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Old 08-11-2012, 09:27 PM
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rcheshire (Rowland)
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Borg Canon Takahashi glass.

Hi. Delving into equipment decisions again???

Are there any Borg users out there willing to share their experience.

In mind is widefield, 71FL f3.9 Astrograph, 5 elements including triplet focal reducer - Cost around 2k. Will atttach an SBIG ST-8300C.

The Tak 4 element Petzval's are superb and in another thread I understand that there was disappointment with the FSQ-85ED @ 3.8k. I'm not sure that the 106 (5.6k), as beautiful as it is, suits my needs - essentially wide field.

Perhaps I should stick with my Canon 200mm for now, which is stopped down with an aperture disk - aka inverted reducer rings - stack thereof.

I would consider a Ritchey Cretien for narrow field if I ever got around to it - happy with wide field right now.

Second question - I understand that the SBIG crop factor is 1.95; that is, 18 x 13.5mm, relating it to full frame - because I am using a camera lens with it - effective focal length is 390mm which is close to the Borg in any case.

For wide field, am I gaining anything by purchasing what is essentially an achromat Fluorite doublet, in the Borg. I don't know if the triplet focal reducer is also a field flattener.

Questions, questions

Last edited by rcheshire; 09-11-2012 at 05:52 AM.
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Old 08-11-2012, 10:12 PM
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DavidTrap (David)
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Effective focal length has little to do with it when it comes to astronomy. The Borg at 390mm will have roughly double the magnification compared to your Canon 200mm.

For a given scope, "magnification" will be the same whether you use a small or large chip camera, but you'll see a wider field of view with a larger chip. The resulting image from a smaller chip camera might appear more zoomed in, but that's only because you've blown the image up larger to fill your screen/print vs the image from a larger chip.

If the math were correct, a longer focal length scope with a larger chip will give the same image as a shorter focal length scope with a small chip, but there will be less detail with the latter combination - eg. and FSQ-106 and ST-11000 might give a similar field of view to an FS-60 and a ST-8300, but the former will give a more detailed image through greater magnification.

Hope this helps!

DT
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Old 09-11-2012, 06:05 AM
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rcheshire (Rowland)
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Thank you David. You are correct , effective focal length is not a consideration. I am more concerned with FOV and resolution and think the 200mm is just about right with a DSLR 1.6 crop factor. This will change with the increased crop factor of the KAF8300 chip. Perhaps I am looking at this from the wrong perspective.

My first consideration consistent with FOV and resolution is the quality of the optics. Finding a small aperture scope that fits the bill is more challenging than I thought. I'm getting to the stage where I'm not satisfied to settle for less than excellence and if buying another scope is merely a compromise then I'll stick with the apprentices tool in the 200mm Canon.

Thinking this through and elucidating a bit more, I'm looking for a small instrument with excellent optics to replace my 200mm in the hope that I can improve on it too.
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Old 09-11-2012, 07:27 AM
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Hi Rowland
I think the Borg works out to be 280mm
Its worth remebering the FSQED can take a reducer giving you a 380 mm FL @ F3.64 and will cover FF chips if you ever upgrade there is also a 1.6x that works well too .
So the FSQED can work at F3.6 , F5 and F8 , you will need a solid mount though , they are heavy.

Zane
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Old 10-11-2012, 05:13 PM
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rcheshire (Rowland)
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You are right Zane. Not only am I confused, but I'm managing to confuse everyone else.

I was hoping Borg owners might chip in with their experiences using the equipment.

Is the Borg a good instrument? Let's leave it at that...
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Old 11-11-2012, 06:36 AM
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Hi Rowland
The only Borg scope I've owned in one of there 5" APOs with the DG-L .85x reducer , nice scope but the Helical focuser was not up to the task and I replaced it with a Feather Touch .
The coulout correction could have been better to but this scope had ED optics not FL ones in the 71mm .
There very vercatile scopes and very light wieght , I would look at a focuser upgrade ( which Borg can do ) to a FT focuser for CCD work.

Zane
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Old 12-11-2012, 07:23 AM
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rcheshire (Rowland)
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Thanks Zane. I took a few shots over the weekend, to tune up my system for the SVAA camp this weekend. The 8300 - 200mm combination produces a nice image. Stars look great, but a flatter field would be nice. It would be nice to take in more sky which is a little restricted in my view. This setup in square mode would frame eta C well.

The idea of interchangable objectives, reducers and so on makes the Borg very attractive. And the .7 focal reducer is also a flattener, as it turns out. Borg don't go into a lot of detail at times. The user group is the best resource.

Rowland.
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