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C8 - Actual focal length w/2" SCT style mirror diagonal...

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#26 Dave Bush

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Posted 13 October 2014 - 07:47 PM

It's the second of your guesses. While the focal length of the basic scope is increasing, the additional spacing from reducer to focal plane is increasing the reduction factor of the reducer. The nominal .63 factor occurs with the 100mm spacing of a 1.25" diagonal. The SCT vignetting report shows an overall focal length of 1150 for 125mm spacing, for a reduction factor of .54 assuming 2125 focal length for 125mm spacing with no reducer. For 150mm spacing the factor is 1050/2200 or .48.

 

Mike

 

Mike, 

 

Where is this SCT Vignetting report?

 

The example you give seems to say that for a 125mm spacing, the focal length is 2125.  That contradicts what Edggie said earlier, namely that a at 125mm it's 2200mm.  



#27 hudson_yak

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Posted 13 October 2014 - 08:00 PM

Ed went back to correct that post, looks like.

 

The SCT vignetting report was originally made available in the files section of the Yahoo sct-user group in 2007, written by Ken Hutchinson. It's still there, and is really excellent. I think Ed has been sending it around to people too.

 

Mike



#28 Dave Bush

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Posted 13 October 2014 - 08:20 PM

Ed=Eddgie?

 

If so, in his post it still says...

 

"In the chart, the graph shows the focal lenght at 125mm to be right at about 2100mm"

 

Am I missing something?



#29 hudson_yak

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Posted 13 October 2014 - 08:28 PM

You cited him saying 2200 just two posts ago, so, yes, I guess so...

 

Note that in Ken's report the C8 numbers were not measured directly, they were taken from a book and may not exactly match every C8 made. There's a slight (very slight) difference in his chart relative to the one posted at the top of this topic, for example. Don't know where this chart came from. In any case the difference between 2100 and 2125 is not much, in practical terms.

 

 

Mike


Edited by hudson_yak, 13 October 2014 - 08:39 PM.


#30 Dave Bush

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Posted 13 October 2014 - 09:32 PM

Oh duh. I see now. I'm tired.

Ok so these are good numbers to use.

Thanks again to all. However.....

I do have another question.

It seems from the info Ed gave earlier that for optimal performance out of my C8 I should use a 1.25" prism diagonal. Am I reading that right?

#31 Jan Owen

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Posted 13 October 2014 - 09:35 PM

Another issue that is present here makes it hard to specify the light path length of an SCT diagonal, because there are several different configurations. This one is shorter than most.

IMG_4488a (1708 x 961)CN.jpg

But the Baader Click-Lock diagonal will screw directly onto the rear thread of either the telescope or the reducer/corrector, for an even SHORTER light path... The Baader diagonal provides the shortest 2" light path that I know of...

http://agenaastro.co...nal-to-sct.html

#32 hudson_yak

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Posted 13 October 2014 - 10:56 PM

It seems from the info Ed gave earlier that for optimal performance out of my C8 I should use a 1.25" prism diagonal. Am I reading that right?

 

I think "should" is too strong a word for what Ed was suggesting as a possibility.

 

Personally, I lost interest in the prism diagonal that came with my SCT early on because I saw additional stray reflections dancing around the field when looking at planets. Perhaps one with better coatings would avoid that issue, though I'm happy with my 1.25" TV Everbright in any case. Around here the atmospheric conditions are always by far the biggest influence on what I see from night to night.

 

Mike



#33 Eddgie

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Posted 14 October 2014 - 06:40 AM

You cited him saying 2200 just two posts ago, so, yes, I guess so...

 

Note that in Ken's report the C8 numbers were not measured directly, they were taken from a book and may not exactly match every C8 made. There's a slight (very slight) difference in his chart relative to the one posted at the top of this topic, for example. Don't know where this chart came from. In any case the difference between 2100 and 2125 is not much, in practical terms.

 

 

Mike

 

It was my mistake because initially I did say 2200mm, but it was a typo, and I corrected it.  I deeply apologize fo the error.

 

The table above was done by one of the forum members and the blue dots represent actual measurements he made.   And as the graph shows, it is in pretty good agreement with the chart that Ken Hutchinson made, and this is because the forumla for focal lenght is little affected by apeture. 

 

In all of the f/10 SCTs, 1mm of back focus increases the focal lenght about 3.1mm.

 

Now keep in mind that while the increase is constant and not a function of apeture, what is a function of apeture is the percentage of focal lenght increase.  In other words, while 100mm of back focus will add about 310mm of focal lenght, if you start with a C8, that representes a much larger increase in the percentage of focal ratio increase. 

 

For example, a 300mm increse in a C6 representes a 20% increase in focal lenght  (1800 divided by 1500).  On the other hand, that same 300mm focal lenght increase in a C11 would result in only an 11% increase in focal lenght.   So, while the back focus changes the focal lenght by the same number of millimeters, the bigger the apeture, the smaller the percentage change that this 300mm makes.


Edited by Eddgie, 14 October 2014 - 06:53 AM.


#34 Eddgie

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Posted 14 October 2014 - 06:52 AM

Oh duh. I see now. I'm tired.

Ok so these are good numbers to use.

Thanks again to all. However.....

I do have another question.

It seems from the info Ed gave earlier that for optimal performance out of my C8 I should use a 1.25" prism diagonal. Am I reading that right?

 

If your SCT has no sperhical aberration with a 1.25" prism diagonal, then the best performance at the center of the field will be when you use a 1.25" diagonal (though you can do much better than the factory 1.25" diagonal).

 

But the best performance at the center of the field would come at the expense of being limited to a 1.25" format, and this means narrow fields of view.

 

The SCT diagonal you mentioned does in fact provide one of the shorter light path lenghts for a 2" diagonal, and if your system has perfect SA correction with a 1.25" prism diagonal, then this 2" would keep you closer to perfect than a 2" refractor style diagonal with a long visual back. 

 

And the change in SA would not be enough to matter.

 

My point was simply that if the system starts with excellent SA correction as it was shipped, then the further you get away from the  light path lenght of the 1.25" visual back and 1.25" prism diagonal, the more SA is induced.   A little SA makes no difference, but get much longer than a standard 2" SCT diagonal and the SA will increase enough that the scope my no longer be able to match a good 5" APO in performance (which an excellent C5 should be able to do).

 

If you want your cake and want to be able to eat it too, the shorest possible light path lengh for the C8 is going to be with the Baader 2" Clicklock diagonal.

This diagonal body thread directly on to the C8 rear port.   It has a light path lenght that is almost the same as the 1.25" prism and standard visual back.

 

This keeps the focal lenght very close to optimal, and if the system started with zero SA, will keep the SA very close to ideal.

 

But again, the difference between the 1.25" digaonal and SCT 2" diagonal is very small, and most people would not see it.

 

When excellent C8s loose out to 5" APOs though, there has to be a reason. 

 

To really know the truth though, you need to test the system before you do anything.  If the optics are perfect with the 1.25" diagonal, then increaseing back focus lowers performance.  Only a tiny bit, but lower is lower.  If on the other hand, there is some undercorrection, adding a 2" diagonal might correct it. 

 

Here is the Baader 2".  For a system that comes out of the box with perfect SA correction using the 1.25" factory prism, this is the 2" diagonal that will keep it closest to perfect and still give you 2" capability.

It will also give you the widest, brightest field possible with a 2" eyepeice becasue the focal lengh will be shorter.

 

http://agenaastro.co...nal-to-sct.html



#35 Eddgie

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Posted 14 October 2014 - 07:01 AM

And again, the chart you see earlier show the plotted line based on the forumla, while the blue diamonds represent the actual measurements someone made from their C8.  Notice that they are very close to the values for the formula.   Even different eyepieces can change the focal lenght, and as I recall, the dots that are clustered togetther represented the difference in focal lenght that resulted just from changing eyepeices.

 

I think it my have been Edz that did these measurments, but they are in almost perfect agreement with the formual for focal lenght in crease, which is in almost perfect agreement with Ted Hutchinson's paper.   

 

If the scope starts with an f/10 primary and -f/2 secondary, the slope will be about about 3.1mm of focal lenght increase for every millimter of back focus.

 

This is why the focal lenght will be shorter for a Televue 40mm Plossl (field stop at bottom of barrel) than it will be for a 25mm Plossl (field stop at the top of the barrel).  That 20mm difference in field stop location changes the focal lenght by 63mm.



#36 Dave Bush

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Posted 15 October 2014 - 02:46 PM

You know, re-reading this thread it just occured to me.   Many years ago when I had my first SCT (Super C8+) I was told that the Celestron Ultima eyepieces (pseudo-Masuyama style) were a near perfect match for it.   And now I think I know why.

 

Using a high quality 1.25" diagonal I'm at the optimal spot (100mm) and with the relatively small apparent FOV that the Ultima's have (51 degrees), SA is almost not there.

 

And that bares out with my experiece.  At f/10 with the Ultimas, I get sharp, in-focus to the edge images.

 

Hmmm....  maybe I should just accept that my C8 isn't gonna give me wide fields of view and enjoy the sharp views it does give.

 

Thanks again all!



#37 Jan Owen

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Posted 15 October 2014 - 03:21 PM

You might want to consider some experimenting before giving up your quest...

Here's two approaches. Visually, I think you'd like what you see in either setup(especially with 8" instead of the 6" shown here)... I can use this setup with either my 6" or 10", and visually, the fields do get significantly wider, while the stars remain crisp to the field stop in both configurations.

Of course there's vignetting in the version with the 2" diagonal & 20 mm T2 Nagler, but you can't see it visually... And in the 1.25" configuration, there should be NO vignetting, and the focal reducer is working with the design-prescribed 105 mm back focus, versus about 118 mm in the 2" Antares version, which is about 13 mm too long (but visually, again, the views look great - and you can get an even shorter light path by substituting a Baader Clicklock 2" mirror diagonal in place of the short, but not quite AS short, Antares unit shown)...

IMG_6315 (1090 x 818)CN.jpg

Attached Thumbnails

  • IMG_6587a (929 x 631)CN.jpg

Edited by Jan Owen, 15 October 2014 - 05:36 PM.


#38 Bill Barlow

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Posted 15 October 2014 - 08:31 PM

Would using a 2" visual back adapter like the AP one that attaches to the C8 that is about 2.15" long negatively affect the SA to the point of degrading the planetary images when compared to using the Celestron 1.25" VB and 1.25" eyepieces?  The rear cell opening on the C8 is somewhere around 38mm I believe.

 

bill 



#39 Jan Owen

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Posted 15 October 2014 - 09:28 PM

The following photo is of a 2" SCT visual back that's about 2.1" long, or quite similar to the A-P one in length... As you can see, it really pushes the diagonal back a considerable distance compared to the SCT setups in the earlier images I've posted above... This will result in a change in effective focal length, and may impact the spherical correction, though I haven't done any testing on this configuration, other than to photograph it, because it's far longer than I was willing to use...

IMG_5408az (1408 x 792)cn.jpg

Edited by Jan Owen, 15 October 2014 - 09:29 PM.


#40 Jan Owen

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Posted 15 October 2014 - 09:54 PM

And to close the circle, here's an image of a more or less standard SCT type 2" star diagonal, so you can get a better idea all in one place how the different units stack up, from a light path length standpoint... Too bad I don't have a Baader 2" mirror Clicklock unit (yet...)...

IMG_5412az (1408 x 792).jpg

#41 Dave Bush

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Posted 15 October 2014 - 10:04 PM

Jan, 

 

Thank you for the illustrated options.   My SCT style diagonal (2") is shorter than the one in your last photo.  So at 125mm of backfocus vs. 100mm of a 1.25" diagonal, the effect on SA would be small enough that I don't think I'll see it.  Thus, no need to spend the considerable money for the Baader just to get a few millimeters shorter. 



#42 Jan Owen

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Posted 15 October 2014 - 10:40 PM

That's pretty much why I've stopped where I am now, too... Enjoy!!!

#43 Bill Barlow

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Posted 16 October 2014 - 09:03 AM

Thanks for posting these pictures, Jan of the different 2" visual backs.  Seems I could just use  a SCT diagonal that screws directly onto the rear cell to get a very short light path.  But then I couldn't use my AP Maxbright with the C8.  I see TV sells a short 2" SCT VB but it only has one locking screw.  Would prefer two screws to get a stronger grip. 

 

Bill



#44 Dunkstar

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Posted 21 October 2014 - 08:21 AM

The Baader Clicklock diagonal + locking ring is a really nice solution for alt az observing. Keeps the path very short, over an inch shorter than the supposed SCT diagonal I had before, and feels very secure.



#45 Jeff B

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Posted 21 October 2014 - 02:43 PM

 

It would be interesting and useful, IMO, to plot effective clear aperture on the same graph using the y-axis on the right hand side. 



#46 RandyC

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Posted 18 September 2015 - 09:43 AM

So if I increase the "Distance from Backport", I increase my magnification. You mean I don't need a barlow for DSOs? Thank you for the information. It's surprising it hasn't come up as a means to increase detail and magnification at existing aperture.



#47 junomike

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Posted 18 September 2015 - 11:17 AM

So if I increase the "Distance from Backport", I increase my magnification. You mean I don't need a barlow for DSOs? Thank you for the information. It's surprising it hasn't come up as a means to increase detail and magnification at existing aperture.

This is true to an extent but  the added Focal length is usually not more than 300mm.

A Barlow can still be required if one's desire is to double the F/L.

 

Mike




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