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Determining back focus requirements for Newtonian
Posted: Tue Feb 23, 2021 6:39 am
by chris_g
Hello all,
Is there a some way to determine how much back-focus a Newtonian will need to to achieve focus based on it's specs, I was looking at a Meade FS85 6" Newtonian, focal length is 750mm with F/5 but they don't talk about back focus requirements, a coma corrector I was looking at says it requires at least 55mm of back focus. Celestron talks about really needing a 2x Barlow T-Ring adapter combo to do it. I understand the newts are primarily an observing scope but was wondering if I got one would it be a waste to get a coma corrector too if it's not going to work. Can I add extension tubes like on a refractor?
Thanks for any thoughts, this newbie thirsts for knowledge!
Re: Determining back focus requirements for Newtonian
Posted: Tue Feb 23, 2021 7:07 am
by Piet Le Roux
I dont think there are but if the coma corrector states that it needs at least 55mm it would depend on how much travel your focuser has, if your nomal focus point is midway with a focuser that has 50mm travel then you have a maximum of 25mm in and out focus. Normally a barlow needs back focus and a reducer needs in-focus, both Celestron and Meade makes a comma corector/focal reducer. What comma corrector/reducer are you looking at?
Re: Determining back focus requirements for Newtonian
Posted: Tue Feb 23, 2021 7:10 am
by SkyHiker
Extension tubes do the opposite of what you need. A mirrorless DSLR will have a better chance of reaching focus especially if you can get a T minus adapter for it. Aside from that, try longer primary mirror screws to move the mirror up. A CC will not fit on that Newt, I think I mentioned that earlier.
Re: Determining back focus requirements for Newtonian
Posted: Tue Feb 23, 2021 8:31 am
by chris_g
SkyHiker wrote: ↑Tue Feb 23, 2021 7:10 am
Extension tubes do the opposite of what you need. A mirrorless DSLR will have a better chance of reaching focus especially if you can get a T minus adapter for it. Aside from that, try longer primary mirror screws to move the mirror up. A CC will not fit on that Newt, I think I mentioned that earlier.
The other was an 8" celestron, the one I'm looking at now is a 6" Meade. Yourr answer on the celestron prompted the question. So it's the physical position of the mirror that's the issue.
I'm set on a fract setup for
AP but was wanting to add a newt since I can get a discount on the tube in a package with the mount. The Newtonian would be for visual but if I can take pictures with it so much the better.
You mention a mirrorless, I'd eventually add one to the rig but the next camera will be something along the lines of a ZWO. Would I have the same issue?
Re: Determining back focus requirements for Newtonian
Posted: Tue Feb 23, 2021 8:40 am
by chris_g
Piet Le Roux wrote: ↑Tue Feb 23, 2021 7:07 am
I dont think there are but if the coma corrector states that it needs at least 55mm it would depend on how much travel your focuser has, if your nomal focus point is midway with a focuser that has 50mm travel then you have a maximum of 25mm in and out focus. Normally a barlow needs back focus and a reducer needs in-focus, both Celestron and Meade makes a comma corector/focal reducer. What comma corrector/reducer are you looking at?
I'm looking at the TPO 2",
https://optcorp.com/products/tpo-2-coma-corrector-cc2 it says it needs 55mm. So what decides if the
coma corrector will work or not is the travel of the focuser it's going in to. If I get a meade it's, best to get a meade cona corrector???
Re: Determining back focus requirements for Newtonian
Posted: Tue Feb 23, 2021 1:57 pm
by SkyHiker
chris_g wrote: ↑Tue Feb 23, 2021 8:31 am
SkyHiker wrote: ↑Tue Feb 23, 2021 7:10 am
Extension tubes do the opposite of what you need. A mirrorless DSLR will have a better chance of reaching focus especially if you can get a T minus adapter for it. Aside from that, try longer primary mirror screws to move the mirror up. A CC will not fit on that Newt, I think I mentioned that earlier.
The other was an 8" celestron, the one I'm looking at now is a 6" Meade. Yourr answer on the celestron prompted the question. So it's the physical position of the mirror that's the issue.
I'm set on a fract setup for
AP but was wanting to add a newt since I can get a discount on the tube in a package with the mount. The Newtonian would be for visual but if I can take pictures with it so much the better.
You mention a mirrorless, I'd eventually add one to the rig but the next camera will be something along the lines of a ZWO. Would I have the same issue?
Ah my bad yes that was the 8". I wonder if you should slow down a bit and learn how to use the AVX first, or otherwise move up to a larger mount. It is easy to spend a lot of money then have to upgrade and lose money on the old gear. One of the best APers is Kathy, she has been using a cheap 8" astrograph with a Paracorr 2 forever, shooting great images. It convinced me to order a 12" Newt astrograph, I have the Paracorr 2 already. There are big quality differences between CCs so you may and up spending a few $100 on one then have to upgrade later. It's like motivating spending money on an 8" Newt by comparing what you would have spent on some other mount. It adds up. When you mention a ZWO, realize that the cheap ones give you a small
FOV so you will have to upgrade again. Astrographs are relatively cheap so you can get a lot of
aperture and a fast imaging machine for cheap and then a quality CC will pay off. In that case first get a larger mount. About the ZWO, there are some color ones for about $1K that are quite reasonable. But I would spend the money on the Newt for it because it will work for your 70 mm frac. Focus on that, there will always be plenty of Newts for cheap. Maybe I am patronizing but it's well known how easy it is to get blndsighted by all the bling and buy stuff that you may not use.
Re: Determining back focus requirements for Newtonian
Posted: Tue Feb 23, 2021 2:23 pm
by KathyNS
The scope doesn't "need" back focus, it gives it. It is the eyepieces and other optics that have the need. The back focus that the eyepieces need varies with the design. The focusers on commercially available scopes are designed to be suitable for a wide range of eyepieces.
DSLR cameras need a lot more back focus than eyepieces, and are usually the devices that have back focus limitations.
The 55 mm spacing requirement for coma correctors is the distance to the optimized focal plane. If you are using an eyepiece with the CC, you will need a "tunable top" to adjust the spacing for the eyepiece. If you are using a DSLR, you just need to ensure that its sensor is the required distance (usually 55mm) from the CC.
Note that this spacing requirement has nothing to do with the focuser. It is the spacing from the coma corrector to the eyepiece or camera sensor. Most coma correctors just drop into the focuser like an eyepiece.
So the bottom line is to not over-think it. If your coma corrector wants 55mm, then just connect the DSLR with a T-ring and you are good to go. If you have a dedicated astro camera, its depth is usually 12.5 or 17.5mm, requiring an additional 42.5 or 37.5mm of spacers from the coma corrector.
Re: Determining back focus requirements for Newtonian
Posted: Wed Feb 24, 2021 4:31 am
by chris_g
SkyHiker wrote: ↑Tue Feb 23, 2021 1:57 pm
chris_g wrote: ↑Tue Feb 23, 2021 8:31 am
SkyHiker wrote: ↑Tue Feb 23, 2021 7:10 am
Maybe I am patronizing but it's well known how easy it is to get blndsighted by all the bling and buy stuff that you may not use.
No, you're not, I'm asking for advice and you're giving it, and it's good advice.