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C6 & FR/FF Image Quality

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#1 Ed Whitney

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Posted 20 August 2014 - 10:45 AM

Hi,

 

Can anyone relate how well the "visual" image quality of the C6 is improved by the Celestron Focal-Reducer / Field Flattener, PN 94175 ?  This is the "standard" 0.63x unit that converts F/10 to F/6.3.

 

Years back I had the C6 and liked it a lot except for the field curvature that my older eyes can not accomodate. That is, when I had sharp focus in the center, the outer 1/3 of the FOV was out of focus. And likewise when focusing the outer 1/3, the center of FOV was no good.

 

Thing is, does the FR/FF allow for sharp focus across the entire FOV? Or, is the focusing -still- compromised somewhat?

 

I have this scope & FR/FF on order and wanted to get some opinions on what I can expect.

 

Thank You and Clear Skies! :)


Edited by Ed Whitney, 22 August 2014 - 08:06 AM.


#2 desertlens

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Posted 22 August 2014 - 02:04 PM

Sorry no answers from me Ed but I have the same question, same scope (in hand) and same reducer (on order). I'm using a Baader T2 prism to keep the path short, so 1.25" EPs only (usually Plossls and Abbes).



#3 Dwight J

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Posted 22 August 2014 - 02:28 PM

The instructions that came with my Celestron F6.3 FR described focusing the centre of the field first and then defocusing 1/24th of a turn counter-clockwise so that the full field is focused as good as it can be.  



#4 end

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Posted 22 August 2014 - 02:45 PM

As far as field flattening goes, I find it to be quite good with the same scope and reducer / flattener. However, there have been many documented cases of very bad internal reflections (which show up as ghosts or rings / arcs of light near the edges of the field of view) in the C6 that are made much more apparent with the use of the reducer. In my case it ended up being a deal breaker. Your mileage may vary.



#5 desertlens

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Posted 22 August 2014 - 03:16 PM

I experienced the reflection problem myself. It disappeared completely when I flocked the baffle tube. I don't expect it to return with the FR.


Edited by desertlens, 22 August 2014 - 03:24 PM.


#6 Ed Whitney

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Posted 22 August 2014 - 07:01 PM

Very interesting comments guys. Thanks! I received the little "6" today and the skies are clear tonight! Can't believe it! :)

 

That "arc of light" sounds disturbing, but I may have an answer for it besides internal flocking. With each scope, I use a dew shield made out of 1/16th inch thick black sponge rubber sort of stuff that you can get at a Michael's Craft store for $1.00 a sheet that is 12in x 18in I think. It is fairly easy to glue the ends together to form a circle with 3M Spray Adhesive. This longer than normal dew shield also protects the lens from collecting up "atmospheric stuff" that just seems to form there, even in a short time.

 

I was kind of figuring the corrector may not quite give perfect focus over the entire FOV. But, that's ok, the F/6.3 speed should give a slightly brighter image. This scope setup will mostly be used on double-stars where the center focusing will be just fine. And I too love 1.25in eyepieces!

 

I did so miss my "first" C6-A terribly. At the time I couldn't afford the corrector, so parted with it.  - - - Never Again! - - - :)

 

Clear Skies and Thanks! :)



#7 charles genovese

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Posted 24 August 2014 - 09:49 AM

Ed- common misconception. The focal ratio has nothing whatsoever to do with the brightness. Only the magnification (exit pupil). So if you match eyepieces to give the same magnification with and without the reducer (ie one .63 X's the other- (not exactly because the distance from focal reducer to focus influences the focal ratio and thus the final magnifaction) the brightness will be the same. Reminds me of when Meade advertised their f/6 SC's (in the 80's) as being brighter. Since they required larger secondary mirrors they actually had less light transmission.

#8 jrbarnett

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Posted 24 August 2014 - 04:58 PM

Hi,

 

Can anyone relate how well the "visual" image quality of the C6 is improved by the Celestron Focal-Reducer / Field Flattener, PN 94175 ?  This is the "standard" 0.63x unit that converts F/10 to F/6.3.

 

Years back I had the C6 and liked it a lot except for the field curvature that my older eyes can not accomodate. That is, when I had sharp focus in the center, the outer 1/3 of the FOV was out of focus. And likewise when focusing the outer 1/3, the center of FOV was no good.

 

Thing is, does the FR/FF allow for sharp focus across the entire FOV? Or, is the focusing -still- compromised somewhat?

 

I have this scope & FR/FF on order and wanted to get some opinions on what I can expect.

 

Thank You and Clear Skies! :)

Yes, it works visually pretty well, reducing FC and to a lesser degree coma, but eliminating neither entirely.  The price you pay though is increased vignetting with wider field eyepieces due to extra focal length being added off the rear by the filter.



#9 areyoukiddingme

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Posted 25 August 2014 - 02:56 AM

I use the Celestron FF/FR in my c6. It does improve things, but it hardly turns it into an "edgehd 600". . . 

 

The more important point is that it's a faff to use if you don't have a good eyepiece collection (most of mine are 2", and I use 1.25" only with the c6 to max portability). I use a 8.8mm ES/82, 15mm Vixen NPL plossl, and 24mm ES/68. For high mags I remove the reducers, which kind of gets old fast. So now I find I rarely use it.

 

Also don't expect a dew shield to cure the 'diamond ring' effect. I have a good astrozap shield for my c6, which is rather long and it doesn't have any effect. The baffle tube is the culprit. But it's easily fixed by adding flocking.



#10 Ed Whitney

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Posted 25 August 2014 - 07:15 AM

Thank you much guys!

 

Yea, I realize my mistake about the "added brightness". I know the exit pupil increases in size with the reducer thou. I'm really anxious for "First Light"!

 

I'll have to look for that "diamond ring" effect. But pulling the corrector right now is not a high priority, since the scope is new. Bob's Knobs will be ordered thou!

 

Also realize about the vignetting effect. The max AFOV eyepiece I'll use is probably a 68 degree, 1.25". I did try an ES 82 degree once in a refractor and didn't like it at all. I had high hopes for it too. The view thru it seemed a bit "strange".

 

Best to you all! :)


Edited by Ed Whitney, 25 August 2014 - 07:16 AM.


#11 mclewis1

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Posted 25 August 2014 - 08:55 AM

You don't need to touch the corrector to try out the effects of flocking the baffle tube. You can simply cut an appropriately sized piece of flocking and insert it from the rear of the baffle tube. It won't fit absolutely snugly against the sides but it will be enough to test whether or not it will ultimately help. The only real downside from this temporary baffling is that it will restrict the size of the baffle tube just a little bit which will cause a little vignetting with low power eyepieces (which usually isn't the way you'll be using the scope when you worry about potential extra light anyways).



#12 areyoukiddingme

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Posted 25 August 2014 - 12:00 PM

You can avoid vignetting by not pushing the flocking material too far up the baffle. Some flat black paint at the end of the baffle probably wouldn't hurt (the celestron stuff is rather reflective).

 

And if you do decided to pull the corrector you can flock the inside of the tube too. Just make sure that you carefully mark the orientation of the corrector to the tube and re-install in exactly the same orientation. It's an easy job, as is collimation when you have a night of good seeing as the diffraction rings are easily seen around bright stars. I have Bob's knobs and they are very good.



#13 mclewis1

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Posted 25 August 2014 - 12:24 PM

There's virtually no benefit to flocking the inside of the ota on commercial SCTs. The design of the primary and secondary baffles does a very good job of preventing off axis light intrusion.



#14 hardwarezone

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Posted 25 August 2014 - 12:32 PM

You don't need to touch the corrector to try out the effects of flocking the baffle tube. You can simply cut an appropriately sized piece of flocking and insert it from the rear of the baffle tube. It won't fit absolutely snugly against the sides but it will be enough to test whether or not it will ultimately help. The only real downside from this temporary baffling is that it will restrict the size of the baffle tube just a little bit which will cause a little vignetting with low power eyepieces (which usually isn't the way you'll be using the scope when you worry about potential extra light anyways).

 

How do I secure the flock roll to the baffle tube ?

The adhesive-backed stuff is too messy to work within the tight space.



#15 jrbarnett

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Posted 25 August 2014 - 01:10 PM

 

You don't need to touch the corrector to try out the effects of flocking the baffle tube. You can simply cut an appropriately sized piece of flocking and insert it from the rear of the baffle tube. It won't fit absolutely snugly against the sides but it will be enough to test whether or not it will ultimately help. The only real downside from this temporary baffling is that it will restrict the size of the baffle tube just a little bit which will cause a little vignetting with low power eyepieces (which usually isn't the way you'll be using the scope when you worry about potential extra light anyways).

 

How do I secure the flock roll to the baffle tube ?

The adhesive-backed stuff is too messy to work within the tight space.

 

Nah, I think non-adhesive, rolled up a bit then allowed to unroll and expand to contact the walls of the baffle tube.

 

- Jim



#16 areyoukiddingme

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Posted 25 August 2014 - 01:15 PM

I used the plastic-backed protostar stuff. I first tried painted sandpaper, but found it very difficult to seat in the baffle without vignetting.

 

In the end, the protostar 'paper' rolled around a drum stick and inserted into the baffle from the rear worked. When cut just right, it unfolds and seats against the baffle without needing adhesive. But it's not particularly easy.  

 

I wouldn't swear that using this stuff to baffle the inside of the tube has hurt the views. I do suspect it has slowed down cooling somewhat though. The celestron flat grey-black stuff is obviously not brilliant. 



#17 desertlens

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Posted 25 August 2014 - 03:06 PM

I used the ScopeStuff flocking and left the adhesive covered. As I recall, I cut an 80mm wide strip, rolled it up and just inserted it into the baffle tube from the rear. I marked the "tube" and cut it to length. I see no evidence of vignetting. It's what I don't see that matters... no more flare from bright objects just out of the field. My FR arrived today and if it clears tonight (50/50) I'll give it a try.



#18 Sarkikos

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Posted 26 August 2014 - 09:46 AM

Flocking the baffle tube is a project that I still need to do for my C90, C6 and probably C5 as well.  Something to keep me busy during cloudy weather ... Cloudy Nights!  I'll use ProtoStar or ScopeStuff flocking with the adhesive backing left on.

 

Mike


Edited by Sarkikos, 26 August 2014 - 09:47 AM.


#19 Ed Whitney

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Posted 26 August 2014 - 09:48 AM

Gee, Thank you guys for this info! I now see the problem. Thought it was just the inside of the ota that needed flocking. It's the small tube that's supposed to block stray light from getting to the eyepiece. I've been away from SCT's a little too long I guess.

 

I'll try a small strip of that 1/16th inch black rubber stuff I used to make the dew shield. I'll only go about 3/4ths the way into the tube to prevent vignetting. The stuff curls up into a nice cylinder and I'll secure it with masking tape, but it is thicker than the ScopeStuff flocking, which BTW is "good stuff" ! :)

 

It's too bad there isn't some sort of flat black paint we could use to fix this. The tube almost seems to need knief-edge light stops, perhaps even one would do it. But making the stop would be a headache.



#20 desertlens

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Posted 26 August 2014 - 11:22 AM

My attempt at testing the FR was a bust last night... soon I hope. The ID of C6 baffle tube is already small. I'd keep whatever material you use as thin as possible to avoid vignetting.



#21 Ed Whitney

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Posted 26 August 2014 - 04:07 PM

Yea, for the C-6, the baffle opening is only 27mm, which is not much to play with. :confused:



#22 Sarkikos

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Posted 26 August 2014 - 05:39 PM

It's even a little worse than that.  I used a digital caliper to measure the baffle opening of the C6 to be 25.6mm.  This "27mm" spec has been around for a long time. I'm not sure where it originated. Maybe Celestron?

 

Mike



#23 rguasto

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Posted 27 August 2014 - 06:25 AM

Hi Ed,

I love the FR/FF in my C6. Much wider FOV. No impact whatsoever on image quality IMHO. I believe SCT should be sold with one as standard equipment.  As for flocking the baffle tube (why are we revisiting this in a FR/FF thread?); I cannot appreciate any problems with my C6 that would cause me to open up the tube and put anything in the baffle and potentially introduce dust, etc into the OTA. Just my  .02

-Rob



#24 jrbarnett

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Posted 27 August 2014 - 09:59 AM

Hi Ed,

I love the FR/FF in my C6. Much wider FOV. No impact whatsoever on image quality IMHO. I believe SCT should be sold with one as standard equipment.  As for flocking the baffle tube (why are we revisiting this in a FR/FF thread?); I cannot appreciate any problems with my C6 that would cause me to open up the tube and put anything in the baffle and potentially introduce dust, etc into the OTA. Just my  .02

-Rob

You don't need to open the tube to install baffle flocking.  You cut it to length, roll it up, slide it in through the rear port and let it unroll and expand to rest against the interior sides of the baffle.  It's easy and easily reversible.  It also quells the hooped glare seen in the eyepiece when a bright target is place far off axis.

 

- Jim 



#25 desertlens

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Posted 28 August 2014 - 01:37 AM

OK, some initial impressions. On the whole the FR/FF works well and as advertised. The most dramatic impact is of course the change in magnification (FL down from 1500mm to 945mm, nominal) and the resulting larger exit pupil and TFoV. Sharpness and contrast are quite good and I see no ghosting or scatter introduced by the 4 extra elements. FC correction is modest so that problem is improved but not entirely eliminated. This was a quick test of about an hour using a 25mm TV Plössl and a 25mm UO VT. Targets were M8, M22, the Double Cluster and eta-Cas. Seeing was my usual 3 of 5 and transparency was excellent. As expected, the first 2 were brighter due to exit pupil. I worked without filtration on M8 but look forward to using the extra brightness with a UHC. The DC was chosen to get a good selection of stars across the field. Focus was good out to about 75% of the field radius where FC and coma began to have some impact. Focusing was done with a star at about 33% of the radius (from center). Eta-Cas was easily split at this mag (~38x, 1.3º TFoV, nominal) and moved around in the field to confirm what I was seeing. I'm pleased with what appears to be a respectable dual FL capability with the C6.








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