Mountain Instruments MI-250 questions

[baileys2611]

Hi Everyone,

I wanted to ask your advice please. I've recently acquired a second hand MI-250 (actually there aren't any new so the second hand bit is a little superfluous). Anyway, I put it together and started testing it and am a bit worried about the DEC worm drive - it grinds and bumps.

I'm advised by someone that these things are part of the learning curve, are normal and don't become an issue when there's a proper load on the mount (i.e a scope and counterweights).

I wanted to ask the learned community here - it that right? Are these kinds of things normal for large direct drive style mounts?

I've got some new bearings coming to me because I separated the worm drive from the worm block to determine what was causing the grinding noise and odd DEC drive behaviour and you can see the state of the old bearings here (http://www.skyslab.info/unboxing-and-installing-an-mi-250/) (or here (https://www.flickr.com/photos/skyslab/albums/72157684393942355)) and hear them operating here (http://www.skyslab.info/mountain-instruments-mi-250/) - but my original question stands - is this normal?

[garymck]

Hi There is a Yahoo group for the MI-250:

https://groups.yahoo.com/neo/groups/MI-250/info


I doubt that grinding noises are standard fare for a mount that was of the quality that this was when new. Never had the pleasure of using one but it was my dream mount...

cheers
Gary

[LewisM]

Simon,

Was this disclosed to you prior to purchase? Was this the one sold on the trader?

I think you need to take it up with the seller, but having pulled it apart yourself, you may have relinquished any recourse there. Looks like it may be simple enough to rebuild regardless, but you shouldn't have had to.

[brian nordstrom]

:thumbsup: I agree with Lewis here , it should be quite simple to clean , re-grease and adjust , these are high quality and built to tight tolerances so it should run perfectly if nothing has been damaged .
Good luck .

Brian.

[baileys2611]

Thanks for the advice.

No is the answer to the first question, second is yes - but, I am confident I can get it running to spec. My new bearings arrived today and they are smooth, fit perfectly and I'll do a re-grease of the DEC drive while I'm at it.

Instructions on the web for this are spot on and have helped. Fingers crossed!

[baileys2611]

After doing some repair work and some adjustments (more needed) it has come better but not near what was advertised.

Advice for those not seasoned and overly trustworthy: Insist on a video of the thing working before you purchase it if you are in any way unsure about the sale. This was an expensive buy for me and I was hoping that the advertised specification were correct as the MI-250 mount is praised all over.

My fortune hasn't been great lately so I've had to spend a significant amount of time 'repairing' it before it comes good. No doubt I'll need to spend more time 'adjusting' to get it close to operating specification because it's not there yet.

[LewisM]

Take it up with Astronomy Alive. I remember the multiple times it was listed - no mention of defects or issues just the usual superlatives.

I just fear now you have disassembled and replaced parts you have no legal recourse.

Good luck Simon.

[alocky]

Disappointed to hear the same story over and over again in this forum. I suppose it could be forgiven if the mount was sold at a significant discount, but if my experience is anything to go by, the price was also a premium one.
Hopefully you will get it operating as well as these mounts are reputed to work.
Regards
Andrew Lockwood.

[issdaol]

I recall the listing as well as I had a brief amount of interest in an additional mount before changing plans....I recall it was advertised as being in Excellent Condition.




I have purchased quite a few items on ice trades without a hitch so largely most sellers are genuine and reliable.

Unfortunately there are the rare individuals that mislead and cannot be trusted to tell the complete picture :mad2:

[LewisM]

Yes, I take it that is the one you bought Simon?

[LewisM]

A brief history lesson:
Most recent: http://www.iceinspace.com.au/forum/showthread.php?t=156390&highlight=mountain+instruments

Prior: http://www.iceinspace.com.au/forum/showthread.php?t=151927&highlight=mountain+instruments (price was removed in edit this year)

Further back: http://www.iceinspace.com.au/forum/showthread.php?t=86892&highlight=mountain+instruments

And then: http://www.iceinspace.com.au/forum/showthread.php?t=77276&highlight=mountain+instruments

[stefang]

Whether the goods bought were from an individual or even a retailer, (check your receipt), you can pursue a refund or return via VCAT (Victorian Civil and Administrative Tribunal)

https://www.vcat.vic.gov.au/case-types/goods-and-services

I would think that the gears / meshing and bearings issue you have experienced would definitely call into question whether the item was of "merchantable quality"

VIC GOODS ACT
GOODS ACT 1958 - SECT 19

Implied conditions as to quality or fitness
Subject to the provisions of this Part and of any Act in that behalf there is no implied warranty or condition as to the quality or fitness for any particular purpose of goods supplied under a contract of sale, except as follows—

S. 19(a) amended by No. 6716 s. 2(Sch. 1).

(a) where the buyer expressly or by implication makes known to the seller the particular purpose for which the goods are required so as to show that the buyer relies on the seller's skill or judgment and the goods are of a description which it is in the course of the seller's business to supply (whether he be the manufacturer or not) there is an implied condition that the goods shall be reasonably fit for such purpose: Provided that in the case of a contract for the sale of a specified article under its patent or other trade name there is no implied condition as to its fitness for any particular purpose;

(b) where goods are bought by description from a seller who deals in goods of that description (whether he be the manufacturer or not) there is an implied condition that the goods shall be of merchantable quality: Provided that if the buyer has examined the goods there shall be no implied condition as regards defects which such examination ought to have revealed;

(c) an implied warranty or condition as to quality or fitness for a particular purpose may be annexed by the usage of trade;

(d) an express warranty or condition does not negative a warranty or condition implied by this Part unless inconsistent therewith.

The relevant Victorian Statutory terms are linked here (https://www.australiancontractlaw.com/law/implied.html).

Good luck
sG

[LewisM]

The latest seller should be well aware of its operational ability based on the fact he claims it was in his observatory replaced by an ASA DDM85 mount .

I would hard save copies of all the sales adverts on it before they are deleted - screen shots at least

[ausastronomer]

It's just the same old story from the same old IIS member.

It just doesn't change and the selling principles don't change.

Cheers
John B

[baileys2611]

Thanks for keeping up with this guys. Yes, that's the unit I purchased.

I'm testing my new bearings and repairs when I next get some clear skies in the hopes that I have repaired it. How on earth it got a PE of +-3 with those bearings and the vibrating worm is beyond me.

IF I've managed to do a good job then the mount will be a keeper (and maybe I can offer a repair service!). Otherwise I will be looking at my options.

To my mind I'm going above what may reasonably be expected in order to make the sale work - in good faith, so that if even after I've attempted repairs its still not of merchantable quality then I will take it further.

I know that's a risk because it could be said that I have made matters worse by attempting a repair. I don't think that has happened yet however, visual and audio tests indicate it's better than it was before. Fingers crossed for a few clear nights to test it.

[alocky]

It took me 4 years and almost as much as I paid to rectify what was advertised as 'the finest example' of a 4" unitron. I was fortunate enough to be able to find or have remanufactured a lot of parts after scouring the world for three years. It's always going to be a risk buying used and obsolete gear. However we now fortunate to have a forum where the facts can be presented objectively, to prevent others from falling into the same trap.
Hope it works out, you've got yourself a beautiful bit of gear, despite the abuse it's suffered.
Cheers
Andrew

[LewisM]

Kudos for sticking with it Simon. You seem to be incredibly composed considering the circumstances.

[Astromelb]

Interesting reading guys.

1. The buyer asked me as seller to assist with the motors, when they didn't work. I told him what to do and they then worked. Astronomy 101 stuff. why was this even asked ?
2. He then asked questions about gearing, and again I assisted with guidance. Then I heard nothing further. I repeat then heard nothing further ?

Then I get a couple of calls from people I know on IIS asking whether this guy knows enough to run this unit. I cannot answer this, ask him.

This thread is the usual tangents going off all over the place without being intimately involved. With one side of the story and totally inadequate actual detail.

Great stuff guys - shooting first without asking any questions :(

Seriously guys, this stuff is premier gear and needs care, and attention to detail, to get operational. I get a little of this from time to time when people buy gear and due to the learning curve of this level of gear have issues.

Sad, but don't bag people when you aren't involved. Poor behaviour from some of you. Particularly a PM received today from someone that knows who he is, and doesn't know me whatsoever, his language was appalling, get a set of manners mate :(

To the others that rang me to support, thanks guys, appreciate your caring :)

[alocky]

There's always two sides to a story, but usually they overlap somewhere in the middle. The ads quoted here all claim it to be in excellent condition. Since it's a photographic mount, it should be easy enough to demonstrate the level of performance it is capable of delivering by showing some of the incredibly sharp and well guided images it must have acquired while serving in the observatory?
Most gear of this quality is usually advertised with PEC graphs, autoguider outputs, or at least an example of an image. +-3" is pretty damn good performance, and as Carl Sagan said - 'extraordinary claims require extraordinary evidence'. If it was only ever used visually, then it is disingenuous to describe it's performance, because these problems will not show up visually.
Having said that - these things are exposed to the elements when they're working, so it's unrealistic not to expect it might need a bit of work. Even ultra premium mounts like the DDM85 require new bearings eventually! Pity mine needed them sooner rather than later...
Anyway credit to Simon for sticking with it, staying within the facts, not getting personal and not enumerating his mates who will come looking after school is out...

[AndrewJ]

I always thought premier gear just required electricity of the right voltage and it would run ( without making odd noises )??????
Sure getting it aligned etc may require a bit of tweaking,but just getting the motors to run smoothly doesnt require a degree in mount maintenance.

Andrew

[baileys2611]

For clarity, I want to show the facts here - I don't want to make assertions about a person, their experience or other qualities as that is not the issue here.

The advert did lead me to understand the condition was good enough to assemble the mount, perform a few tweaks and then use it without maintenance.

Here (https://www.youtube.com/watch?v=DHNwM75r4o0) and here (https://www.youtube.com/watch?v=dVIJrWAKXQE) are videos of the DEC operating without load. On testing the mount, when I used it to slew this sounded like metal grinding on metal. This may have 'disappeared' under full load, but in my experience if an issue is present, putting a full load on what I suspect to be a failing component would make me negligent and risk the equipment and the mount.

On manually testing the worm, it did not move smoothly but bumped and halted regularly - this was while disengaged from both the motor and the DEC gear.

When I removed the worm block and inspected the bearings, I found them worn and in one case missing a raceway entirely and with dried grease. Here (https://flic.kr/p/U5BfaY) is the photo of those bearings, here (https://flic.kr/p/UYugis) is a photo of the replacement bearings and here (https://flic.kr/p/VeRgdC) is a comparison shot.

I have since replaced those bearing, re-greased the DEC gear and worm block, re-seated it and adjusted it under load (evidenced here (https://flic.kr/p/VyEuka)). Here (https://www.youtube.com/watch?v=rh-sr8kowcE) is what it sounds like now. There are no bumps when I manually test the worm drive and no grinding noises when it slews (please excuse the frogs, it's night time).

This leads me to believe that the issue was worn bearings and that this was present when I took ownership of the mount.

As to periodic error - when I get a chance and have correctly polar aligned the mount, I will post the results here. It's cloudy tonight and I'm going away with my family for the weekend, but I'm eager to get this working to specification so will persist.

[LewisM]

Perfect reply Simon, and thank you for not sinking to the level of personal retorts.

There was a mechanical issue with the mount as received. You amply demonstrated your mechanical aptitude that was called into question in not only diagnosing it but potentially fixing it.:thumbsup:

[issdaol]

Yes two versions that are worlds apart.

One by the purchaser who has ample skills to troubleshoot and repair, without disparaging anyone in the process.

The other by the seller who attempts to make the buyer appear inexperienced or incompetent ....backed up by bedtime stories of "mystical skills" required to master the use of premier grade equipment.

In my experience people pay for premier grade equipment for the extra features, reliability, expecting it to work as advertised so they don't have to fiddle endlessly to get it working properly.......especially when it's advertised in Excellent Condition which the buyer photos and videos show that it clearly wasn't.

[baileys2611]

I had a LOL moment yesterday evening. Still testing the mount and getting it PE correction-ready.

Removing and re-adding my scope to the mount seems to have taken it's toll on the knob (https://flic.kr/p/VXEGo1) for tightening up the front most saddle-clamp (https://flic.kr/p/VPFpJR)...but, in another show of mechanical application I've come up with a tool (https://flic.kr/p/UMDpHe) to use in place of the affected knob. :P

[LewisM]

That's an original Murrambateman Knob Shifter right there.

[g__day]

To the original question - I would expect used high quaity gear to work exceptionally well and be pretty bullet proof. I would expect a seller to state in advance if there are real problems with the gear being sold. I expect any high end mount has a non trivial learning curve to reach peak performance - not to do routine actions.

Grinding noises on first use sound like a major whoopsie that the seller and buyer should both know about before making the purchase decision. If the seller is unaware of these - or states they never happened for him - that's one thing. If they knew but didn't disclose that is another.

These mounts have a killer reputation at the price point, especially for robustness. If it's more than normal wear and tear I would ask what has this thing carried to be making the noises you have observed?

Hopefully the buyer and seller can come together on this and reach a great final result.

[baileys2611]

Yes, I agree that those things are important and thank you to all who are providing advice on this, I appreciate it very much.

However, rather than discuss the commercial aspects - the mechanical ones are more important to me, I can fix those, the commercial ones are painful to all involved, I'm not a fan of pain.

That being said I think I've made progress on the mechanics. A long update is below but for those that don't want the long read I have a question on Gemini servo motor gearboxes for the good people here: Should the shaft be able to shift in and out a little bit? This is from me using my finger to gently push on the end of the shaft coming from the gearbox and it has play in it, not sideways at all, just in and out by about 2 or 3mm (really need to get some calipers!). I'm guessing that this is ok but is that normal?

Long update:

It's hard to find bearings that are high quality in Australia. I realised after a little research that bearings ain't bearings. The ones I purchased were pretty cheap and apparently not high quality. My fault for not understanding and researching that in the first place so I'll correct that. Having realised that, I scoured local suppliers for high quality bearings, something closer to ABE7 or ABEC9 (P4 or P2). ABEC 3 is about as close as you get here 'in the sticks'. So I have some coming from Boca in America (ABEC7, ceramic balls, stainless steel housing).

Also, I noticed that the Oldham couplers and the plastic in-between them is yellowing with age so I ordered some replacements from Bintel which are designed to fit Losmandy mounts. I'm guessing that they will fit but will need to do a close comparison when I remove the worm drives and motors just to make sure. I probably should invest in a Vernier caliper I think...

I'm going to re-do the bearings *again*, replace the Oldham couplers and then re-jig it all for the last time.

Polar alignment is problematic. I've used many programs including TheSyX's Tpoint polar alignment report and they all seem to differ. I'm putting that down to potential mechanical inaccuracies in the mount at the moment. The best test I managed to complete so far is using PEMPro and their equivalent of a drift alignment. They tell me I'm good to go but TPoint tells me I'm 2 arc minutes out in RA and about 7 arc minutes out in DEC. I'm not going to fuss around with that right now until I've got the mechanics of the mount to where I think they are most accurate.

However, I did a test the other night using an autoguider and the results are promising. I'm getting between 0.9 and 1.7 pixel shifts between 4-second exposures on the autoguider (no idea what that means yet in terms of arc-seconds) and took a set of pictures of the Trifid Nebula (just to test if I could) using an SBIG ST-8300C. Here's (https://flic.kr/p/V78nok) the result which is promising. The stars are a little eggy. I expect to iron out the mechanical errors with more precision in the bearings and replacing the oldham couplers.

[baileys2611]

Quick update: After some good nights lately I think I've managed to get the DEC axis to within +- 4 arc second error before PEC (using PEMPro to measure, GSO RC 10" and an sbig st-8300c). I think that's pretty good for now.

RA is another story completely which seems to vary wildly between +-5 to +-20 with each separate measurement, it's not consistent which is frustrating. Time for some fiddling on that axis now. DEC only took 2 months to get to within original specification, RA probably won't take as long.

[PRejto]

Simon,

I'm glad you are making some progress with your mount!

I've never heard of using PEMPro to measure error in declination. Might you explain what you have done here? Since the declination axis typically doesn't move, except for guiding corrections, how can there be meaningful PE? Sure, if the axis were always rotating it would matter, but that isn't the case.

I personally wouldn't have any concerns about T-Point not agreeing with PREMPro's drift alignment. You wouldn't actually expect them to agree! T-Point is taking all sorts of things into account such as refraction, and specific mechanical issues with the OTA and mount that can be modeled. I have found T-Point to be somewhat sensitive to polar alignment wrt where and how many points are collected. I have obtained best results only when the points are spread equally on both sides of the meridian. I have been told specifically that this should not matter very much but my experience has been otherwise.

Peter

[baileys2611]

No problem Peter,

Yes, you are right. I'm making an assumption here based on a selection in PEMPro.

When you are measuring the RA error (the tab where it shows a graph) by default it shows the Y axis error, there's a drop-down box just below the graph, a bit to the left that allows you to select the X axis also. You can do that without affecting the measurements (i.e. it doesn't stop the process). When you select X it changes the graph. I'm assuming that the X axis graph is measuring movement of the centroid in the opposite direction.

For a practical demo, I used PEMPro to measure error, then tightened up the worm block by tiny amounts until the error in X was literally below +- 1 arc-second and the Y error was within +- 4.

Using relays on my autoguider, I was getting 0.5 pixel movements in the star centroid during autoguiding, pointing at the Cats Paw nebula while measuring this. Pointing east is not as accurate, getting around 1.5 pixel movement which I have a few ideas on how to reduce (slight imbalance for example might work there).

Re T-Point, yes I've read somewhere that it takes a lot of modelling parameters into account. Problem is, which measure to use? One saving grace is that they both seem to agree on vertical measurements - i.e. raising or lowering the mount to point to the SCP works well (nothwithstanding mechanical things like tightening up the mount moving the axis by a fair amount and coming up with techniques to counter that). They differ a lot in recommendations to move the mount horizontally. I've also read that it does not matter so much if you are autoguiding because small errors in polar alignment can be corrected through autoguiding corrections - cannot remember where I read that but it does sound reasonable.

[PRejto]

Hi Simon,

I've got no clear idea of what you are measuring in declination. It would seem to me that since dec isn't moving it would just be a measurement based on seeing. Anyway, what matters is whether you can guide reliably in dec...i.e., little backlash or stiction.

FWIW, because my location prevents much viewing to the East, I never could get a repeatable or reliable T-Point polar alignment. I use PEMpro to drift align and then I build a model with T-Point and completely ignore the T-Point polar alignment recommendations. I end up with great pointing, working Protrack, and have no trouble guiding. This all got discussed more than a few times on the Software Bisque Forum and the author of T-Point stated that extremely accurate polar alignment was not necessary in that Protrack would compensate, and he said that a drift alignment was an acceptable method to get reasonably close. The bottom line is the fact that there is no polar alignment that is best or works in all situations. There are alignments that minimise drift, others that minimise field rotation, etc, etc. I personally think that if you can drift align in PEMpro to the point where after say 7 minutes you are within 30 sec of aligned it is good to go.

Peter

[AndrewJ]

Gday Peter/Simon
Just to clarify, did you get a good calibrate with PEMPro???
If not, and the camera is not perfectly aligned, the X and Y axes can cross contaminate each other
ie RA PEC will show up in the "DEC" trace.
In normal operation with PEMPro, the DEC axis data ( as noted ) should be a relatively straight line, with only "seeing" ripples on it.

Andrew

[baileys2611]

Hi Andrew,

Yes, realised that after my first few attempts. Made sure that the X axis was smooth (seeing aside) and am fiddling with the RA worm drive to reduce error. It's a pretty good process :)

[baileys2611]

After a few months of fiddling and testing, I'm not getting much better results so I'm guessing I've hit the limit of my abilities to fine tune or the mount is about as far as it will go. I'm quite pleased with the DEC, very little movement in that direction and when I trained PEC it was only moving tiny amounts (Canberra's seeing is not so good).

The RA on the other hand flops about a bit. I've tried tightening the worm against the RA gear but I'm at the point where if I tighten the worm against the gear any more, I get errors from the controller (too tight). So, attached is what a typical evening looks like while tracking now.

Stars are not pin-point but then for a mount that was 1/4 to 1/5 of my previous mount and 1/4 the price of a Mesu (mmmm.....Mesuuu), it's ok.

Any suggestions for further improvements?

[alocky]

Although the G11 is a different mount, getting the right amount of play between the worm and wheel is an art, and you also need to do it at the tightest part of the wheel. Again, with the G11 I get the best performance when the RA has some play in it, your mount will do its own thing.
Assuming your polar alignment is good and your guide camera is correctly resolved into ra and dec, there should be very little correction in dec. Since you've got similar amounts of error in both you might well be looking at the seeing. It might be worth guiding no faster than once every 4 seconds and seeing how that looks.
Cheers
Andrew.

[strongmanmike]

From where you started Simon, good to hear you have improvements :thumbsup:

The average guide errors look to be around +/-0.5 pix..? What's the image scale for that graph in "/pix?

Mike

[Paul Haese]

I know this story; I experienced it myself quite a few years ago with my PME. That was in perfect working ordertoo; except the worm gear had been changed out with a new one (nice), but it had not been fitted correctly (the grub screws were loose and that created slop in the pointing and guiding). It took some time to find the problem and fix it. It gave me the you know whats having to sort something that was in perfect working order. To be frank when someone sells a piece of kit and says it is perfect working order, that means it works as per design parameters. Grinding, poor tracking etc is not as per design parameters. It grinds my gears (yep pun intended) when people do this and I read about it here and on other sites. I don't know either of you involved really, but the evidence is certainly compelling here. Clearly the mount was not in perfect working order and it should not have been sold as being the case. Some transparency would have been the right thing to do.

[baileys2611]

Thanks Andrew - that an interesting idea! It would also improve the resolution of my guide stars. This was taken at 1 second intervals with corrections coming from the ST4 port on an SBIG ST-i directly to the Gemini (L4 of course, I've read that if it were anything earlier I would need some sort of optocoupler).

I'll give that a try!

[baileys2611]

Thanks Mike. By the way, Ron Abbott from Land of Oz Observatory mentioned your name recently and said to say hi. :hi:

Re your question - Gah! Maths! Here goes:

SBIG ST-i is 7.4 microns, I'm using off-axis guiding so the scope focal length comes into it. The scope is 2032mm f/8 (I like it a lot!).
I was binning at 2x2 during this guiding.

Based on the formula here (http://www.wilmslowastro.com/software/formulae.htm): I get a calculation of 1.5 arcseconds per pixel. So, the graph would show an average error of 0.7 arcseconds error in RA and 0.5 in Dec I think?

[baileys2611]

Thanks Paul, agreed. The PMX I used to use was heavenly (ahaha). But on the bright side I think I've learned a lot from this exercise! Anyone else with an MI-250 that wants some tips, I'm good to offer them.

[strongmanmike]

Ok well that's certainly not terrible :)

BTW how would you rate the overall astro conditions in the Murrumbateman area? ie. seeing, sky darkness, frequency of fog etc?

Cheers

Mike

[Atmos]

That actually seems pretty good!
There were times I could get that with the EQ6 (for short periods) but then there would be a random spike here and there that would mess things up. And it would only happen on nights of good steady seeing!

If you're getting that level of guiding without any random spikes then it's pretty good.

[PRejto]

Simon,

I agree with an earlier comment that the errors you are seeing might be due to seeing given RA and DEC look similar. I think you could judge this by looking at the guiding graphs while not sending guide corrections. Since DEC isn't moving everything you see in terms of excursions would be due to seeing whereas RA would be PE + seeing. Another tool worth experimenting with is the Guiding Assistant in PHD2. It basically does the above and analyses the result making suggestions for exposure duration and min move settings. It also has a tool to measure DEC backlash. On Paramounts the backlash tool doesn't really work (It seems to measure stiction) but it might prove useful with your mount and could quantify adjustments.

Peter

[baileys2611]

Mike, summary for you because I think I've rambled on a bit below: the overall conditions have been 'good' - but not 'great' and that's changed recently. About three years ago it was clear days and cold clear nights in winter, very little humidity or moisture. Lately it's hard to pick.

Some detail
We have great dark skies, very little to no light pollution unless you are pointing south and low on the horizon - that's where Canberra sits and you get some interference there. I can see the large and small Magellanic Clouds on most dark nights (when there's little to no moon and I don't have superman eyes).

Seeing varies because of a few factors, the relative humidity is high in Spring and the upper atmosphere wind conditions are different to those near the ground - we often have a pretty high jetsteam. I use this (https://www.meteoblue.com/en/weather/forecast/seeing/canberra_australia_2172517) site to gauge upper atmosphere conditions and temper that with those on the ground.

Fog happens but normally in the mornings. The weather has changed a bit recently, it used to be many mornings in winter, and was often a 'pea souper' but lately it's harder to pick. Nights - not so much, if it's a clear day it's normally a clear night and winter in Canberra (and Murrumbateman) has normally been clear days and cold nights.

Poor Paul in Adelaide soaks up a lot of the moisture before it comes here, we often get the 'leftover' weather from Adelaide in winter. In summer an easterly comes in from the coast and brings the moisture with it. Spring, we're getting jetstreams off the south so it can be frustrating and Autumn is normally good results.

[baileys2611]

Excellent. I need to do some reading and a bit more research on PHD2 I think - I get what you've said at a broad level and it definitely makes sense but I'll need to practice a little with PHD2 to understand it's output - haven't used it much (just a little).

[Octane]

Sorry to hear of your mount woes, Simon. Hopefully you get it all ironed out for the summer objects.

Just wanted to thank you for the link to that excellent Meteoblue web site. Wow!

H

[baileys2611]

Ah, thanks Humayun. It's all good, a long(ish) life and opportunities aplenty ;). No problems re the site, it's pretty good :D I used to use Sky Harbinger (got pointed to it on the IceInSpace forums) but alas, things change. Sunday night is looking good!

[gregbradley]

Something must be not right here.
That sort of guiding is impossible with anything less than the absolute very best. PMX, PME specifications are 7 arc seconds of error. AP mounts are similar perhaps a bit tighter.

If you had guiding anywhere approaching that sort of accuracy you would have perfect round stars in any length of exposures.

PE of most high end mounts would be around 3-6 arc seconds. MI 250 has the diamond grease smoothed out worms so it may be around the 5 arc second band.

But if you are having some sort of mechanical problem it would be nowhere near that.

I don't quite trust PHD2 graphs. I have used them before that showed awesome guiding yet the subexposures had eggy stars. Not sure why that was but it did.

If you need more advice about your MI250 you should give Larry at MI a call. He's a pretty helpful guy or send him an email. I don't know if they are still in business though.

Greg.

[baileys2611]

I thought that too Greg. This is after PEC is applied. I cannot remember what the error was before PEC (a figure like +-12 comes to mind) - but I looked at the calcs and wondered about how this was possible also.

[Octane]

Eggy stars could also be due to a slight imbalance? I know I've had that. Even with my little setup while self-guiding. MaxIm DL guide graph was quite flat but the stars were elongated. Checked my balance and I was out a fair bit.

H

[GC - South Aus]

G'day Simon

Been reading this thread with great interest.

The MI-250 is a beautiful piece of machinery, it was the mount I always wanted (dreamed about - scary) and even looked at freighting one from the US a few years back from a CNC post. But, freight costs were prohibitive, as was my wife!

Hope you get it all worked out, I am sure it will serve you well when you do!

Regards

Gav

[strongmanmike]

Thanks a lot for that Simon. We are contemplating a move back out to a rural location near Canberra and obviously Murrumbateman is an option. I have had feedback from Anthony Wesley and Dave Herald already and you all seem to confirm that the main potential problematic issue is fog..? The development of Fairly could be problematic in the future too, so that would need to be avoided and will likely degrade the quality of the night sky as time goes on if it were to in anyway mimic Googong and Bungendore....never the less the area is probably a good compromise.

Thanks for that seeing site, hadn't seen it before. My last OT question, have you found it to be relatively accurate?

Mike

[gregbradley]

Even with PEC applied it sounds highly unlikely in my experience. The proof would be in the images. If you are getting perfectly round stars in long exposures then you know the PE is very low. If you are not then the numbers are not correct.

Greg.

[gregbradley]

I noticed mist and fog in some areas was common around Goulburn in the cooler months. The solution to fog is being on the top of a hill. I have never been fogged out at Bigga even when there has been fog in lower lying areas.

Greg.

[baileys2611]

Yeah, I might need to check that. I did do the whole balance thing until you can move it with your pinky and it stays there, in a few different directions. Problem with the MI-250 is that it's actually quite tight, even with the ... things ... knobs that tighten stuff... forgotten their names... those things unscrewed a lot. I wish this had a balance function similar to the HEQs or Ioptrons. I've read that if you move it manually, it throws out T-point out because the encoders aren't very accurate.

So, I'll try re-balancing and do another t-point after that. Cannot hurt anyway, it's pointing accuracy is down to 50 arc-seconds so it will be good to see if that's repeatable.

I'll also get a tak I think and check the collimation and will need to read a little bit on making sure that the optical train is aligned nicely. I'm not getting significant astigmatisms that I can see, but I think I'll take a look through a tak scope and see what comes of it.

[baileys2611]

Fog is more of an issue now than it used to be - but that's also changed in the last 10 years. I've lived in and around Canberra all my life and when we were in an el-nino we had better clear weather (higher pressure). Over the last 10 to 12 years that's changed as the humidity has been higher. I wonder if that's a permanent future?

Light pollution will be interesting when Fairly is in full swing. The estate's western border is right up against my eastern, but I haven't noticed much change yet even though the street lights are all on now.

Sorry Mike - what does OT mean?

[baileys2611]

Last stub I took was 2400 seconds and the stars were lovely :) Then the one after that was a bit eggy, I can't pick it.

[gregbradley]

That's a bit trickier when you get good guiding then poor. I have had that happen.

Apart from mechanical issues with the mount such as some slop in the bearings or the worm is not at the correct pressure against the gears it could be other factors

1. Balance of OTA at the angle you image at. That is if you have loaded gear on top of your OTA like guide scopes etc that are heavy and above the centre of gravity it can balance when horizontal but be quite imbalanced at 60 degrees.

2. Cable drag. A common hidden cause of erratic guiding.
3. I recalibrate when I do a meridian flip.
4. First thing I do if I get poor guiding is pick another guide star. Its amazing how much better the guiding can suddenly become. Double stars are sometimes hard to notice until you start guiding on them.
5. Wrong autoguiding settings. You can have too short an exposure for autoguiding on high end mounts. The weaker the mount probably the shorter the guide exposure. On the weekend I noticed I got better guiding do a bit longer on the guide exposures with my AP1600. That's because natively it has such little PE. If your PE figures are even close to what you quoted then longer guide exposures may be the go.

Using the right min/max settings, the right aggressiveness settings.
SkyX now allows you to alter X Y plus and minus for each aggressiveness. A bit trial and error but often one gets worse errors than the other and you can attempt to improve that.

6. Spot on polar alignment is the ultimate key. Also if you have been monkeying with the bearings etc you would need to redo your PEC as the older one would no longer be valid.

Sky X accurate polar alignment routine with TPoint is the best method I have come across for truly excellent polar alignment. Every sophistication seems to be built into it.

Greg.

[baileys2611]

Hi Everyone,

Thanks for all your input. It's been an education. Worth the price? Not sure...but then it's been good to understand the mechanics in detail and all the factors in play.

My last shot at this had me taking photos all night (automated of course) and had some good results, some odd ones but I put that down to cables, the camera and niggling things that happen with tracking from time to time (hot pixels...I hate them!!).

Anyway, it's been a good experiment and something to learn from. I think I'm going to look at upgrading however to something that is more suited to what I want.

Having re-done some of the mechanics, it is probably below spec by a small fraction but is now much closer than it was when I received it. Having said that, I wouldn't recommend it for remote operation long duration astrophotography - which I like to do.

If I'm going to pass this on to anyone, it would be more suited to visual astronomy or wide field and fast photography.

I thought I would post this here though to get a feeling for if this is reasonable to look at selling. Given the history and work I've done on it - would anyone benefit from buying it or would it be best for me to shelve it and call it a good learning experience?

I know the answer may be 'put it in the equipment sale forum, be open and honest about the history and see if there are any takers' which is logical - but am after a sanity check to see if this is actually reasonable in the first instance. If it's not, I won't consider it.