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Prime Focus with a Fast Newtonian

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#1 tinyaiRarKs

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Posted 16 September 2020 - 09:07 AM

Hi! This is my first post here in Cloudy Nights. Hope you are having a clear sky today. 

My current (first) setup is a Zhumell 114mm, 456mm focal length reflector. It's a fast f/4. The mount is an alt/az dobsonian so my images are always untracked. My current camera is my Redmi Note 7 smartphone and I think of upgrading it to a second hand Canon 600D(or maybe a ASI120/290 color). For the mount, I'm planning to upgrade to a AZ-GTi or Star Adventurer. 

My question is, can I achieve prime focus to be able to use a DSLR? I can't find any formulas or calculations to determine if I can or not so I'm resorting to asking here and maybe someone can help me. 

Thank you and stay safe y'all. 



#2 tjschultz2011

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Posted 16 September 2020 - 09:26 AM

It's pretty much a guess and check type deal for figuring that out. At least it was for me. Best way to try it for the first time is to try and focus on the moon since it will be easy to tell if you have enough in-travel to reach focus. If not, you have a couple options. You can move the primary mirror forward in the tube if you're comfortable doing so. Also, if you're very close to getting focus like I was, you could try a Televue Paracorr (if you already have one or are comfortable buying one). They have a special adapter that allows the optics part of the Paracorr to attach to a DSLR camera T-ring if you go that route. It increases focal length by a factor of 1.15x while also correcting the coma that will be present in a fast reflector. This was just enough to allow me to focus without having to use a barlow or anything like that. You can also try a low-profile focuser if your scope doesn't already have one. That will allow the camera to be closer to the optical tube. I use a moonlight focuser on mine. Good luck!


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#3 Gipht

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Posted 16 September 2020 - 09:31 AM

Welcome tinyaiRarKs!

 

Since a DSLR sensor sits about 40 mm back from where your eyepiece sits, you may be able to just measure how much inward travel you have from the focus position of an eyepiece.  Most reflectors set up for visual are not able to be brought into focus for a DSLR.

 

The ASI120 is a very small sensor and is seldom used for imaging other the solar system objects.

 

The reflector is likely too heavy for the camera tracking mounts, but a camera with a lens works great on those.


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#4 tinyaiRarKs

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Posted 16 September 2020 - 09:43 AM

It's pretty much a guess and check type deal for figuring that out. At least it was for me. Best way to try it for the first time is to try and focus on the moon since it will be easy to tell if you have enough in-travel to reach focus. If not, you have a couple options. You can move the primary mirror forward in the tube if you're comfortable doing so. Also, if you're very close to getting focus like I was, you could try a Televue Paracorr (if you already have one or are comfortable buying one). They have a special adapter that allows the optics part of the Paracorr to attach to a DSLR camera T-ring if you go that route. It increases focal length by a factor of 1.15x while also correcting the coma that will be present in a fast reflector. This was just enough to allow me to focus without having to use a barlow or anything like that. You can also try a low-profile focuser if your scope doesn't already have one. That will allow the camera to be closer to the optical tube. I use a moonlight focuser on mine. Good luck!

For the Paracorr one, I think I may opt to upgrade my OTA than to buy that since it can cost almost as much as a decent reflector. I might try using a barlow since I already have that. Thank you for your inputs!
 

 

Welcome tinyaiRarKs!

 

Since a DSLR sensor sits about 40 mm back from where your eyepiece sits, you may be able to just measure how much inward travel you have from the focus position of an eyepiece.  Most reflectors set up for visual are not able to be brought into focus for a DSLR.

 

The ASI120 is a very small sensor and is seldom used for imaging other the solar system objects.

 

The reflector is likely too heavy for the camera tracking mounts, but a camera with a lens works great on those.

I guess it is still trial and error to determine if I can achieve prime focus but it isn't good news since most reflectors for visual can't achieve prime focus.

Oh, AZ-GTi is mostly for Camera/Lens combo. Got it. Thank you!


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#5 sg6

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Posted 16 September 2020 - 10:06 AM

I doubt that you can achieve getting the focal plane and the DSLR camera to coincide.

The focal length of the scope is probably too short and in a way if you can get an eyepiece to focus you are very unlikely to get the focal plane and sensor to match up. Just the way the eyepiece optics operate and the nature of the scope.

 

Rather unfortunately for ease of use and ease on pockets for all imaging is a scope and a camera it is not any scope and any camera. And it is steadily becoming more specialised.



#6 DubbelDerp

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Posted 16 September 2020 - 10:15 AM

If you like to tinker, one solution to get the newt to focus with a DSLR is to move the primary mirror up the tube towards the secondary. This effectively moves the focal plane outwards to where it can align with the DSLR sensor. Depending on your mirror cell design, it may be as easy as getting longer collimation bolts and springs, or it might require you to drill holes in the tube to remount the mirror cell. Proceed with caution.. but it is possible.

 

That doesn't do anything to solve the problem out the mount though.


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#7 Alen K

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Posted 17 September 2020 - 08:29 AM

If this is your telescope, I think lunar images using your phone is about all you can reasonably accomplish with it. (Maybe some passable snaps of Jupiter and Saturn, as well.) I certainly wouldn't be putting any more money into it in an effort to do astrophotography. And even if you were to modify the OTA as described and put it on a decent EQ mount, you would still have to contend with a weak 1.25-inch focuser, for which AFAIK you cannot get a coma corrector (they are all 2-inch). 



#8 tinyaiRarKs

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Posted 17 September 2020 - 09:07 AM

I know I will outgrow very easily my scope that's why I'm planning to use a DSLR or maybe an ASI290/224 as a starter cam so that I can carry over it to my next setup. I don't have the capacity, for now, to buy 1000 USD equipment as my starter gear. I also don't have a high expectations with my gear so anything I can produce with my gear is always a treat for me. Thank you for your inputs. Clear skies.

These are some sample shots with the current setup.

Saturn

Saturn
Jupiter
Jupiter
M13
M13, Hercules Globular Cluster
M57
M27, Dumbbell/Apple Core Nebula

 

 


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#9 DubbelDerp

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Posted 17 September 2020 - 09:44 AM

Hey, that's way better than any of my shots of Jupiter and Saturn! If your goal is to add to your existing kit with an eye to having stuff you can use in the future, what about going with the ASI camera? It'll be complicated using a DSLR with your setup, but you can probably avoid that by going with an astro cam. But with an eye on the future, if you go with a mono version, that could be used later as a guide camera. Just something to consider.



#10 Alen K

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Posted 17 September 2020 - 10:06 AM

 

I know I will outgrow very easily my scope that's why I'm planning to use a DSLR or maybe an ASI290/224 as a starter cam so that I can carry over it to my next setup. I don't have the capacity, for now, to buy 1000 USD equipment as my starter gear. I also don't have a high expectations with my gear so anything I can produce with my gear is always a treat for me. Thank you for your inputs. Clear skies.

These are some sample shots with the current setup.

Well, well. You have blown me way with what you have been able to accomplish with that little scope and your phone. The DSO shots certainly show why you need a coma corrector. But without replacing the focuser with a two-inch one, which will cost far more than the telescope, I don't think you can.     



#11 tinyaiRarKs

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Posted 17 September 2020 - 10:10 AM

It'll be complicated using a DSLR with your setup, but you can probably avoid that by going with an astro cam. But with an eye on the future, if you go with a mono version, that could be used later as a guide camera.

Yeah, that's what I've been thinking also. I'm just sacrificing wide field imagery with going low-tier ASI than a DSLR but it saves me the hassle of finding prime focus and benefits me in the future in a guiding system. Thank you for the wonderful insight!



#12 tinyaiRarKs

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Posted 17 September 2020 - 10:14 AM

Well, well. You have blown me way with what you have been able to accomplish with that little scope and your phone. The DSO shots certainly show why you need a coma corrector. 

Yup, I don't plan on buying a coma corrector which is twice or maybe 3x the cost of the reflector or upgrading the focuser to two inch. That is quite a peat considering how much I bought for this scope. Thank you!



#13 DubbelDerp

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Posted 17 September 2020 - 10:37 AM

Yeah, that's what I've been thinking also. I'm just sacrificing wide field imagery with going low-tier ASI than a DSLR but it saves me the hassle of finding prime focus and benefits me in the future in a guiding system. Thank you for the wonderful insight!

That's my rationale for adding guiding to my Skyguider Pro tracker. I keep hearing reasons for why it's not worth it, but now I know how to guide and I have the equipment already for when I upgrade the mount.

 

In that same line of thought, as long as you don't overload the tracker and damage it, you might want to consider the star adventurer. It's better than shooting from a static tripod. And if/when you invest in a DSLR and some prime lenses, you'll continue to find uses for the tracker as a light, portable setup. So I don't think there's a wrong answer here, as long as you keep your expectations in line with the capability of your gear.



#14 kevinrfrancis

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Posted 17 September 2020 - 06:53 PM

How to Move Prime Focus YouTube video.

 

How to Move Prime Focus Blog Post




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