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Old 29-10-2019, 05:19 PM
poider (Peter)
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Prime focus??

I hear a lot about prime focus and how some manufacturers optimise their scope by shortening the tubes, Is this just something that can be done to any scope, could I just shorten my 750mm tube and move the focuser down?
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  #2  
Old 29-10-2019, 09:01 PM
Startrek (Martin)
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I image at prime focus with my DSLR in both a 6” and 8” Newtonian reflector
I had to move the primary mirror further into the tube by about 4 or 5mm to achieve focus
I just used some compressed cork flat spacers and this pushed the mirror into the tube enough
My focuser now has 2 full turns of in focus and out focus
Problem fixed
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Old 29-10-2019, 11:51 PM
poider (Peter)
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Thank you Startrek, How do you attach the DLSR to the scope, is it just the t ring? I have a nikon adaptor that screws into a Barlow, and attaches to the camera I aslo recently got t ring with no glass, I assume the t ring would attach to the adaptor then both would attach either end to the scope and camera... (after I shorten the tube or move the mirror of course).
What is the benefit of prime focus, is it just another way to attach to scope.
Prime focus sounds like the best way but i don't want to muck up my scope if it is just another way to attach.
Peter
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Old 30-10-2019, 04:37 AM
glend (Glen)
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You need to know whether your scope is optimised for visual or imaging, and most new scopes advertise what sort of setup you are buying. Yes, visual optimised newts may require slight movement of the primary mirror location, and people can work this out, but imho it is preferable to purchase an imaging optimed scope as there is no "work" to do, and these scopes typically have imaging capable focusers ( which can handle the weight of the imaging train). Every scope has a back focus distance, one where the image falls on your camera sensor in focus. Many scopes require the use of spacer rings, used to adjust the precise distance required to achieve focus. Variable spacer rings are available, these use a locking ring to deliver precise distances. Spacer rings might screw into DSLR bayonet adaptors, via M42 or M48 threads, and provide a rigid imaging train.
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Old 30-10-2019, 07:59 AM
Startrek (Martin)
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Peter
A few questions,
Does your scope have a 2 inch focuser ?
If not , then you cannot insert a DSLR camera into your telescope
If yes, is it dual speed or single speed ? Dual speed is needed for astrophotography for fine focus
If you want to use your DSLR in your telescope for Astrophotography what objects will you be imaging ?
Just the Moon ? Moon and Planets ? Or everything Moon , Planets, Clusters , Nebula, galaxies etc...
Depending on what you want to Image you need to work out your Prime Focus FOV ( field of view)ie: whether an object will fit into the image frame
You also need to work out your “image scale” so your sampling is satisfactory for imaging
Lots of things to consider in DSLR prime focus Astrophotography
See photos of my DSLR inserted into my 8” (200mm) f5 Newtonian reflector telescope plus photos of Canon DSLR , T ring adapter and T ring unassembled
Hope the above is helpful
Martin
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Old 30-10-2019, 12:15 PM
poider (Peter)
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I have a cheap Australian Geographic Newtonian, 150mm x 750mm, It has a 1.25 inch single speed focuser and a few basic eyepieces with a 2x Barlow, I have a Nikon crop sensor D7200 1.5x crop I have the adapter Togo intobayonet of camel and it is threaded to fit the Barlow, also i have a smaller threaded tube/ ring 1.25 inch.
I am looking at buying a skywatcher ed 100 doublet with a HEQ5PRO mount, I would use the Refractor fo moon and planets and the reflector for clusters and nebulae, I am just curious to know if it is worth adapting for prime focus or just use the Barlow setup, I would update the focuser to a dual two inch to utilise the both scopes eyepieces
Peter

Currently I can use the scope with the Barlow, but my star adventurer is not really up to the weight, so for tracking i will go with the HEQ5PRO and should be fine for both scopes separately of course

Last edited by RB; 30-10-2019 at 01:18 PM.
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Old 30-10-2019, 03:55 PM
Startrek (Martin)
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Peter
My advice ( don’t take it personally) but ditch the cheap retail reflector and buy a Bintel 150mm f5 or f6 Newtonian reflector , they are only $300 and will last you a lifetime. They come with a great 2” focuser and can be upgraded to dual speed later
I use my Bintel 150mm f6 when I’m in Sydney and can image everything in the universe ( check the beginners astrophotography section )
That way you have more choice in imaging anything now and in the future
By all means buy your refractor but they are pricey buggers in compared to the newt
Cheers
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Old 31-10-2019, 08:32 AM
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Retrograde (Pete)
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Hi Peter,

Another option for you to consider:
Why not pick up a cheap planetary cam like the ZWO ASI120 (currently $279 at Bintel but I've seen it on sale even cheaper) and give it a go on your 150mm Newt?

The back-focus won't be nearly so much of an issue as with a DSLR, it has a small sensor so should work fine with a 1.25" focuser & it weighs less than most eyepieces. Resolution is a function of aperture anyway so you'd probably get more planetary detail with the Newt than you would with a 100mm refractor.
It will help you learn what's required in terms of planetary photography (processing etc) & will do double duty as a guide camera for deep sky. You can then decide which direction you want to head equipment wise down the track.

Last edited by Retrograde; 31-10-2019 at 09:04 AM. Reason: clarity
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Old 31-10-2019, 09:57 AM
spaceout
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https://stellafane.org/tm/newt-web/newt-web.html


Poider you may find this calculator useful for visualizing (ray trace tab) what needs to be to done to make a particular telescope achieve focus.



It will also give you an idea of the performance of different configurations under the performance tab. In particular this will show you if you can fully illuminate a DSLR sensor or not.



Put some coffee on, its a lot of fun.



You will see that raising the mirror, or lowering the focusing tube radially or axially has the same effect of pushing the image plane out of the telescope. That is what is happening when people shorten their OTA length. Of course there are caveats with regards to secondary size etc. but you'll get the idea.
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