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I am using a Bresser AR102s 102/600/f/5.9 refractor for visual.
I am having to use 2 spacers screwed into to the eyepiece barrel to achieve focus!
Questions;
1. Has anyone had the same issue with this or any other refractor?
2. What’s causing this?
2. Is there another fix apart from spacers?
Any thoughts,
JackIf I understood correctly, I would say that the travel of the eyepiece holder tube, i.e. the focuser, is too short for the diverter (prism or mirror) not to be inserted. It is obvious that without a diverter, the focal plane remains very far out in relation to the extrusion of the eyepiece holder tube. I have the same problem with a Russian telescope. That is if I have not misunderstood.
Saluti
GiovanniYes, this is typical and a diagonal is normally required. All my modern refractors are the same. It allows for lots of backfocus for Cameras, filter wheels and OAGs.
Ian.
Giovanni,
You have understood correctly.
So, I am using spacers to achieve focus.
I was not aware this could be an issue with some telescopes!
Regards,
JackIan,
I was using it with a camera no issue.
I am using a diagonal with 2 spacers screwed into to the eyepiece barrel to achieve focus for visual.
I was not aware this was an issue with some refractors.
That’s a question people should ask when buying for visual use as I suspect it won’t be mentioned in the specs!
Regards,
JackYes, it is not even a particularly modern development. I recall a Japanese 50mm refractor that I tried in a department store in the late 1970s that did not focus without the diagonal, and of course gave an upright image with the diagonal. I had read in all the astronomy books of the time (by Moore etc.) that astronomical telescopes always gave an inverted image. As this issue was not mentioned in any of the books (Moore himself seems to have never used a diagonal) I was much puzzled, as were the shop staff, and eventually I bought a Newtonian.
So, I agree, it is a detail which should be explained in specifications, but never is.
Giovanni,
You have understood correctly.
So, I am using spacers to achieve focus.
I was not aware this could be an issue with some telescopes!
Regards,
JackI wish it was an issue with my telescope!
There is insufficient back focus to use an active optics unit with the current camera, off-axis guider and filter wheel. However, I have a cunning plan …
The Zeiss Telementor (63mm OG, 840mm focal length refractor) of the 1970’s is like this. If you screw the eyepiece holder directly into the telescope tube you won’t be able to achieve focus, either an extension tube or a diagonal is required. Zeiss manufactured six different extension tubes (in the range 20mm to 80mm, and usable singly or in multiple) to cover all circumstances.
‘Classic’ Newtonian reflectors of a few years ago had the opposite problem because the eyepiece tube couldn’t be too long as it would cut into the light path inside the main tube. (i.e. use of a diagonal was impossible.)
The Tomline Refractor at Orwell Park also suffers from poor prime focus image position (too far up the tube) which similarly makes the use of a diagonal impossible.
On the other hand a by-product of a ‘focal reducer’ or a ‘Barlow lens’ is a shift in the position of the focal plane image. The image is moved away from the objective with a Barlow and toward it with a focal reducer. This allows you to get the image plane nearer where you want it, but you have to accept a reduction or increase in magnification for any given eyepiece.
Jack,
Over the next year I will be bringing another scope into action. I already have another camera with integral FW; this has a much shorter back-focus requirement than the SX equivalent. Also purchased was a very slimline OAG pickup which takes up only 9mm of back-focus.
With judicious mix and match it should be possible to get decent guiding on both scopes and the use of the SX AO unit on one of them.
If I could figure out how to drive an SBIG OA through either MaximDL or EKOS without needing a SBIG camera it may be possible to get both systems fully equipped. OTOH, if anyone would like to purchase it … 😉
Last year I investigated changing the Dilworth’s transfer lenses to give a longer back focus but that would have been expensive, included a return trip for a delicate piece of kit to Switzerland, and would have spoiled the resolution slightly though that would have been minor as it is seeing limited in practice.
The other scope is an elderly and not very good Meade LX75 25cm Schmidt-Newtonian which will live in a ROR observatory presently under construction. That one will be fully robotic when commissioned.
Paul
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