Guiding MN190

[Sp@ce_d]

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

I thought the FLO one was 13mm.. Isn't your spacing requirement 11mm ??

[Sp@ce_d]

There's quite a good explanation of cone error in the Skywatcher owners manual

[Sp@ce_d]

Here if anyone wants a read Appendix A

SynScan 240406V2.pdf

[Gina]

I thought the FLO one was 13mm.. Isn't your spacing requirement 11mm ??

Currently I have 3.5mm (TS 48mm to T2 adapter) + 3mm + 8mm = 14.5mm total spacing between focal reducer and filter wheel. The FLO OAG screws directly onto the reducer I'm told so if the FLO one is 13mm I'll need an extra spacer. It looks just like the TS one which is 9mm but I'm expecting it to arrive tomorrow morning so I'll find out then.

[JohnC64]

Hi Gina,

I'm a bit late to this thread and you look like you are going down the OAG route, but I have the MN190 and use the QHY5 connected to the finder with an adapter to do my guiding. It guides no problem at all and I have been able to easily manage subs at 10mins. i bought the adapter from Bern at Modern Astronomy (towards the bottom here http://www.modernastronomy.com/camerasGuider.html#guiding_accessories )

Might be another option to look at.

John

[Gina]

Hi Gina,

I'm a bit late to this thread and you look like you are going down the OAG route, but I have the MN190 and use the QHY5 connected to the finder with an adapter to do my guiding. It guides no problem at all and I have been able to easily manage subs at 10mins. i bought the adapter from Bern at Modern Astronomy (towards the bottom here http://www.modernast...ing_accessories )

Might be another option to look at.

John

Thank you John I have one of those adapters already so can try that if I find any problem with the OAG.

[opticalpath]

Gina, you mentioned that you might replace the focuser with a Moonlite CR2. I did so with mine - it's a big improvement. If you order a CR2, you may need longer mounting screws than those supplied with the MN190 installation kit (which Ron at Moonlite will supply)

PM me if you want more info about this and/or advice about swapping the focuser - I learned some things the hard way!

Adrian

[Gina]

Gina, you mentioned that you might replace the focuser with a Moonlite CR2. I did so with mine - it's a big improvement. If you order a CR2, you may need longer mounting screws than those supplied with the MN190 installation kit (which Ron at Moonlite will supply)

PM me if you want more info about this and/or advice about swapping the focuser - I learned some things the hard way!

Adrian

Thank you Adrian Will do.

[IanL]

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.

James,

Your intuition that cone error would have no effect on tracking is right. First of all, it is worth understanding why it is named cone error; the diagram in the SW manuals makes it obvious if you have one to hand. Here's a diagram of what is going on (best I can do, it's early):

The 'cone' is the difference in orientation between the scope's optical axis (green line) and the the RA axis (purple line), which should be polar aligned. Things to bear in mind:

1. Cone error can be in the RA direction, the Dec direction or (usually) both at once. If you have no cone error, the red line and the green line would be the same. If you have cone error, then the green line (optical axis) is off the red line (a line that is parallel to the RA axis).

2. Cone error in Dec does not matter at all, since you can simply adjust your mount's home position to compensate for it by rotating the Dec axis until there is no Dec cone error and calling that the home position. If the scope is pointing away from the pole once you have completed polar alignment, you can simply rotate the mount's Dec axis to compensate before you start your star alignment procedure.

3. Cone error in RA is more of a pain. The only way to deal with it is to physically adjust the dovetail bar/scope rings/scope somehow. That is what the 'cone error' screws at the far ends of some dovetails are for. They allow you to raise either the front or the back of the OTA to correct for RA cone error. If you don't have cone error screws on your dovetail, the usual fix is to use washers or shims between one of the scope rings and the dovetail to raise the appropriate end of the scope.

So why does cone error matter? Well it affects alignment and pointing. When the mount starts up, the pointing model used by the handset or computer software assumes that the optical axis of the OTA is perfectly aligned with the RA axis. Since the mount has no other means of figuring out where the scope is pointing in the sky, it assumes you are in the home position and both the scope and the RA axis are pointing directly at the celestial pole.

(Now pedants amongst you may notice that if both the scope's optical azxis and the mount's RA axis are pointing at the pole they can't be parallel, but since we are dealing with an imaginary point at infinite distance, even though the lines are not parallel they will never meet, so they are parallel for any practical definition of the word).

Next you slew to your first alignment star, manually adjust the pointing of the scope and add an alignment point to the handset/software's pointing model. It now has a better idea of where you are pointing. Once you have added three alignment points, many currrent handsets and software can calculate what your cone error is and correct subsequent slews to allow for it. The main problem with cone error is prior to having a three point alignment model, or if you are using an older mount which cannot compensate for cone error.

In the case of a modern mount, your slews to the alignment stars may be way off target, especially the first one until you have a three point model. Usually the three points need to be on the side of the meridian where the scope is aimed. If you do a meridian flip you usually have to get three alignment points on that side to get accurate gotos again. If you have an older mount, cone error will mean your slews are always going to be off target and you will have to manually adjust.

Whether any of this this is a problem or not depends. If you are experienced and have a wide FOV eyepiece or a well aligned finder, you can usually get back on target without too much trouble. If you are a beginner, or you are using a narrow FOV (like a camera) and don't have a finder, you may struggle. Before getting in to plate solving, I fell in to the latter category, which is why I boned up on cone error

Does cone error affet tracking once you are on target? Your intuition is correct, it absolutely does not. Assuming the mount is polar aligned, with the mount tracking in RA, the OTA will describe a circle on the sky in RA regardless of any cone error.

In the absolute worst case case scenario, you could have 90 degrees of cone error in RA, so that your scope is pointing straight out along the Dec axis (orange line). How you'd mount it like that I leave it to you to figure out becasue I can't!. Assuming you managed it, once you are on target, the scope would still track a circle in RA (in the 90 degrees case it would be along the the celestial equator).

That of course leads to the other problem of cone error in RA, which is that in extreme cases (hypothetical) there are places in the sky where you physically can't point the scope regardless of any pointing model. In the real world however, assuming you have only a few degrees of cone error, this wouldn't be an issue in practice.

[Gina]

So if you use say AstroTortilla to align to target and then guiding to stay accurately on target then slight cone error doesn't matter - is that right?

[JamesF]

Thank you for the explanation Ian. I was fumbling my way towards the realisation that cone error is more about pointing and alignment than tracking I think, but your explanation makes it much clearer.

James

[misterp]

Hi,

I use a 190 mn on an EQ6pro. I guide using the finder guider, 10min subs are no problem. I use the standard focuser but with a Lakeside motor focuser attached. I have NEVER collimated this scope.

Another plus for the finder guider is if you align it with you scope as you would a visual finder you can easily align your scope. M51 appears in the finder guider in about 2 secs (using QHY5) If I remember correctly.

good luck

Ian

[JamesF]

So if you use say AstroTortilla to align to target and then guiding to stay accurately on target then slight cone error doesn't matter - is that right?

I don't think it should matter, GIna. Once you've found your target and you're guiding I think you should be fine. The problem cone error causes, if I've understood Ian's explanation correctly (and I will read it through again as I'm feeling a little fuzzy this morning) is that it causes a discrepancy between the mount's idea of how its axes are aligned and their actual alignment, thus messing up its calculations of how to get somewhere else based on where it ought to be now.

I imagine it might be more desirable to "fix" it if you're imaging with more than one scope on the same mount so that the framing is as near identical as possible, but I'm not even completely convinced about that.

James

[Komet]

I have NEVER collimated this scope.

Assuming it is collimated OK now then you are very lucky. Mine came with quite woeful collimation causing me to learn what a swine it can sometimes be.

[IanL]

So if you use say AstroTortilla to align to target and then guiding to stay accurately on target then slight cone error doesn't matter - is that right?

You got it. I still have all the cone error on the 80ED that caused two or three abortive outings in December. I never fixed it (mainly because I have zero differential flexure with the guider and don't want to risk it), I don't have a finder on the scope and I don't really care. I just use AT to get on target (and build a pointing model using append on sync in EQMOD as it goes) and that's it.

I don't think it should matter, GIna. Once you've found your target and you're guiding I think you should be fine. The problem cone error causes, if I've understood Ian's explanation correctly (and I will read it through again as I'm feeling a little fuzzy this morning) is that it causes a discrepancy between the mount's idea of how its axes are aligned and their actual alignment, thus messing up its calculations of how to get somewhere else based on where it ought to be now.

I imagine it might be more desirable to "fix" it if you're imaging with more than one scope on the same mount so that the framing is as near identical as possible, but I'm not even completely convinced about that.

Well strictly speaking, and assuming you have a reasonable polar alignment, the mount knows exactly what the mount axes are doing, it just doesn't know the relationship between the optical axis and the RA/Dec axis until you have built a three point model on the correct side of the meridian. It's also worth noting that an OTA's optical axis may not be perfectly aligned with the mechanical axis (i.e. the tube), so it can look perfect to the eye but be off through the eyepiece or camera.

If you are using a multi-imaging setup, whether you want to align all the scopes/lenses depends on whether you are using them to image one target (e.g. different filters simultaneously) or different targets (e.g. several panels of one mosaic at once). Probably desireable to have at least one of them with no cone error if the second case, but if you use plate solving to find your targets, it is largely irrelevant

[Gina]

Thank you I was considering the possibility that cone error could have cause the elongated stars I go on my last imaging session with the 105mm lens but seems not. I'm sure a wobbly mounting of the widefield rig was the cause and I'm fixing that.

[Gina]

As for the OAG, I was hoping to recieve that today but I think FedEx just popped a card through our letterbox without waiting for a reply. It says they tried to deliver the goods at a time when I was having lunch Now I have to wait until Tuesday before I get it - drat!! I stayed in specially to take that delivery! I was hoping to see how it would work with my ED80. Anyway, I think I'll probably stick with my present setup for the ED80 and widefield rig using the ST80 for guiding. So I expect to leave the OAG on the MN190 and unscrew the FW/camera from the OAG and unscrew the QHY5 from the OAG and transfer to the ST80 - and vice versa.

[nightvision]

Do you plan to use a reducer with the 190?

[Gina]

Do you plan to use a reducer with the 190?

Not for the present. What may happen later is anyone's guess

[fwm891]

Hi Gina,

I have an MN190 and pillar mounted NEQ6 in my obsy.

I started with an 80mm guide scope and QHY5 but could never get tracking/guiding very tight. I went down the DIY OAG route and the guiding improved immediately, like others I could get rid of the counterweight extension bar and everything balanced up much better. Lately I have changed the QHY5 for a Lodestar and the guiding has improved again. Whether I guide from PHD or a second instance of AA5. 20 minute subs now don't hold the fear they used to. (I'd love a Mesu).

Collimation has been mentioned by others, I've only had to collimate mine once and to do it I basically removed everything below the corrector plate from the tube and carefully put things back one at a time aligning each as I went. Being on a pier mount I don't change OTA's around so collimation has remained good.

Best scope I've ever had. The mount improved after the belt mod but if anything is updated then it will be the mount (Lotto permitting!)

Francis

[nightvision]

Francis, interested in what you mean by DIY OAG?

[fwm891]

Tony: this thread -

[Gina]

Hi Gina,

I have an MN190 and pillar mounted NEQ6 in my obsy.

I started with an 80mm guide scope and QHY5 but could never get tracking/guiding very tight. I went down the DIY OAG route and the guiding improved immediately, like others I could get rid of the counterweight extension bar and everything balanced up much better. Lately I have changed the QHY5 for a Lodestar and the guiding has improved again. Whether I guide from PHD or a second instance of AA5. 20 minute subs now don't hold the fear they used to. (I'd love a Mesu).

Collimation has been mentioned by others, I've only had to collimate mine once and to do it I basically removed everything below the corrector plate from the tube and carefully put things back one at a time aligning each as I went. Being on a pier mount I don't change OTA's around so collimation has remained good.

Best scope I've ever had. The mount improved after the belt mod but if anything is updated then it will be the mount (Lotto permitting!)

Francis

Thanks Francis I think I may well upgrade to a Lodestar later as it's such a big improvement.

Glad to read about the collimation At most I shall only carefully take scope off mount and carry it to a storage bracket in the obsy and vice versa. It will not get any rough treatmant so collimation should stay good.

I might look at the NEQ6 belt mod later.

I'm certainly looking forward to getting it - should be next week. The extra photon collectring power should certainly help for whenever we get those rare minutes of clear night sky!

[Gina]

I now have the FLO OAG and have been measuring up in case I want to use the OAG with the ED80 and focal reducer. However, the TS adapter seems to be thoroughly "welded" to the reducer and the current setup with ST80 guide scope seems to work pretty well so it's unlikely. I can't use the OAG with lenses for widefield so will want the ST80 guide system for that so I might as well stick with the current arrangement for the ED80 and widefield setup and remove the whole assembly when I want to use the MN190.

Anyway, here are the measurements in case anyone else was thinking of using the FLO OAG with the ED80 and focal reducer plus EFW2 and 314L+ :-

• OAG 48mm face to T2 face = 16.2mm
  
• Atik T2 to 54mm thickness = 1.5mm
  
• FW thickness = 22mm
  
• Atik 314L+ front face to sensor = 13.5mm
  

Total = 53.2mm

55 - 53.2 = 1.8mm extra spacing required. That's an awkward amount but delrin spacers might be a possibility particularly as a bit wants taking off to account for the filter.

Also been checking the current ED80 reducer with FW and 314L+

• Reducer face to Atik adapter = 17.9mm (measured with digital calipers) includes TS adapter and extension tubes
  
• Atik T2 to 54mm thickness = 1.5mm
  
• FW thickness = 22mm
  
• Atik 314L+ front face to sensor = 13.5mm
  

Total = 54.9mm

[portedGoblin]

Great info there Gina. I have ordered a Borg 77 and will probably buy a 314L also and have been eyeing a OAG solution. I have to go with car for each session and would rather not have an additional scope.

Here's a stupid question: Why is correct backfocus so important? Cant the focuser move the focal point a few mm if its not perfect? I feel like I am missing something really obvious.