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Effects on moving the primary foward


neil phillips
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Ive just fitted a 9 point cell on a f6.3 1600 mm focal length 245 mm newtonian. the new cell compared to old is 30 mm further foward, im interested in the effects this will have, i assume the focus position will now be 30mm further back in the focusser. If anyone could confirm thats correct ? im also wondering if my focal length will change, assuming the focus position is 30mm further out. then the focal length will still be the same, again if anyone else could confirm this is right. Is there any undue effects on planetary imaging, by moving the primary 30 mm further foward, does anyone know ? any detrimental effects. Or is this change to small to make a noticable difference ( of course thats what im hoping for ) the cell itself is sitting quite far back, but now without a closed back of the old Europa design, so the primary is still 30 mm further inside the tube than the old design, i suspect airflow will be very good, as spx cells dont normally sit flush at the back of the tube. i dont think this is a problem, infact im hoping air will circulate better than most spx scopes being further in still, with those designs

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Yep no change in focal ratio.

Possible vignetting as the light cone is wider than normal at the secondary but 30mm out of 1600 is small so shouldn't bs a problem.

Cheers for the info guys

I assume vignetting would be less of a problem with a healthy sized secondary. which i think mine is ? certainly hoping the effect will be negligable. as the plus of a open back design and better 9 point support, was worth striving for. but to get the same distance as the old Europa design would have meant a new tube. not something i particulaly wanted to pay for, if the effect would be un noticable

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Moving the mirror 30mm further forward makes the light vone 30/6.3 = (a bit less than) 5mm broader at the point where it intercepts the secondary. Therefore either the secondary needs to be enlarged by 5mm, or the diameter of the fully illuminated field will be reduced by 5mm.

A marginally too small secondary is much more of an issue than a marginally too large one, as the effective aperture of the scope will be reduced. There are also potential issues with vignetting if the scope is to be used for visual or CCD photometry.

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Ok thanks for the input everyone, on Orions webpage they are showing secondary sizes of 50 mm for a 10" f6.3. i cant get to my secondary yet, but from what i can tell from the back the secondary is considerably larger than this either 63 mm or 75 mm. so in either case clearly this 5 mm shift wont be a problem, even if the flat is 63 mm deducting 5mm, will still leave this setup with 58 mm 8 more than Orion use on there CT range for 10" f6.3. i will measure the flat properly at some point, if anything it looks too large to me. are there any other undesirable effects by moving the primary foward like this do you know Brian ? again thanks for the discussion.

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are there any other undesirable effects by moving the primary foward like this do you know Brian ?

If the focuser is good & rigid mechanically, no. If the focuser is a bit "sloppy" it's better used with less extension which might mean buying an extension tube. But this may be the price that you have to pay if you need to bring the focus out far enough to get to primary focus with a DSLR.

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Ok thanks Brian, all is looking promising then, clean mirrors on a 9 point cell and ( never before open back ) should really cool, and stabillize like never before. the focuser drawtube is a problem that i will have to work with, as mentioned tube extensions might help slop a bit. so will have to experiment. havent got a power supply for the fan yet. so likely first light will be un powered. but with a open back, it will be more effective from the off

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Call me stupid Brian secondarys are not something i think a great deal about, occured to me last i measured the major not minor axis lol, as such this is a 50 mm flat and your advice likely stands, i will be testing the scope soon as i can, and will look for signs of vignetting. I cant afford it yet, but i will as soon as i can get hold of a 55mm secondary. Its a weird size so might have to have a custom job, David lukehurst oldham optical springs to mind.

I may try a wanted thread, to see if anyone has a good quality 55 mm flat being unused they might be able to sell to me. Though i would prefer if the wavefront was excellent even though oldham optical say the flats PV can be much less than say my Primarys 1/10th pv figure. Not sure what my current flats figure is, but orion told me once there matched to the primarys PV figure, so its likely 1/10th pv or higher, maybe i can sell my flat or part x it with David for new one. Just to be certain Brian how sure are you of the 5mm light cone increase ? are you certain if i get a 55mm flat. then all will be similair to what it was before ? thinking maybe i should have waited till saturn has gone before doing this. But if i can get this sorted. then likely my results will improve a touch.

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