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help!! auto focuser for 127 slt mak?

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bmc

bmc

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i'm looking for an auto focuser for my celestron 127 mak. i've been looking around and as yet there seems to be no indication of such a thing. i see the sky watcher auto focuser seems to be popular, but is it plausible to use?

many thanks

keep looking upward :rolleyes:

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hobsey

hobsey

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I purchased one of these off of eBay last week.

It was listed as refurbished when in fact its something he has knocked together himself.

Not entirely happy with it so I will be tweaking and modifying it to make it better.

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jambouk

jambouk

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The one i have isn't as good as the one in the link above, and it was very home made. But it was only £30 or so, and i wasn't expecting anything fancy. It does the job, but to be honest, mounting the scope on a more sturdy mount massively reduces the vibrations and makes focusing it much easier by hand anyway.

James

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JamesF

JamesF

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I had a Skywatcher motorised focuser before I bought my 127 Mak and modified it to fit:

http://www.tanstaafl.co.uk/2012/02/skywatcher-127-mak-focuser-motor-drive-conversion/

For imaging it has been fantastic.

What a motorised focuser won't do for you is get rid of the "mirror shift" that happens when you change the direction of travel.  You'll always be stuck with the situation where changing direction first moves the mirror slightly and then brings focus in the opposition direction.  That is unfortunately a "feature" of the 127 and many of the other Maks and SCTs available today.

James

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bmc

bmc

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thanks everyone for their advice.

jambo & hobsy, i think iv'e seen the device on e-bay, and yeah it looked home fabricated, i didn't know how much i could trust it. though, jambo while researching,  found a usb controller for an auto focuser at "astronomyshed.co.uk" that seemed to give a more smoother actiion and drive.

steve & james f, thanks for your advice, your remedies seem to be the solution if i want to use an auto focuser. though i'm resit-ant because i have no diy skill. the last time i tried to hammer a nail into a piece of wood, i nailed the hammer into the wood, and now my books keep falling of the shelf!

unless i go with adapting the focuser, i feel stumped.

would a feather-light focus be a soulution? and is there such a thing for a 127slt mak?

keep looking up :rolleyes:  

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Steve Ward

Steve Ward

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I use one of Dion's USB controllers to adjust focus on my scopes that have all had the SW Autofocus motors fitted , wouldn't be without it now ...  :p

For my Mak180Pro I fitted a Revelation DS focuser on the back first then added the Autofocuser to that , eliminates all the mirror shift that James mentioned and gives a much , much finer touch .

Not sure if you can fit an extra focuser to the 127 , unsure of the size of the thread on the back , but if you're looking at imaging it's definitely the way to go in my opinion. 

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JamesF

JamesF

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For my Mak180Pro I fitted a Revelation DS focuser on the back first then added the Autofocuser to that , eliminates all the mirror shift that James mentioned and gives a much , much finer touch .

Not sure if you can fit an extra focuser to the 127 , unsure of the size of the thread on the back , but if you're looking at imaging it's definitely the way to go in my opinion. 

It is possible to get a converter to take the baffle tube thread on the 127 Mak out to the 2" SCT thread, at which point in theory at least it should be possible to fit an external SCT focuser.  At least, mine had such a converter.  Newer 127s have a different size thread, I think.

I'm not sure however that an external SCT focuser wouldn't foul the existing focuser knob.  It would certainly be very tight.

The one unfortunate thing about fitting an SCT focuser (which I'd otherwise almost certainly have done with mine) is that to achieve focus with a DSLR the focal length is increased (the exact focal length of a Mak changes as you move the primary mirror to focus) to the point where it is no longer possible to fit the full image of the Moon or Sun on the sensor.

James

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JamesF

JamesF

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Steve, what is the difference between mirror shift and mirror flop?

James

Pinching Steve's question :)

The mirror in a Mak or SCT is on a carriage that slides up and down on the outside of the baffle tube to achieve focus.  Obviously if it were an interference fit it wouldn't move, and in mass-produced OTAs there are no bearings, so the carriage is ever so slightly loose on the baffle tube to allow it to slide.  Only a small amount, but this means that at times the mirror can rock by a tiny fraction.

"Mirror flop" is often used to describe this movement occuring as a result of the telescope being moved on the mount.  As the orientation of the OTA relative to the ground changes on an EQ mount, the mirror can shift its weight and move.

"Mirror shift" or "Image shift" on the other hand is often used to describe what happens when you change the direction of movement of the focuser.  The focuser pulls or pushes on one side of the carriage, so when you change direction there's that little bit of slack in the fit of the carriage that needs to be taken up before the mirror starts moving again.  In normal use it's barely visible.  If you're imaging with something like a 3x or 4x barlow in the optical train, it can move the image completely across the camera sensor (or even off it altogether).

There's no absolute agreement on which term means what however.  They often get used interchangeably.

The shift on focusing can often be circumvented by fitting an external focuser, so you get focus roughly correct by moving the primary mirror and then do the fine focusing with the external mirror.

Preventing mirror flop is more tricky and often involves locking the mirror in place.  Some of the high end SCTs have such a locking mechanism.  You don't tend to find it on cheaper ones.

James

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jambouk

jambouk

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Thanks james.

I have a similar sct focuser in my 180 mak. I've noticed some mirror shift on my mak, but not been aware of flop, but then i suspect that is harder to see as one may just think the object is just off centre with inaccurate goto rather than thinking it's moved because of flop.

I've often wondered if using the external sct focuser moves the image as a flat sheet, whereas using the internal mirror moves the image as a curved thing, and as such if the sct focuser may give some artefact similar to coma observed with newtonians. I've never seen or looked for it, but wondered if it's there or not.

James

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JamesF

JamesF

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I may have to invest in one of them if I can't get this motorized focuser working as I want it.

Most of the ones I've seen are similar, and need (I think) at least an inch and three quarters clearance from the centre of the baffle tube.  You'd need to check you have at least that much space between the centre of the baffle tube and the edge of the focuser knob closest to the tube.

James

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JamesF

JamesF

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I've often wondered if using the external sct focuser moves the image as a flat sheet, whereas using the internal mirror moves the image as a curved thing, and as such if the sct focuser may give some artefact similar to coma observed with newtonians. I've never seen or looked for it, but wondered if it's there or not.

James

The external focuser won't move the image plane at all.  It moves the camera or eyepiece.  The internal focuser does move the image plane, as a result of moving the primary mirror and changing the effective focal length of the entire optical system.

James

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Steve Ward

Steve Ward

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Thank you to James for answering the question much more eloquently than I could have done ....  :p

Hadn't considered the clearance issue regarding the focus knob , on the 180 there's obviously room to play with due to the size of it's tube , but not that much to spare .

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jambouk

jambouk

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A crayford style focuser isn't cheap. I suspect one could pick up a 127mm mak for the same price.

Might it be time to think of a more sturdy mount? And/or a different scope rather than putting significant amounts of money at this one? Just a thought.

James

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