Sky-Watcher ED72 Focus Question

[Rod-AR127]

Hi everyone, beautiful sunny day here in Perth, should get some observing in tonight hopefully.

I've heard the term back focus and I'm not sure exactly what it is referring to.
I have a problem with my 420mm FL ED72 when l have higher powered EP in use, usually with a Barlow, in that l cannot get focus due to the EP being too far away in the light path. ie: focusser wound all the way in and still can't reach focus.
I'm using a GSO 2" SD, would a 1.25" make much difference? l don't have one to try at the moment.
Thanks.

[Nikolas]

These do work much better with a 1.25 inch diagonal

[Joshua Bunn]

Hi Rod,


Backfocus is referring to where the image, formed by the telescope, comes to focus. And, photographically speaking, its some distance behind a focuser mounting surface for example.


Are you sure you need to rack the focuser in? With a Barlow, that will push the focal plane further back.


regards
Josh

[Rod-AR127]

Well, l must have some interesting EP's.
When using my 2" GSO 2x barlow both my 1.25" GSO 20 and 15 EP's need racking in and only just come to focus on stars, the 15 more so with less FL.
My 1.25" Orion Edge on Planetary 5mm EP racks out considerably to get focus and its 2 to 3 time the length of the GSO's.
Doesn't make sense to me but never mind, getting a cheap 1.25 SD to test the focus range.

[astro744]

Here’s a response I provided to a similar question a couple of years ago. (I wasn’t sure how to post a link the the earlier thread).

Note whist the Barlow may push the focal plane out, to do so it needs to be optimally placed and this is usually inward. (Barlow design and different eyepiece can affect result). A better choice for Barlowing large 2” low power eyepieces is by using a Tele Vue Powermate as it wont upset the exit pupil. For higher power eyepieces (typically 1.25” anyway) use a 1.25” Barlow.

From 16-05-2020.

“If you need inward travel then an extension tube will only make it worse for then you will need to add the length of the extension to the amount if inward travel that you need.b

I just tested with my ED80 (not original focuser as I bought used and previous buyer replaced the focuser for better astrophotography). In any case I was using a Bintel 2x Quartz Star diagonal and Bintel 2x Barlow early non-ED model. Without Barlow I have 33mm inward travel remaining. With Barlow I have 11mm inward travel remaining. I tried with the following eyepieces:; 30mm 1rpd, 40mm Meade 5000 Plossl, (both 2") and also with 25mm 1.25" Bintel Plossl. The 2"-1.25" adapter I used was the one that came with the Barlow, recessed type with zero length added. I got the same result to within 1mm.

I put the diagonal into the telescope first and then the Barlow into the diagonal. This gave me the above result. I then tried with Barlow into telescope first and then diagonal into the Barlow. I could not get focus at all, not even close at both ends of the travel.

I just packed it away and as I'm writing this I thought I should try with a 1.25" diagonal and 1.25" Barlow as well as a 2x, 2" Powermate. I'll try this later and report back.

Additional content:
I checked with ED80 with 2" diagonal with 2"-1.25" adapter and 25mm Plossl and then the same but with Tele Vue 1.25", 2x Barlow insterted into the 2" diagonal with 2"-1.25" adapter and remaining inward travel was 33mm. The Tele Vue 1.25" 2x Barlow is parfocal and adds zero length.

Your best bet is to use a 1.25" Barlow especially if only using 1.25" eyepieces. The 2" Barlow is taking up too much focuser travel that your ED72 does not have to spare.”

[Rod-AR127]

Thanks for the info Astro744.
I don't know how powermates work, I'll go google that, but l do understand what you're saying.
I have some 2nd hand 1.25 SD on their way so l will soon learn a lot from them it seems.
Luckily my 127achro has a focusser with more travel than anyone needs on a 1200mm FL and my 2" barlow and 2" 30mm EP work well with it.
I guess I'll have to just buy some more stuff like barlow and quality SD.
Thanks for the reply, greatly appreciated.
Rod.

[astro744]

My quote from an older post is a little bit out of context but most is relevant to your situation. Plain and simple though is you do not have sufficient focuser travel to use some accessories. This is not a factory fault with your telescope, just a design limitation for the short tube refractor that it is.

Adding a 2” Barlow simply chews up more inward travel. Keeping the 2” Star Diagonal but using a 1.25” Barlow may work but you won’t be certain until you try it on the ED72. If that doesn’t work then using a 1.25” SD with 1.25” Barlow will almost certainly work but again it depends on just how much travel you have to play with. In the latter case the 2”-1.25” adaptor goes in first to the end of the focus tube.

There are a lot of factors that will subtract from the available travel you have. You have thickness of 2”-1.25” adaptor, light travel path length of diagonal, design of Barlow, I.e. long tube, short tube and whether it is parfocal with the eyepiece. A Barlow that is parfocal with the eyepiece simply means that if you focus with a particular eyepiece, then add the Barlow to the sane eyepiece, the focus shift required is zero or very minimal.

You could easily spend a lot of money testing things and a 2”, 2x Powermate could cost as much as your telescope and you still may have the same problem. I wouldn’t go that path, main reason being ergonomics and weight. Powermates work very well on all types of eyepieces unlike Barlow’s which through the exit pupil out too far on the longer focal length eyepieces, but they are not necessary or ideal in all situations.

You have to ask yourself what magnification do you want and which shorter focal length eyepiece will give you that. No need for a Barlow at all then. If you want a Barlow I highly recommend the Tele Vue 2x Barlow (3x also available) as it is parfocal and of high quality. There may be others that also work for you. If this still doesn’t work then you will also have to get a 1.25” star diagonal and one with a short path length just to be sure. Once again the Tele Vue 1.25” mirror diagonal is a good solution but if over budget there will likely be others that work.

See https://www.televue.com/engine/TV3b_page.asp?id=52&Tab=_back

and https://www.televue.com/engine/TV3b_page.asp?id=53&Tab=_back

and https://www.televue.com/engine/TV3b_page.asp?id=62&Tab=_90EB


I find the information on the Tele Vue site invaluable whether you have or use their solutions or not.

There will no doubt be lower cost solutions from other manufacturers but I can only comment on what I have used (except not on ED72, but ED80). Perhaps some others can chime in with worked (not theoretical) solutions.

Take your time and ask lots of questions as buying accessories gets expensive real quick especially if you are just experimenting. Hope this helps.