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3" Focuser Support for Orion 8" Reflector

Equipment Focuser Reflectors
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texasastrophotography avatar
texasastrophotography avatar
Hi folks,

So I've been recently looking to finally upgrade my camera train. I've been looking at the 6200mm / Zeus-M cameras, but learned that going to such a size would require a 3" focuser to ensure I don't have vignetting issues… Which is making me run into questions that I can't seem to find clear answers to online about.

I've got this 8" f/4.9 (1000mm) orion telescope from a while back, which currently is installed with a 2" Feathertouch. My concerns are as follows:

1. I don't know if this is actually something valid to think about, but would the secondary mirror become an issue for a 3" focuser? In other words, do I need to think about this - and if so, how can I make sure my secondary mirror can handle a 3" focuser? My secondary mirror is ABOUT 50mm x 80mm (rough estimate, wasn't exactly in a position to get precise numbers at the moment)

2. I noticed that the 3" Feathertouch adapters don't have anything available for newtonians? This is making me wonder if a 3" focuser / train on a (non-custom) newtonian is even something I should consider or not. Or are there adapters and/or recommended focusers I should look into instead of Feathertouch that would be compatible AND reliable?

Does anyone know how I can figure these out, if the answer's not already known by someone else? I'd really love to upgrade to one of these cameras, but at the moment it sounds like I might have to go with non-full frame imaging unless I were to go to a different type of telescope and basically do a whole new setup… Hoping to get some answers first though smile
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andrea tasselli avatar
andrea tasselli avatar
I think you got the carriage in front of the horse(s), in a manner of speaking. You should ask yourself what is the coma corrector that would yield an unvignetted 44mm circle at the desired focal ratio. From THAT you would establish whether the secondary and/or the focuser need replacing (most likely the focuser, less likely the secondary). As for the secondary, you would need the focus extraction above the tube (once the CC is in place) and the tube diameter as fixing parameters, if the focal length is known. As a minimum I expect you'd need a 2.5" focuser but that depends also on the CC you are going to use.
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texasastrophotography avatar
texasastrophotography avatar
andrea tasselli:
I think you got the carriage in front of the horse(s), in a manner of speaking. You should ask yourself what is the coma corrector that would yield an unvignetted 44mm circle at the desired focal ratio. From THAT you would establish whether the secondary and/or the focuser need replacing (most likely the focuser, less likely the secondary). As for the secondary, you would need the focus extraction above the tube (once the CC is in place) as a fixing parameter. As a minimum I expect you'd need a 2.5" focuser but that depends also on the CC you are going to use.

Thanks Andrea,

At this moment, I have the Sky-Watcher Quattro Coma Corrector (S20204). According to the Starizona page, this provides a 30mm imaging circle.

For 3" coma correctors, I was looking at as a potential option was the SharpStar 3" 0.85x, although I'm not 100% sure it'd be compatible with a 1000mm aperture or not. Was mainly looking at it since the f-stop range falls into what I'm looking for. It does mention that it has a "44mm fully illuminated imaging circle".

As for the 3" recommendation, this was something provided to me from the Player One support when asking about the Zeus-M, so that's why I was researching that size.

That being said, if I find a coma corrector that's 2" that would be able to provide a 44mm imaging circle, it sounds like the next thing would be to check the focus extraction? It seems the distance from the plane to diagonal is roughly 6.8". The focuser has a 2.5" draw tube, so it seems like that distance is roughly 6.8" - 9.3" (roughly 170mm-240mm). I'm not sure if that'd be the necessary range for such a setup or not. I was attempting to use the calculator found here: https://www.bbastrodesigns.com/NewtDesigner.html#diagonal - but wasn't 100% sure I understood what to fill out in some of the fields (or rather what the results meant) 
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andrea tasselli avatar
andrea tasselli avatar
In these things is best to carry out the actual measurements and the tube outer diameter should be rather easy to measure (dress-maker tape meter or a piece of string). The present focus extraction can also be quickly established remembering that the SW quattro CC moves the focal plane out by 20mm and measuring the current position of the focuser flange above the tube and add the spacing with the current setup and remove the aforementioned 20 mm.  Once you got hard data I can be in a position to calculate the various bits and bobs. 50 mm of minor diagonal is however very small and it is just a wisper from vignetting the aperture, never mind the imaging circle.
texasastrophotography avatar
texasastrophotography avatar
andrea tasselli:
In these things is best to carry out the actual measurements and the tube outer diameter should be rather easy to measure (dress-maker tape meter or a piece of string). The present focus extraction can also be quickly established remembering that the SW quattro CC moves the focal plane out by 20mm and measuring the current position of the focuser flange above the tube and add the spacing with the current setup and remove the aforementioned 20 mm.  Once you got hard data I can be in a position to calculate the various bits and bobs. 50 mm of minor diagonal is however very small and it is just a wisper from vignetting the aperture, never mind the imaging circle.

Thanks - I'll look into taking a look at this. I'm going to actually start with trying to dig up a 2" (or 2.5") coma corrector that would work for my current scope, since if one doesn't really exist for this, no point in checking for this specific case. However, I agree on the 50mm comment as I was imagining that would indicate I need to be 'fairly close' to keep the imaging circle at 44mm, but then that'd result in other issues. 

More and more sounds like I'd either need a different scope for this kind of camera/setup, or just get something else like the 2600mm or Poseidon-M (which I'm leaning towards the latter). 

If you don't hear back from me in the next couple of days, It'll be safe to assume I just went with the other camera instead for this scope, most likely because I was unable to find a coma corrector for this particular hardware/setup... But also if I get an 'exact' measurement of my secondary mirror and find it's even less than the estimated 50mm, I'll just assume 'nope' lol. 

Just doing this research before making this kind of purchase has helped me learn quite a lot in a little amount of time about the hardware and what to look for, and your responses have also helped me understand a few more things - so I really do appreciate it!
andrea tasselli avatar
andrea tasselli avatar
Sure, take your time and if in need just call out.
Tony Gondola avatar
Tony Gondola avatar
What really matters here is what is the nature of the field illumination that the optics provide before it even hits the coma corrector. If you have the following information:

I.D of the tube
Length of tube ahead of the secondary
Diameter of the secondary
Distance from the secondary to the focal plane without the corrector

If you can measure all that just plug it into:

https://stellafane.org/tm/newt-web/newt-web.html

and your answer will be under the performance tab. It will give you diameters for 100% and 75% illumination. It won't matter how large the focuser is if the field illumination you need isn't there to begin with.
Werner Stumpferl avatar
Werner Stumpferl avatar
I´m using a fullframe camera with a 2" focuser. My CC was a Maxfield 0.95 which I have replaced now with a Baader MPCC. Much less vignetting as the Maxfield and there is no need for me to change anything at my telescope.
8" Newton f/4, QHY600M + ASG PhotonCage, Indigo Filterwheel and OAG, Baader MPCC, Baader Steeltrack 2" focuser, secondary mirror is 72mm
texasastrophotography avatar
texasastrophotography avatar
Werner Stumpferl:
I´m using a fullframe camera with a 2" focuser. My CC was a Maxfield 0.95 which I have replaced now with a Baader MPCC. Much less vignetting as the Maxfield and there is no need for me to change anything at my telescope.
8" Newton f/4, QHY600M + ASG PhotonCage, Indigo Filterwheel and OAG, Baader MPCC, Baader Steeltrack 2" focuser, secondary mirror is 72mm

Oh nice, surprised this one didn't come by my searches.

However, I did find the secondary mirror on mine to be at 58mm (so I slightly under estimated it) and at prime focus, I measured the imaging circle roughly somewhere between 30mm and 32mm (Orion's not around anymore for me to confirm that with them, unfortunately), so with that, I decided I'll probably target a full frame in the future when eventually getting a newer scope. By then, maybe the next best thing will have been made too. 

I did have some interest in the Quattro 300P, but Skywatcher informed me the image circle is only at 28mm... Seems newts are tough when it comes to full frame compatibility with minimum vignetting lol. I'd be curious what kind of imaging circle you have on that newt though - I may have to keep it on my watch list if it's on the larger side!
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Tony Gondola avatar
Tony Gondola avatar
I did have some interest in the Quattro 300P, but Skywatcher informed me the image circle is only at 28mm... Seems newts are tough when it comes to full frame compatibility with minimum vignetting lol. I'd be curious what kind of imaging circle you have on that newt though - I may have to keep it on my watch list if it's on the larger side!


It's not a problem f you don't mind a larger obstruction. What makes it challenging is that the focal plane needs to be placed so far outside the tube to accommodate typical imaging trains.
texasastrophotography avatar
texasastrophotography avatar
Tony Gondola:
I did have some interest in the Quattro 300P, but Skywatcher informed me the image circle is only at 28mm... Seems newts are tough when it comes to full frame compatibility with minimum vignetting lol. I'd be curious what kind of imaging circle you have on that newt though - I may have to keep it on my watch list if it's on the larger side!


It's not a problem f you don't mind a larger obstruction. What makes it challenging is that the focal plane needs to be placed so far outside the tube to accommodate typical imaging trains.

Oh, that's something I didn't think about either... Distance to gain that focus. I'll have to keep that in mind too if I want to continue with a new newt and full frame 
Stephen Jones avatar
Stephen Jones avatar
You might like to have a gander at YingtianZZZ gallery.  He is using an 8" SW Quattro and FF camera.  https://app.astrobin.com/u/YingtianZHANG#gallery
andrea tasselli avatar
andrea tasselli avatar
As per the below excerpt from the TS website shows the SW/TS-GPU CC *can* cover the FF format, depending on the image train e.g., the size of the secondary and its position w.r.t. the focal plane (surface, in reality).
texasastrophotography avatar
texasastrophotography avatar
Hmmm, okay you all got me reconsidering again, haha. I'm going to have to dig even more into this then!

It's interesting to see Yingtian used the same coma corrector as me for the image of M104. I know flats can clear out a lot of vignetting, but would be curious in how much there was in the 'flats' taken for that image. Regardless, this is all pretty promising information you all have provided me and has me getting my hopes up smile

I sent out a contact form to TS in regard to the above coma corrector to see if they can help me determine what kind of vignetting, if any, I'd experience with my current hardware, and to let them tell me if they need more details or not. The little "suitable for full-frame format" snippet is pretty exciting to see. I can report back once I'm gathered more information with that (for anyone else in the future looking for this same kind of information).
Stephen Jones avatar
Stephen Jones avatar
When you get time why not send Yingtian a private message with you questions.