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I use my 20mm EP for  general star watching  but  find the process of  accurate in situ  focussing extremely trying owing mainly to the tripod which gets a fit of the trembles  when the scope is touched.. Also, being of the alt / az variety , it makes for some difficulty in maintaining  a star image at centre stage, so to speak, when focussing.  ( When funds allow the tripod will be replaced  with something more substantial.)

Just recently, and by way of an experiment I set up tripod and scope at an open window and was able to take great care in achieving an accurate focus on objects ( single trees ,  animals in fields etc )  some  3 to 4 miles distant across a valley.   The careful retention of this pre- focus   was found to be very successful as proved later .......the hard bit is remembering  to avoid touching the focusser. !!.

As a complete contrast to the above and something which I find very puzzling, is that the same pre focus ritual with my 10 X 50  bins does not seem to work. I can get pin point focus on my tree across the valley ,  and even after using the bins focus lock , I  end up  fiddling  because a star  image appears to have lost its sharpness.

Pete

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Hi Ya Crossway, I think there are many variables when focusing - as you say that a not so steady mount has a lot to do with it, also, with the bins, its a combination of mount and optics - as with everything Astronomy you tend to focus on infinity.  I think that the steadiness of the atmosphere is one of the main culprits, there are videos about showing how the observatory class scopes like the might Kecks use adaptive optics to deform the main mirrors in conjunction with the unsteady atmosphere to produce pin sharp stars, the stars we're looking at are at vast distances and are single points of light except for the massive stars within the realms of the Kecks and the like.  Just lock onto a star on any given clear night and the de - focus the star and have a look at the "airy disk", on real poor nights the image will be bubbling and boiling away before your very eyes - so even trying to focus the star is a real problem - but as you know - it just comes with a little practice - the larger objects like the Planets/Moon tend to show good focus a little better (but not always the case).  As said previously, magnification has a role to play - you can tend to focus a smaller brighter image better than an over magnified object - so as with magnification, atmosphere, optics, mount and an individuals eyesight, all of these factors play a part in focus and all of these come into play under far from perfect skies.

Paul.

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Hi Pete.  You could make sure that nothing is loose on the tripod and mount, this would add to the focusing 'jiggles'.  Another thing you may try is to add weight to the tripod, anything may work, but a large bottle of water is a quick and easy method.  Attach it firmly so it's not swinging.

If the focuser is not smooth, that will make focusing more difficult. If you are a DIY sort, you may be able to improve that, but go carefully if you are not sure, you don't want to turn a working telescope into a pile of bits.

The binocular focus point on the tree was probably not quite at infinity, so stars will need a focus tweak after viewing land objects. The same should apply to a telescope, so not sure why that did work ok, but I wouldn't worry about it, just a small adjustment to get sharp focus with whatever you are using.

If you are viewing across a valley, you are very fortunate, I am surrounded by houses for miles in every direction   :sad:

Regards, Ed.

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Hi Ed and Paul,  Thank you for your very  constructive replies and help .   There is quite a lot of broken cloud cover here tonight and  therefore a bit of a pain trying to use my binoculars skywards  but I did at least manage to illustrate your point  Ed, that  there is a small difference between focus across the valley and  a star.  Still a mite puzzled, though not about to lose sny sleep,  but how far is infinity as seen by 10 X 50s.  The cross valley objects are  3 miles away

My ' scope is an Astromaster reflector of modest aperture ( and incidentally I am delighted with it ,  apart from the  tripod ! )  but  there is some noticeable "play" in the focussing assembly .  There are  no visible means  of adjustment   but I think it could be improved if something like very very thin  brass or copper foil was used .

I tried the  suggested remedy  of adding weight to the tripod  and it does work !.  Looks a bit ungainly ,  a  milk carton full of water ,  but it seems to improve steadiness if the weight is suspended below the accessory tray rather than  immediatly below the  tripod head.  I 'll enjoy looking at other weight options !!

Regards ,Pete.

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Hi again Paul.  Glad you've had some success with adding weight.

I'm surprised too, if 10 x 50s focused on 3 miles away need a tweak to focus stars, but as you say, not really a problem.

Is the Astromaster focuser play between the drawtube and the focuser body ?  As there are no visible adjustments, you could indeed use something to take up the slack.  I have some Teflon tape. I'ts self adhesive, 12.5 mm wide, 0.2 mm thick after the peel off backing is removed. If you would like some, you could PM your address, I'll pop some in the post (no charge).

But please be careful with dismantling, and if you keep the tube at a downward angle that would mean nothing could fall onto the mirror.  You may be able to eliminate the play, or greatly reduce it, and experiment is key, may need a few tries before you are satisfied. Ideally, you need to remove the drawtube to add the tape.  One or two strips opposite the rack on the draw tube, maybe so that looking on the end of the drawtube, the two strips and the rack are equidistant could work, but if you get too much tape in there, the focuser may lock solid. You may have to compromise with a tiny bit of play.

Regards, Ed.

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I find using a bahtinov mask helps to focus on a wobbly mount, you can turn the focuser, let the vibration settle and you can see if you've gone too far or not far enough.

It's not a solution as such but it's a cheap band aid, and personally I find it stops me from tweaking the focus constantly trying to improve the view.

James.

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When I owned a light mount I decided to take (if this is possible on yours) the rubber stoppers off the tripod legs and I filled the legs with sharp sand and loads of little stones, it did quite a reasonable job, along with as has been mentioned adding a bottle of water underneath the mount, you may just get it stable enough for now.

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I tried Ed's suggested solution and hung a bottle of water off the tray and it does seem to improve matters  but was interested in  proflight2000's idea of filling the tripod legs with sand / gravel but these extend telescopically  and just wondered if harmful damage could result.  Another idea  came from a neighbour here who asked me what the water bottle was for  ( think he thought that beer would be a better option !)  he suggested getting in touch with,  for instance,  an electrical contractor who might well have some junk lead sheathed electric cable which could be inserted into the legs.....the heavy duty stuff is certainly weighty.

What is  "bahtinov " mask as mentioned by  JED- E3....  I  noted that some double sided self adhesive tape has a "backing " on one side which in itself seems to be a sort of silicon film,  tough and with very little surface resistance.

Thank you Ed, for your very kind offer and I 'll  send you a PM.

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I find using a bahtinov mask helps to focus on a wobbly mount, you can turn the focuser, let the vibration settle and you can see if you've gone too far or not far enough.

It's not a solution as such but it's a cheap band aid, and personally I find it stops me from tweaking the focus constantly trying to improve the view.

James.

This is the first time I've heard of a bahtinov mask, well with me being new n all, so had to google it  :grin:

 see learning new tricks of the trade everyday.. So Thanks for that info mate.. :grin:

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I am not suggesting you buy one but if you are at all half decent with DIY you could make something similar to Televue's focusing arm. this creates a wider diameter wheel and therefore will provide lighter tuning of the focusing. might help.

tv85-10.jpg

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