Skywatcher ED 80 - what am I doing wrong

[jabberwock48]

I recently bought an ED 80 secondhand but in good condition to mount on my 12in LX200 as a guidesope/imager. Having acquired ADM guide rings, counterweights etc I set it up for the first time this afternoon with the intention of aligning the two scopes. So first to focus the ED80 using a Meade 4000 26mm Plossl eyepiece. But however much I wound the focuser in and out there was just a complete blur. The focuser moved in and out smoothly with about 3in travel. When I removed the eyepiece and sighted throuth the scope the focus point was perhaps 12-15in beyond the extended focuser. Help!

[SteveP]

Jab

Are you trying to focus the ED80 with or without a diagonal?

[Cornelius Varley]

Without a diagonal there maybe insufficient backwards travel in the focusser to bring the image in to focus.

Peter

[jabberwock48]

Yes perhaps that is the problem - it did not occur to me at the time that perhaps it will only focus with the diagonal in use. I am used to the LX200 which of course has a good deal of adjustment available.

[MishMich]

I have a Celestron Onyx 80 ED - won't focus for visual without a diagnonal. With a DSLR in prime focus, yes, not an eyepiece. Alternative is to put a 2" extension or two between the eyepiece and the focuser, although a diagonal makes life easier looking through the eyepiece.

M

[jabberwock48]

I will have another go at setting it up with the diagonal. Perhaps somebody could answer my next question though. If used with a QHY5 autoguider as opposed to visually will the diagonal be required. Thanks

[Merlin66]

I have the same set-up...

You will need a 50mm extension tube to get focus without the diagonal, either with an eyepiece or the QHY5 guide camera!

Ken

[MishMich]

It gets even more fun when you start sticking barlows on - the other day I had a 2" x 50mm extension, 2" barlow, then the tube from a 1.25" barlow, with the lens itself in the end of the T-adapter tube, and the EOS hanging off of that. The length behind the focuser ended up as long as the OTA itself.

[SteveP]

JW

Every design of scope has a particular point at which the image is brought to focus. The distance of this point from the rearmost point of the OTA, usually the focuser, is known as the Back Focus Distance. Most refractors are intended to be used for visual purposes with a diagonal in place. If a diagonal isn't being used then you will need to 'fill' that gap with some form of adapter/coupling/extension tube in order to reach focus by positioning the EP or the camera's CCD at the correct BFD.

You may not have come across this issue when you've been using your SCT because its design has such a focusing range that it makes up any shortfall in the BFD. On the other hand, refractors and especially Newtonians have more limited focusing range.

Bottom line is as Merlin says - you'll need an extension tube

HTH

Steve

[Mick UK]

I did the same thing when i bought my skywatcher 80ed pro, within minutes i was panicing on the phone to the supplier thinking the lens must have been knocked out of allingment while in transit or worse still, as it was my first double APO i looked down the lens and could only see one lens!

O my god, i thought, they've only put one lens in and it wont focus!

I soon stopped trembling when the kind gentleman at Sherwoods told me i had to use the Diagonal! (PHEW!)

[Mick UK]

I did the same thing when i bought my skywatcher 80ed pro, within minutes i was panicing on the phone to the supplier thinking the lens must have been knocked out of allingment while in transit or worse still, as it was my first double objective lens scope, i looked down the lens and could only see one lens!

O my god, i thought, they've only put one lens in and it wont focus!

I soon stopped trembling when the kind gentleman at Sherwoods told me i had to use the Diagonal! (PHEW!)

[Mick UK]

I did the exact same thing when my Skywatcher 80ed Pro was delivered!

I was on the phone to the supplier within minutes in a panic!

It would'nt focus so i immediately thought the lens has been knocked out of alignment while the telescope was in transit!, then to my horror (this been my first double lens scope) i looked down the front of the telescope and i could only see one lens!..by this time, i was in a right old state!!

A quick phone call the the nice gentleman at Sherwoods soon had my pulse back down to a bearable 200bpm ! (phew)

You do indeed, need the diagonal to achieve focus!