refractor rejected ?

[rory]

i had a chap travel over to pick up my old refractor today. i was at work but my partner was home. but the guy didnt take it ,saying it is badly out of collimation ?

my partner said it was taken in the garden , ha looked through it . asked if there was another diagonal. which there isnt, and then explained its " out of collimation".

my question to you guys is, what was he checking and how ?

i used it friday night with no issues at all

im not suggesting he made an excuse,i doubt he'd travel 40min's if he wasnt genuine.

but surely id notice a collimation issue when looking at saturn, jupiter and the moon wouldn't i ?

i feel bad about wasting his time and petrol .

[StevieDvd]

Not sure how he checked it but I've read how to and checked my refractors.

You need to check that the light path through the diagonal-focuser-objective is all aligned. I placed a mirror on the objective end and a laser collimator in the diagonal to see if the bounced back light was central - it was.

The diagonal can be out and some higher end refractors have adjustment screws to centralise the focuser.

Unless of course he compared the finderscope and telescope views and thought that was how to check collimation

[Capricorn]

Suspect that whoever called was trying to get it reduced.

Part of "collimation" is that the mirror is centered along the optical axis, if the mirror is a little off then "collimation" is off. BUT if the mirror is of sufficent size it would make no difference - as long as the optical cone falls on the mirror and the eyepiece is also on the optical axis you have the same same quality image.

It is this that I suspect they were refering to but that might be a case of the mirror was not placed in the ideal position (maybe couldn't be placed in the ideal place owing to mechanical constraints).

As they asked about another diagonal that is all I can think of.

[rory]

the diagonal is a prism diagonal so a normal terrestrial view is avaialable. dont know weather to remove it from sale until ive researched more. but its only a astromaster, so i doubt theres collimation screws. and ive never had a problem with viewing. dont know what to make of it all to be honest.

"Suspect that whoever called was trying to get it reduced."

im not sure. id already agreed a reduced price .

the main problem i suppose is i wasnt home to witness his testing . the misses has no idea of scopes and such.

ive e mailed the chap , but no reply... he must be miffed !

[rory]

okay ive had email reply now. it goes .

"Thank you for your apology but it really wasn't required. The astromaster range is aimed at the budget astronomer so a collimation error is often expected but in this case I considered it to be some what more than I would have desired. Your views of Saturn & M13 will be fine but if the scope was aligned correctly you would see a subtle but noticeable improvements. Unfortunately as I pointed out to your other half these scopes are not the easiest to collimate so sadly I was unable to continue with the purchase on this occasion. If you have no luck selling the scope and would be happy with me making you a much lower offer then please get back to me as I would always consider it for my son as I doubt he would really be able to tell any difference in the views."

genuine i think, although the reduced price offer seems strange to me !

[John]

I think he is bluffing. To check the collimation of a refractor you need either i) to star test it or ii) to test it using a cheshire collimation eyepice as per these guidelines:

http://spacealberta.com/equipment/refractor/collimate.htm

or iii) to check the centering of the focuser you use a collimated laser collimator in the focuser and note the point of emergence of the laser at the objective end.

If he did none of the above I can't see how he could know the state of collimation of the scope.

[JamesM]

So now you know you have a scope that is good enough for a child - charming! Not sure how he cold effectively test its collimation during the day. To be honest and not wishing to be rude, the astromaster is a scope that is made to a modest price but having said that, it is more than sufficient to get someone started in astronomy. Will the scope show false colour (colour fringing) most probably and will certainly show up during the day where there is high contrast, for example whilst focusing on a distant TV ariel but that is to be expected given its short focal length and minimum objective lens coatings. However this scope would perform well on brighter DSO's and despite some false colour will show you the planets and detail on the moon. Personally, I would wait for further offers as you haven't got anything to lose in waiting.

James

[JamesF]

I think he's trying it on to try to get it cheap, personally.

If the refractor is a long way out of collimation I'd have thought you'd notice it yourself. If it's not far out I think you'd need at least a cheshire or laser, or to do a star test, to find out. Given that it's something like f/11(?) it should be fairly tolerant of collimation errors in the first place.

Anyhow, you can check it yourself easily enough. Point the scope at a nice bright star and get it in sharp focus. Then defocus the image slightly. You should see sets of diffraction rings around the star. If they're all nicely concentric then there's no problem. If they're skewed to one side then there's a problem.

In fact, see this:

http://www.spacealberta.com/equipment/refractor/collimate.htm

James

[JamesF]

Ah, my post crossed with John's

James

[rory]

mm, well he may well have had a cheshire or laser ,im not sure . ive not mentioned prices ect. as i dont wish to appear im advertising here. but lets say i put a price a little higher than i expected to get for it and he knocked that down by over a third !

a few squid more than a bst explorer shall we say.

ive a laser and cheshire tool on route to me for my new dob ,so if it aint sold by the time they arrive, i'll have a play.

note:- just a thought, best not diss the guy , he could be a member on here.!

[sologuitarist61]

Sounds like a rapid change of heart to me.

Isn't it strange that some people cannot tell the truth but have to make something up? If it wasn't what he wanted then why not just say so instead of saying something so patently absurd?

[rory]

well ,seems to have worked out fine. another buyer wants just the o.t.a for more or less the same price the other guy was going to pay for the whole set up !

so i can either flog the mount, or keep it for attaching some bins or something at a later date.

[AndyH]

Pleased for you. Sounds like a good outcome.

[x6gas]

well ,seems to have worked out fine. another buyer wants just the o.t.a for more or less the same price the other guy was going to pay for the whole set up !

so i can either flog the mount, or keep it for attaching some bins or something at a later date.

Good outcome. I think the chap just got cold feet for some reason. It shows the measure of the man to be concerned about his time and petrol rather than the hassle for you but fair enough if the buyer didn't fancy the scope for some reason.

If someone was so expert as to be able to check collimation of a 'frac, in daylight then I am a little surprised they were looking at this scope... Anyway, as said above, easiest thing to do is star test it.

[knobby]

How on earth do you collimate a refractor ?

Sent from my Nexus One

[RobH]

How on earth do you collimate a refractor ?

Sent from my Nexus One

You can't, unless it's an expensive model with a collimatable lens cell.

Nor can you tell if it's collimated or not by simply looking through it in daylight

Rob

[John]

How on earth do you collimate a refractor ?

Sent from my Nexus One

Some have their objectives mounted in collimatable lens cells which allow you to tilt the lens as you would with the mirror on a reflector. You can also adjust the focuser tilt to ensure it's square on with the optical axis on some models.

I've a couple of refractors that don't have the above adjustments but their collimation seems fine when tested.

In general, refractors seem to need much less in the way of collimation adjustments and, when in collimation, tend to hold it pretty well.

[RobH]

Like John says, refractors very rarely need collimating.

I have 5 ( 2 x 80mm, 1 x 100mm, 1 x 120mm & 1 x 152mm) and have never needed to adjust any of them.

[LukeSkywatcher]

Refracs simply do not need collimation.

[Damo636]

Refracs simply do not need collimation.

I have owned two Tal refractors and both needed

collimating!

[jimmyjamjoejoe]

Likewise for my evostar 102

[Macavity]

I did once put a Cheshire eyepiece in my ST102: Shock Horror... Well, not quite! <G> BUT few of the various "rings" were concentric. No collimatable objective cell, but much was easily fixed by curing "focusser droop" - by tightening screws etc. I also added a bit of Dymo Tape around the focusser flange, to better seat / pad it within the OTA. No real "before / after" measurements, but nice to make the BEST of what you've got?

[todd8137]

Sounds like he did not want it ,it would be hard to tell if it was out with simply just looking in the day I would not worry about it

[JamesF]

IME most of the StarTravel scopes can have their focusers considerably improved with a little tweaking here and there. None of mine have been stunning out of the box.

James

[Alma]

I once travelled over 50 miles to inspect a Megrez 110 prior to purchase. Good job I did; although it was in reasonable condition and owned by an avid astrophotographer, a quick star test with my own eyepieces revealed a significantly egg shaped airy disk and substantial miscollimation.

The owner was quite insulted inisisting there was no problem and that refractors do not need collimating. Wrong - if the lens cell gets out of whack then there IS a problem and you would be well advised not to go ahead with the purchase.