causes a problem and exactly what sort of problem it causes. Lots of people seem to say "it's bad, it causes problems with tracking and this is how to fix it", but I've not seen an explanation of its effect that makes. I suspect it's related to changing declination between alignment and finding your target.

James

[Gina]

Done a Google and here's a YouTube tutorial on Cone Error which I think explains it well enough :-

[JamesF]

Done a Google and here's a YouTube tutorial on Cone Error which I think explains it well enough

Yes, I've seen that that one. It explains the cause, and the effect it has on tracking, but not how and when the one results in the other. It's that link between the two that I'm not getting at the moment.

For example given a mount with highly accurate polar alignment carrying a telescope with some degree of cone error, if you pick a star and align the telescope with that star and then set only the RA axis moving, at sidereal rate, will the telescope track the star properly or not? Intuitively I think it will because the optical axis will still describe a circle in the sky which should still be centred on the celestial pole, just like the star. I'm certainly not denying the effect of cone error, but in this example I don't see that it causes a problem and therefore the relationship between cause and effect is more complex than suggested.

On the other hand I'm quite happy to be persuaded that I've missed something or can't see the wood for the trees

James