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Thoughts on mounts


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After last night at Belper setting up an EQ5 mount I got thinking(dangerous I know)about pointing errors.

One we may overlook is the “coning†effect of the telescope not being square to the RA shaft ,or the dovetail not matting correctly or the saddle not being machined ,the scope not in its rings and of course the rings flexing on one side of the mount o the other

All these can and do introduce errors into the mix

I did suggest last night that a bit of planning may help by doing an alignment on one side of the mount and doing all your observing on that side of the meridian and do another alignment the other side when you flip, some imagers stay on one side and say pick 3 objects per night and image them after they pass the meridian and down to a reasonable elevation before moving to the next object and combine the results over several nights (or years)

Further thought also lead me to the fact that these errors can be accounted for using software such as “T†point (expensive)which takes an image plate solves it moves on and it does this for a number of points all over the sky the software can then counter the mount errors

But I believe that the EQMOD system can do this (correct me if I’m wrong those that run EQMOD) and this can improve pointing accuracy vastly.

Another most of us do not do is check the polar alignment scope is central (daylight job) we assume its factory set perfectly

Regarding last nights errors the mount did put most of the objects in or just outside the field of view, frustrating yes but please remember that the field of view is around ½ degree so if we move from one side of the sky to the other say 180 degs and the object is on the edge of the FOV it is only ¼ of a degree out which is less than 0.2% out so any apology may be in order to that mount

Or you could get a dob and just observe :¬)

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The specified accuracies of these goto mounts is really quite a feat of engineering, These are quoted assuming all other setting up factors are perfect. As Steve has pointed out there are numerous factors that can substantially mess up this accuracy which are often not initially envisaged but can cause much frustration. Especially with a portable setup, some of these are likely to re-occur. Other factors could include a little settlement of the tripod into that usually squelchy cricket pitch turf or accidental nudging of the tripod.

At least for visual use this can be made a little less frustrating if we can accept these inaccuracies and combat it by initially using a wide angle eyepiece - I would say that wouldn't I :) . For instance, with a 200mm f5 scope, your typical 50 degree apparent field 10mm focal length eyepiece will give a real field of only a half a degree, which is not very forgiving of the possible errors we have discussed. A 25mm 70 degree eyepiece like the one I recently bought from Damian for a very cheap price would provide a real field of about 1.75 degrees which is ( and I've double checked it ) a massive 12 times the area covered by the 10mm eyepiece. Therefore much more likely to put the required object somewhere in the field of view despite any possible errors.

So although most of you probably already know the above, I just thought it would be good to stress how a wide angle ep might make a bigger difference than expected to the ease of finding things. You can then centre the object in the wide angle ep and put in your higher power one. So a longer focal length and or wider angle ep will usually make it easier to find objects with or without goto.

Hope this helps.

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OK, the errant EQ5 mount a further story. I managed a 3 star aligment last night with Vega as the first which I had to maunally correct as Steve did. I then did Caph and Alkaid and I think a managed it? Trouble was once done I decided to follow the sky tour and the first thing was Andromeda. It slewed to roughly where it should have been, I looked through EP and saw a fuzzy area. Then had a thought I didn't know what Andromeda should look like. So i got down off the steps (I'm a but short) to check star map to make sure I was looking at what I thought I was. I stepped on the power cable, nudge the scope as I tripped and it disconnected from the power! I did cuss a lot as it meant a complete blank screen. After a 4.30 am end Friday I called it a day, so still don't know if is was Andromeda in the EP as I have no idea what it should have looked like!

So will now have to await another clear sky and start again :angry:

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In our scopes Sheila it is a smudge, nothing else.

Sounds like you have it sorted.

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Not too sure in a 28mm, but if the seeing is good then in my 24mm it fills over half, perhaps 2 3rds of the fov.

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This was fuzzy bit in the middle of EP covering between 1/3 and 1/2, so sounds promising, wish I had not have messed up so i could have looked for a bit longer. Will teach me not to put wires where I can stand on them :o


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M31 covers an area twice the size of a full moon, all you would have seen from a light polluted area would be the bright core.

But yes, it's fuzzy.

Sounds like it's getting there.

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