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Equipment Discussions >> Cats & Casses

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Michael2
member


Reged: 06/01/11

Loc: Australia
Celestron F6.3 Reducer for visual with C8?
      #5602067 - 01/02/13 10:28 PM

I am considering purchasing a Celestron F 6.3 reducer for use with my 1977 vintage Orange Tube C8 from OPT .
This will be for visual use only. I am working with Stephen James OMeara’s series of books where he used a Televue Genesis Refractor and the eyepieces gave powers of 23x, 72x, 105x plus the addition of a 1.8x Barlow and a 3x Barlow.
The eyepieces I will be using are GSO 50mm =40x, Celestron Silver Top Plossols 36mm=55x, 26mm=77x, 17mm=117x, and 10mm=200x.
Can anyone see a problem with trying to gain some lower power views by using the reducer in this set up?
I have a 2 inch GSO SCT Diagonal that I will be using. Where exactly is the Reducer placed?
Any advice or comments would be welcome.

Michael

Australia.


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Patrick
Postmaster
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Reged: 05/16/03

Loc: Franklin, Ohio
Re: Celestron F6.3 Reducer for visual with C8? new [Re: Michael2]
      #5602096 - 01/02/13 10:49 PM

Hi Michael!

The reducer is screwed onto the back of the scope where you'd normally put the visual back. The visual back is then screwed onto the reducer.

Adding a reducer is a fine solution to get you to a little more field of view.

Patrick


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Jim-M
super member


Reged: 10/09/09

Loc: Southern Calif.
Re: Celestron F6.3 Reducer for visual with C8? new [Re: Patrick]
      #5602248 - 01/03/13 12:55 AM

Michael,
The 6.3 FR works well with the C8 to give wider and flatter field views. Consider though that the C8 field is limited to about 1.3 deg. FOV by the baffle tube. Believe the 50 mm you have will be close to that without the FR.
Also consider the spacing. The FR needs to be spaced close to 105 mm from eyepiece front element. That might be difficult with the 2" diagonal. The 1.25" diagonal that I use gives near correct spacing.


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Eddgie
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Reged: 02/01/06

Re: Celestron F6.3 Reducer for visual with C8? new [Re: Michael2]
      #5602426 - 01/03/13 06:58 AM

Hello Michael,
Just be aware that when you use a 2" diagonal in a C8 behind the 6.3 focal reducer, it will have the effect of turning your telescope into a much smaller telescope.

You get a alightly wider field of view, but the brightness will only be that of a 6.5" telescope, and the contrast for things like planets will be reduced somewhat.

The apture is reduced because to get the focal plane to go far enough to the back to reach the field stop of the eyepiece, the mirror must be moved very far forward using the focuser. When you do this, the outside of the light cone converging on the focal plane is cut off by the baffles in the telescope. And because the outside of the light cone is cut off, rather than being a 34% obstruction, the seconary acts as a 37% obstruction.

The net result is that the field is a little wider, but dimmer, and the view will be vignetted to some degree.

Please keep this in mind and consider removing the reducer when you don't need the entire aperture, or your scope will bascially be working as a 6.5" and performance on most deep sky and solar system targets will be reduced.

So, when you don't need it, it is best to take it off.

And the problem with this is that you will get tired of doing it.

My own recommendation would be to get a 55mm or 56mm Plossl and leave the focal reducer out. You get a slingly narrower field of view, but the quality of the view is far better.

If you choose to leave it in all the time, the next best thing to do is when you don't need 2" eyepieces, remove your 2" diagonal and put your 2" to 1.25" adapter into the visual back and put in the 1.25" diaognal. This will put you back to almost your full apeture, and it is easier than removing the focal reducer every time you want to go to hgher powers.

Also, I think your powers are too compacted into the low power end of the spectrum. The step between 40x and 55x is so small that you will amost always skip int, and the step between 117x and 200x is to big because for deep sky work, you will want some powers intermediate to this.

The reducer goes on to the port at the rear of the SCT.

Your 2" visual back screws on to this. Or if you are using an SCT dedicated diagonal, that would screw on to the focal reducer.

Don't forget though, that when the focal redcuer is used in front of the 2" diagonal, you loose quite a bit of brightness.

Good luck.


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yonkrz
scholastic sledgehammer


Reged: 11/16/06

Loc: SW Minnesota
Re: Celestron F6.3 Reducer for visual with C8? new [Re: Eddgie]
      #5602466 - 01/03/13 07:44 AM

Not trying to hyjack the thread,but i am having the same dilema.So lets say you leave out the FR and keep the 2" diagonal,what EPs would work best for low power,not necessarily wide field.

On the other hand i dont yet have a 2" diagonal but do have the FR,is a person better off getting a good quality 1-1/4" diagonal? Rod helped a while back with pms about a FR,but this just keeps getting more confusing, all i want is the best possible visual views i can get from my c-8.

As i stated above im not trting to hyjack the thread,i just didnt realize all the options and dont know what i need.


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bouffetout
scholastic sledgehammer


Reged: 11/21/12

Loc: Canada
Re: Celestron F6.3 Reducer for visual with C8? new [Re: yonkrz]
      #5602486 - 01/03/13 08:18 AM

Putting a focal reducer Will bring Yuor C8 from f/10 to f/6.3 and 2032mm focal length to 1280mm.
Personally I have one on !


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Eddgie
Postmaster
*****

Reged: 02/01/06

Re: Celestron F6.3 Reducer for visual with C8? new [Re: yonkrz]
      #5602494 - 01/03/13 08:22 AM

Form an absolute best performance across the field standpoint, the 35mm Panoptic is a perfect perfect perfect match for the C8. The field stop of the 35mm Pan is almost the exact same size as the rear baffle, so the edge of the field is as bright as the design allows.

But you can go wider, and in fact, much wider, if the lowest power and biggest exit pupil is your goal.

For the best brigtness, I advise a 56mm Meade Superplossl. This will give a very nice, bright, wide field with a 5.2mm exit pupil.

Remember, you can get lower magnification with the focal reducer, but because the aperture is reduced when you use a 2" eyepeice, you get a esentailly the same size exit pupil because you are using a 6.5" scope. In other words, you won't get any added brightnss using the focal reducer that you would get just using the 56mm Plossl!!!

And there are two functions of a telescope... Expand the angular size of the target by magnification, or make the target brighter by making the exit pupil larger, but at the expense of angular magnification.

This is why it doesn't make sense to use a focal reducer. You get a smaller image scale, but you loose the exit pupil that the lower magnification would give because the aperture is reduced. My personal belief is that the wider view is no advantage if you can't get a brighter image from it for DSOs. And if you want to get the Pliedes into the field, get a 4" achromat. It is a much better way to get there.

The field of the 56mm plossl will be vignetted near the edge. If you drift a faint star to the field scope, it will wink out just before it gets to the very edge, but it is hard to notice the loss of the faintest stars.

But there is this. The bigger exit pupil often comes at a price. If your sky conditions are not very dark, the bigger exit pupil will cause the sky to look brighter and brighter as it gets bigger and bigger. The sky can appear a bit washed out, and even though deep sky objects will keep the same amount of brightness against the background, to the eye, the view seems less contrasty. It really isn't but it could appear that way.

I had a 56mm Plossl and a 35mm Panoptic in my C8 and did side by side comparisons many times from my central Austin back yard. I felt that while the field was very slightly smaller in the Panoptic, the view seemed more pleasing because of the darker background.

Under darker skies though, and for large objects like Lagoon, Orion, Rosette, and many other very large Nebula, the 56mm Plossl was clearly better because of the very large exit pupil that it offers vs the 35mm Pan.

It is like turning your 8" SCT into an 8" dob.


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rmollise
Postmaster
*****

Reged: 07/06/07

Re: Celestron F6.3 Reducer for visual with C8? new [Re: Eddgie]
      #5602614 - 01/03/13 09:45 AM

Quote:

Hello Michael,
Just be aware that when you use a 2" diagonal in a C8 behind the 6.3 focal reducer, it will have the effect of turning your telescope into a much smaller telescope.




We've been down this road before.

As a regular user of this combination, I will again say that you will never notice the difference. The only real difficulty you will experience is that longer focal length eyepieces, above about 25mm will vignette. But with this combination, you shouldn't have to use longer focal length eyepieces.

This is personal preference, mind you, but I have a hard time imagining a less comfortable eyepiece to use than a pea-picking 50mm range Plossl.

Edited by rmollise (01/03/13 09:47 AM)


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Celestron F6.3 Reducer for visual with C8? new [Re: Eddgie]
      #5602683 - 01/03/13 10:25 AM

This is a popular and complex topic. You may want to read the responses I got in a similar thread.

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Patrick
Postmaster
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Reged: 05/16/03

Loc: Franklin, Ohio
Re: Celestron F6.3 Reducer for visual with C8? new [Re: rmollise]
      #5602829 - 01/03/13 11:52 AM

Quote:

We've been down this road before.




Yep...a well traveled road, too.

I personally don't like long focal length plossl's even if they do give a wider field of view. The view may be wide, but the mags are low as well. Plus a long plossl still feels like I'm looking through a straw.

I think there is a trade off between slight light drop off with a reducer and higher mags, versus no light drop off and low mags. I prefer the higher mags myself. Quite frankly I don't see any light drop off when I use my Denk Power X Switch single eyepiece diagonal in 0.66x reduction mode and a Panoptic 27mm eyepiece. I'm not saying it isn't there, but I just don't see it.

Patrick


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yonkrz
scholastic sledgehammer


Reged: 11/16/06

Loc: SW Minnesota
Re: Celestron F6.3 Reducer for visual with C8? new [Re: rmollise]
      #5603015 - 01/03/13 01:33 PM

Thanks again for the input Rod,so with all the repeatedness put aside. Lets say i want to use a 2" diagonal and was starting fresh with no eps,and wanted two eps for the time being, a low power and mid/high power,what would be the suggestion,i realize eps are a complete other area of discussion but am looking to see what is prefered for this particular setup if i can only have two eps,i wont ask any more questions,i will get what is suggested and just observe.

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rmollise
Postmaster
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Reged: 07/06/07

Re: Celestron F6.3 Reducer for visual with C8? new [Re: yonkrz]
      #5603173 - 01/03/13 02:59 PM

Quote:

Thanks again for the input Rod,so with all the repeatedness put aside. Lets say i want to use a 2" diagonal and was starting fresh with no eps,and wanted two eps for the time being, a low power and mid/high power,what would be the suggestion,i realize eps are a complete other area of discussion but am looking to see what is prefered for this particular setup if i can only have two eps,i wont ask any more questions,i will get what is suggested and just observe.




How much moola...dineros...money are we talking? If on the reasonably modest side, you could do worse than the 82-degree jobs from Explore Scientific.


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Patrick
Postmaster
*****

Reged: 05/16/03

Loc: Franklin, Ohio
Re: Celestron F6.3 Reducer for visual with C8? new [Re: yonkrz]
      #5603180 - 01/03/13 03:02 PM

I know what I'd suggest because what I have is just about perfect. But no one generally listens to me so spend your money on eyepieces instead.

1) Denkmeier S1 Power X Switch Diagonal for SCT (Scroll down to the S1 for SCT).

The S1 will give you 0.66x focal reduction, 1.0x (straight thru), and a 2x barlow. With one eyepiece you will get three magnifications.

2) 24mm Televue Panoptic eyepiece (or 27 Pan?).

With the S1 and 24 Pan you will get a 36mm eyepiece, a 24mm eyepiece, and a 12mm eyepiece. In your C8 that will give you approximately 55x, 83x. and 166x

Then get an inexpensive plossl or two for your high power planetary observing using the built in barlow.

Patrick


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Michael2
member


Reged: 06/01/11

Loc: Australia
Re: Celestron F6.3 Reducer for visual with C8? new [Re: Patrick]
      #5603255 - 01/03/13 03:41 PM

Thanks very much for the effort everyone has gone to in these replies.
I am thinking that this may come down to a "Suck it and See" scenario for me.
Now I am aware of possible limitations when used with different Eyepiece/Diagonal combinations there will not be any nasty surprises.
Another option is to go back to using the Celestron Multi ocular Holder with the four Plossols. This may put the eyepieces a bit closer to the Reducer and eliminate some of the vignetting (if I understand correctly). The only 2 inch eyepiece I have is the GSO 50mm which might not be needed often if the Reducer works with the Plossols.
How do I work out the magnifications I will have with my current eyepieces when using the Reducer?
Thanks again for all the help and patience.

Michael


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Steve Cobb
member


Reged: 04/26/10

Re: Celestron F6.3 Reducer for visual with C8? new [Re: Eddgie]
      #5604685 - 01/04/13 12:26 PM

A year or so ago you tried to fill me in on this point. I appreciated your patience and have kept copies of your e-mail posts. This brief message does a superb job of explaining what I initially found to be a rather mysterious loss of aperture when using a two inch diagonal and a focal reducer/flattener. Hang on to it for future use. Those of us pretty new to the field will always need it.

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JamesL
member


Reged: 10/19/12

Loc: Pacific,MO
Re: Celestron F6.3 Reducer for visual with C8? new [Re: Steve Cobb]
      #5607432 - 01/05/13 10:38 PM

After reading this I might sell my focal reducer after it comes in the mail, unless someone can come up with a reason to keep it for a c8. I allready have the 56mm meade super plossl!

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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Celestron F6.3 Reducer for visual with C8? new [Re: JamesL]
      #5607506 - 01/05/13 11:19 PM

Quote:

After reading this I might sell my focal reducer after it comes in the mail, unless someone can come up with a reason to keep it for a c8. I allready have the 56mm meade super plossl!




Try it before you sell it. It's probably fair to say that using both a focal reducer and a two-inch diagonal puts each at cross purposes, but there are folks who leave focal reducers on always, and Celestron designed the scope with the focal refucer in mind. Although Meade's new LX600 breaks the trend, it was thought that SCTs natively faster than f/10 had harmfully large obstructions. I just bought a focal reducer and would like to hear what you think of yours.


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Rick Woods
Postmaster
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Reged: 01/27/05

Loc: Inner Solar System
Re: Celestron F6.3 Reducer for visual with C8? new [Re: Joe Cepleur]
      #5607565 - 01/05/13 11:46 PM

Quote:

...Celestron designed the scope with the focal refucer in mind.




Where on Earth did you hear that?? The C8 came out in the early 70's; a good two decades before anyone thought of an f/6.3 focal reducer.


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Celestron F6.3 Reducer for visual with C8? new [Re: Rick Woods]
      #5607628 - 01/06/13 12:36 AM

Fair question. The early focal reducers were 0.50, not 0.63. I'll have to research my source, but it's true. The thought upon design was that the central obstruction became too large below f/10, so for wider fields or faster exposures, the f/10 native optical tube would be modified with a focal refucer.

I'll try to find and post the source.


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rmollise
Postmaster
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Reged: 07/06/07

Re: Celestron F6.3 Reducer for visual with C8? new [Re: JamesL]
      #5608058 - 01/06/13 10:53 AM

Quote:

After reading this I might sell my focal reducer after it comes in the mail, unless someone can come up with a reason to keep it for a c8. I allready have the 56mm meade super plossl!




That would be silly. It works great, and it works great with a 2-inch diagonal. It was designed for 35mm full frame imaging after all. Yes, if you use eyepieces, 2-inch eyepieces longer than 25mm, it will vignette, but so what? You don't have to use eyepieces that long at f/6.3, and, most importantly, the field edge WITH the corrector is better than it is WITHOUT.


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