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Short focal Newton with binoviewer

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#1 AlbertoJ

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Posted 23 August 2019 - 03:42 AM

Hello, I´m planning buy a 16", 18" or 20" dobson, with focal between f/4 and f/5.

I´m going to use it permanently with binoviewer.

What´s the minimal focal can you use it without coma corrector and don´t notice coma?



#2 junomike

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Posted 23 August 2019 - 06:24 AM

If you're not planning on using any type of Barlow/OCS I'd go with F5, however IMO there might still be some Coma seen in longer F/L eyepieces such as the 24mm Pans.

IME F6 is the threshold for me for Coma



#3 Eddgie

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Posted 23 August 2019 - 08:49 AM

Hello, I´m planning buy a 16", 18" or 20" dobson, with focal between f/4 and f/5.

I´m going to use it permanently with binoviewer.

What´s the minimal focal can you use it without coma corrector and don´t notice coma?

Well, you will have to use an OCS of some kind for f/4 and perhaps at f/4.5 depending on the BV you are going to use, so this will raise the power and cut the field size.

 

At f/4.9, I did not feel the need for a coma corrector when using a 1.3x OCS but there was a bit of coma there (Binotron with 45m OCS).  It was only a tiny bit though, and not enough to bother me. 

 

Here would be my advice.   Once you have the telescope, look at the power of your OCS.  For example, if you get an f/4 telescope and you have a low power eyepiece that will give the same power as you would get using the binoviewer and the lowest power eyepiece you intend to use in the BV with the OCS, and see if it bothers you.  

 

For example, if the longest focal lenght eyepiece you are going to use in the BV is 24mm, then with a 1.3x OCS, that would be the same as using about an 18mm eyepiece without the OCS.     So, you would get an 18mm (or as close as possible) 1.25" eyepiece and see what you think.  If you can live with the coma in the 18mm (or as close as you can get) then you would probably be OK using the BV with a 1.3x OCS and the 24mm eyepieces.

 

Coma is very subjective.  There will be coma at even f/4.9, but the amount of OCS power you use and the resultant true field will determine how far off axis you can see, and that will determine how much coma is present.

 

If you are using the Baader 1.7x Newtonian GPC, it will correct the coma pretty well.   If you are using a Powermate or the Televue 2x binoviewer amplifier, then I doubt you would have any problem at f/4.5 due to the high power, but I can't tell you what would happen at f/4.

 

So, a lot depends on how much amplifier you are going to use.   At 1.3x, I suspect that at f/4.5 or f/4, coma is going to be an issue.  If you go with the Baader 1.7x, that will correct coma.  If you are going with a Televue 2x BV amplifier, you are probably going to be OK at f/4.5 but I have no idea of what would happen at f/4.

 

If I have a clear night tonight, I will try to get my f/4 scope out with the BV and the Televue 2x BV amplifier and report back.



#4 AlbertoJ

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Posted 02 September 2019 - 08:31 PM

Then, in a 16" - 20",  f/4  dobson, 

Isn't there any way to observe without coma at medium power (about 100x) with a binoviewer?



#5 Eddgie

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Posted 03 September 2019 - 07:10 AM

Well, a lot depends on your tolerance to coma.

 

You will need an OCS for sure, so that would mean working at a minimum of 1.3x (more likely 1.4x) with something like the Denk or Earthwin. 

 

If you see coma with a 1.25" eyepiece having a field of view that would be equivalent to your lowest power eyepiece at the amount of amplification you think you will have to use and you don't like the coma, then the best choice would probably be the Baader 1.7x Newtonian GPC. This unit has coma correction built in. 

 

If you are using the Televue Binoveiwer 2x amplifier, then you probably will have a narrow enough true field not to have a problem. 

 

You will need some form of OCS though, so until you know how much OCS you are going to use, it is hard to know how much field you will see, and the more field you can see, the more likely you will see coma. 

 

The Baader 1.7x Newt GPC though will do a good job of correcting coma, but it may not reach focus in all scopes.



#6 AlbertoJ

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Posted 03 September 2019 - 09:34 AM

Ok, but is Baader 1.7x Newt GPC effective at f/4,  f/3.5, or f/3? Or otherwise at these short focal ratio is Paracorr II the only effective coma corrector?



#7 Eddgie

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Posted 03 September 2019 - 05:42 PM

I am pretty sure it works at f/4, but I don't know what the fasted it will work at.  I suggest you contact Baader and ask them.

 

Remember, even if you use a Paracorr, you are still going to need some kind of Barlow or GPC or something.  

 

If you are custom building, keep in mind that a standard 1.25" binoviewer won't give fully aperture at focal ratios faster than about f/4.4.   For anything faster you have to use 2" binoviewer or a GPC, Barlow, or amplifier of some kind. 



#8 noisejammer

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Posted 03 September 2019 - 05:49 PM

A slightly dissenting opinion

 

The problem you may encounter is that all binoviewers will vignette a f/4 field down to about f/5 giving you no light from the outer 36% of the scope's aperture anyway. This means you may as well either start with a f/5 scope or use some sort of Barlow to increase the f-number.

 

(For completeness, Eddgie's right about the f/4.4 but that means only a spot at the centre of the field is fully illuminated - an effective f/5 or even f/6 is preferred.)

 

Let's assume the scope's effectively a 16" f/6 (after the Barlow), then the focal length is about 2400mm so that 100x needs a 24mm eyepiece. That's easily found - in fact the 24 mm Panoptic is many people's favourite eyepiece for binoviewing. (It's not mine.)

 

To conclude, there has been some discussion in the ATM forum about the Baader coma corrector. It was a while back but I seem to recall the users weren't that impressed.



#9 AlbertoJ

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Posted 08 September 2019 - 06:30 PM

Thanks for your answers.

I think, the best is not to use a Paracorr or any coma corrector, because I'd be obliged to use a 1.8x or 2x amplifier or barlow, besides the possible vignetting.

All I have to try is what's the suitable focal ratio for me, between f/4.5 and f/5, so that coma does not bother me and use the telescope with 2 step or less at zenit.

Perhaps, the best could be a 18" dobson.



#10 Miranda2525

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Posted 13 September 2019 - 08:20 AM

If you're not planning on using any type of Barlow/OCS I'd go with F5, however IMO there might still be some Coma seen in longer F/L eyepieces such as the 24mm Pans.

IME F6 is the threshold for me for Coma

When using binoviewers, the end user is well over F/5 and F/6. More notably when using a Newtonian and a barlow to reach focus, (which is needed in a Newt). Even if using an SCT the FL will be much longer and coma is not an issue at all. 

 

I used WO binoviewers with a barlow piece from a 2" barlow lens and I was running at F/12. Coma at F/12 was non-existent. I could let planets go right to the edge of the FOV and the images were still 100% sharp. I was using a 10" Orion XTi.


Edited by Miranda2525, 13 September 2019 - 08:21 AM.


#11 Jon Isaacs

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Posted 13 September 2019 - 12:49 PM

My thinking as the owner of some larger Dobsonians but not a binoviewr user.

 

- The spacing of a coma corrector is important. The Paracorr has a Tuneable Top with a range of 0.7 inches with steps of 0.10 inches. With a binoviewer, you be far beyond the optimal range for coma correction, it's probably not a reasonable choice.

 

-  The binoviewer should not be your first choice, avoiding the need for a ladder should be your number priority. A 20 inch F/5 will have an eyepiece height of about 100 inches. My 22 inch F/4.4 has an eyepiece height of 96 inches. I am 6 feet tall and I need to a ladder for any object greater than 40 degrees elevation.  With my 16 inch F/4.4, I don't need a ladder or stool but shorter friends do.

 

Jon




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