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#1 dpippel

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Posted 15 October 2013 - 06:09 PM

Well, it's not like I needed another scope or was even shopping, but this 6SE just fell into my lap and I couldn't resist.Bought it for a song from a 70 year old retiree who found it too cumbersome to move around. He even gave me the original shipping boxes. It's been used 3 times and is in brand-new condition. First light tonight...

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#2 labmand

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Posted 15 October 2013 - 07:14 PM

Congrats, looking good! Hoping you have clear skies, enjoy

#3 dpippel

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Posted 15 October 2013 - 07:39 PM

Thanks Dave. Now that the monsoon has blown out for the year, skies in the Phoenix area HAVE been clear. Tonight is no exception. I'm looking forward to getting out with the new acquisition. :)

#4 dpippel

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Posted 16 October 2013 - 12:17 PM

I was able to get out with the C6 for about an hour last night and am duly impressed. This is a GREAT little scope and a great package with the mount. Even with poor seeing the views were very nice indeed. I did have some trouble getting Sky Safari to control the mount properly via my iPad and Skyfi, but that can wait. I ordered a 2" visual back for it so I can use either one of my good 2" diagonals, which have got to be superior to the cheap 1.25" that Celestron includes with this scope.

#5 dpippel

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Posted 19 October 2013 - 11:44 AM

My 2" Blue Fireball VB arrived yesterday and I was able to get out with it last night for a test drive on the 6SE. Works like a charm!! I did a variety of observing, from doubles to globulars to open clusters to planetaries and the Moon. I used a 2" GSO diagonal and my eyepiece selection consisted of an 1.25" ES 68* 24mm, a 1.25" Tele Vue Type 6 13mm, and a 2" Baader Hyperion 31mm. With the stock VB/diagonal the distance from the OTA backplane to the center of the eyepiece is 73mm. With the 2" Blue Fireball VB and 2" GSO diagonal that distance measures 89mm, and increase of 16mm or .63 inches. I was looking very carefully for vignetting but didn't see anything noticeable, even with the 2" 31mm.

The Blue Fireball is very well made from quality materials and the dual compression ring makes it easy to rotate the diagonal when needed. IMO this is a HUGE improvement over the stock Celestron 1.25" VB and diagonal that ships with the 6SE. A great $35 investment and a very nice piece of kit...

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#6 Eddgie

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Posted 19 October 2013 - 01:43 PM

How did you come up with those figures for light path length.

I have never seen a 2" diagonal with less than about 90mm by itself, and most 2" visual backs add at least 30mm, and often as much as 40mm.

Even with a Televue Short SCT to 3" (30mm on my C5), with my Televue 2", total light path is over 130mm.

So, not at all sure how you could get 40mm less than that.

Most 1.25" diagonals though are about 65mm to 70mm, and the standard 1.25" visual back is 35mm, so that would be right at 100mm, and that is the design spec for the C5, C6, and C8.

Anyway, my C5 with the 30mm light path Short SCT adapter is (30 + 97 for the Televue 1.25" dielectric) 127mm.

The Blue Fireball looks to me to be longer than the Televue SCT to 2" short visual back, so just curious how the light path can be so short for your configuration.

Again, I am using a Televue Short 2" diagonal with a light from the front of the mirror box to the top of the eyepiece holder light path of 97mm.

The Short Adapter is 30mm. Total is 127mm or 27mm more than the 1.25" diagonal. That is about 100mm of focal length change.

#7 Eddgie

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Posted 19 October 2013 - 01:53 PM

Actually, I just re-measured my Televue dielectric.

I get 32mm from front to center of mirror, 32 form center of mirror to top of box, and 38mm from the top of the box to the top of the eyepiece holder. Total is 102mm just for the diagonal.

I checked the path of the Televue short adapter and was 30mm exactly and it was 27mm.

This is a total of 129mm.

Just curious how you made your measurements, but I don't think it is possible to get a light path of less than about 120mm in an SCT (this is what the standard SCT 2" diagonal has).

Your adapter looks like one I had that had 35mm of path length, so I would estimate your total path to be closer to 130mm.

#8 dpippel

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Posted 19 October 2013 - 02:01 PM

Eddgie - As I mentioned in my post, I measured from the backplane of the C6 mirror cell, where the VB threads exit the cell. I didn't say I was measuring light path length. ;) I was looking for the ballpark difference in length from the back of the mirror cell to the center plane of the eyepiece using the two different VB/diagonal combos. So in essence the new 2" VB + 2" GSO diagonal adds 16mm to the light path length over the stock configuration.

#9 Eddgie

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Posted 19 October 2013 - 02:04 PM

Ah, I think I know what you did. Did you measure from the back of the scope to the vertical line of the centerline of the eyepiece? That is not the way to do it.


In my scope, I measure 71.6mm if I do that.

My visual back is 27mm, and if yours is 38mm (which I think is how much the adapter I had that looked like yours was) then that would be about 11mm further back than mine, and that would be.. Drum roll please.. 88mm.

But that is not the light path length to the focal plane.

#10 dpippel

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Posted 19 October 2013 - 02:06 PM

Yes, that's what I did. I was looking for the amount of added length, not the total length of the path.

#11 Eddgie

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Posted 19 October 2013 - 02:09 PM

Ah, so you were only concerned about clearance and not light path length?

Otherwise, the light path length has gone up to about 140mm, and your focal length is increased 120mm over the 1.25" visual back.

But that is clearly not what your concern was, so forgive me. I thought you meant that you were only increasing the light path by a few millimeters.

For some of us, light path length is very important, and I thought that was your concern because you were measuring it.

Pardon me for butting in.

#12 dpippel

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Posted 19 October 2013 - 02:28 PM

No please, I'm glad that you did! Maybe my understanding here is faulty. My math skills are admittedly weak. :)

I'm definitely concerned about light path length as well as clearance, and I'm not understanding how my focal length has increased by 120mm. Since I measured from the point that the light path exits the OTA, and that light is only hitting the single reflective surface in the diagonal before exiting the eyepiece, how can that be? It seems to me that the distance the light travels from the hole in the mirror cell to the surface of the mirror in the diagonal has gone from 73mm with the stock configuration to 89mm with the 2" setup, which is only a total increase of only 16mm. How are you calculating 120mm? I'm a bit confused.

#13 dpippel

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Posted 19 October 2013 - 02:37 PM

Well, I just found this:

http://www.cloudynig...hp?item_id=2410

Time for some reading me thinks. :)

#14 Midnight Dan

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Posted 20 October 2013 - 07:55 AM

Well, it's not like I needed another scope ...


Blasphemy! :poke: :grin:

Hi dpippel and congratulations on the new scope! Looks like you got a winner. Yes, as Eddgie points out, moving the focal plane on an SCT does indeed have a significant effect on the focal length of the scope.

On the other hand, for many of us, it's not that big a concern. Most folks do use an SCT-threaded diagonal which reduces clearance issues as well as minimizing increases in the length of the light path. But if I already had a refractor-style diagonal, I'd probably do the same as you and get an inexpensive visual back rather than buy another diagonal.

-Dan

#15 azure1961p

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Posted 20 October 2013 - 08:33 AM

I've had mine about two years now and like a great scope it continues to please repeatedly - I never get used to great views through a superb set of optics . Last mars swing around I was getting sharp 350x views . A caveat - the GOTO is very accurate and the tracking for visual as well as lunar and planetary imaging is good - BUT - make sure you level the tripod well before you start a session. I got lazy and sometimes eyeballed the tripod to what seemed level - and it kinda was . Trouble is the tracking now was kinda compromised and the locating ability was kinda off. Always take a few moments with a decent (magnetic) bubble level to make that thing FLAT. Too, pick alignment stars that have a good spread across the sky. I personally find the one target align like on a planet or moon is rather lousy tracking. Always go the distance with the non-shortcut alignment methods.

This system is a very very good doublestar instrument as well. I still have a hard time reconciling the 37% obstruction giving such great views on a number of demanding objects but it does so frequently seeing willing.

Glad you joined the club.


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#16 azure1961p

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Posted 20 October 2013 - 08:45 AM

Well, it's not like I needed another scope ...


Blasphemy! :poke: :grin:


I kind of wonder if Im finally all set with scopes. I still - annually - think about a 12-16" newt but practicality always pulls me back. More inclined to accessorize than go the distance on another instrument which isn't a bad thing. I'm almost never seek g my diffraction limit in the 8" all winter and even in the summer its often very good but rarely excellent to the point a 16" would show a Dawes split . This is more comment than your humorous line but it made me think about where I an with scopes. I'm mostly lunar, planetary and doublestar with some deepsky. Ehhh. The cyclical want , rationalize - dismiss rises now and again though - usually in fall . Part if what puts me off is the possibility of big junk optics. Mind you Im not thinking Celestron in that vein.

Just mulling out loud Dan.

Pete

#17 dpippel

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Posted 20 October 2013 - 11:37 AM

Yeah, I don't know what I was thinking when I made that comment about not needing another scope!

:roflmao:

Anyway, I can see why you all like this little beast so much. I was out with it for a few hours again last night and got the collimation dialed in. It was quite a bit off as received, and after I got it tweaked star images tightened up very nicely indeed, particularly on-axis. I spent some time with a few doubles and Albireo was an absolute jewel in the eyepiece, as was Almach. This little scope continues to impress and really does seem to be something of a sweet spot in the aperture/portability/cost department. The accuracy of the little NexStar mounts go-tos are also commendable. It's usually almost dead-on. I just love the fact that I can pick this whole thing up, walk out to the backyard with it, spend 5-10 minutes leveling and aligning, and be good to go. Great stuff!

As for the VB and diagonal thing, well, I realized last night that I've been thinking *GEM* all along here. One of the reasons I went with a VB and refractor-style diagonal (other than already having the diagonal that is) was to make it easy to rotate the diagonal if need be to put the eyepiece at a comfortable viewing angle. With my C11/CGEM DX this is a huge concern. However, with an OTA this small and short on an alt-az mount, the need to do that is practically non-existent. I may have to look into a SCT threaded diagonal after all... :grin:

#18 Eddgie

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Posted 20 October 2013 - 05:21 PM

The back focus of an SCT is the difference from the flat at the back of the treaded SCT port itself to the focal plane.

The focal plane will be at the field stop of the eyepiece when the eyepiece is in focus.

The back focus is measured from the rear cell and follows the axial center line.

The diagonal bends this line that would normally extend straight back so that it can reach the eyepiece.

To measure light path, you would measure from the flat at the rear of the SCT port to the rear of the visual back.

In your scope, I am guessing this is 38mm because I had an adapter like yours and that it what it measured.

Next, you have to measure the light path though the diagonal.

This takes three different measurements.

The first is to measure along the side of the mirror box. Measure from the front edge to where you think the front of the mirror is at its center. For diagonals with a back, the mirror face is usually at the seam where the back screws in. You would measure to the center of the seam lengthwise from what you think would be the centerine of the nose.

The, you measure from the center of the mirror to the top of the mirror box.

Last measurement is from the top of the mirror box to the top of the eyepiece holder.

For your setup, this is going to be about 30mm from the front to center, and 30mm from the center to the top of the box. My guess is that the eyepiece holder will be about 35mm to 38mm.

This is the length of the back focus.

There are to different aspects of excessive back focus.

The first to increase the focal length but about 30mm for every additional 10mm of back focus you add.

If your scopes was 1600mm with a 1.25" diagonal, it will be about 1720mm now.

The other effect it to possibly increase the spherical aberration of the system.

The SCT is designed to have one exact mirror spacing for perfect spherical aberration correction.

If you add back focus, you move the primary mirror further forward, and this means that the spacing to the corrector (which is what adds the negative spherical aberration to cancel the SA of the spherical mirror) is changed, so the system SA changes. If the SA is perfect with the system at 100mm of back focus, it will become overcorrected as you ad back focus.

The question of course is "How much does it change?"

The answer is that if you only increase the light path a little bit, it only changes a little bit.

40mm Of light path extension is not going to make enough difference to be seen.

But the more you add, the more SA is induced. For this reason, it is best to avoid long light paths for high resolution (planetary) observing.

How can you tell if SA has been induced?

Put on the 1.25" visual back and the 1.25" diagonal (which may look cheap but optically is likely more than decent).

Put in a 10mm or 12mm eyepiece. Does not matter that it is not exactly this length. Anything close is good.

Focus the scope at infinity.

Slip the eyepiece out 4.5mm (does not have to be exact, but if you can make it exact, then hey, why not).

Look at the size of the secondary shadow in relation to the overall defocused pattern. It should be about 34% of the diameter of the entire pattern.

Now, without touching the eyepiece, go to best focus.

Now, without touching the focuser, loosen the eyepiece and slip it all the way in (which will give you the exact same distance inside of focus).

Once again, you are looking at the ratio of the secondary shadow to the Fresnel pattern, and in a scope that is well corrected for SA, it will look pretty similar, though it may not be perfect. The test is very sensitive.

Now, put the 2" diagonal in and repeat the test.

If the patterns on either side retain their ratios, then there is not enough SA to worry about.


If on the other hand, you see that they have become imbalanced, then this is an indication that some SA has been introduced into the system.

It should not be much though, if you can even see it at all, because 140mm of back focus (vs 100mm) is just not that much different.

When the light path gets to about 200mm though, the SA can become more pronounced.

For you this is not happening, but for people that use things like Crayford focusers and 2" diagonals, it can be a bit more pronounced.

And for people that use a 6.3 focal reducer and a 2" diagonal, it can be very severe.

So, my bet (if you care to measure it), is that your light path will be 138mm to 140mm.

Don't let this scope you from enjoying it.

My advice is to keep the back focus as short as possible, but that still allows you to do what you want to do.

If you want to use 2" eyepieces, try to use short adatpers (Televue Short SCT to 2" and short nose diagonal, or true SCT diagonal) if you can afford it.

Avoid things like external focusers, unless you feel that the utility is simply far more valuable to you than the 50mm to 90mm of focal length that they add, and the SA that is incurred is still small enough that you don't think it matters.

And to get the most out of an SCT, try to use it as close to the recommended back focus as possible. For the C5, C6, C8, C9.25, and C11, this figure is about 100mm.

For the C14, this figure is about 150mm.

For the EdgeHDs, it varies by model (133mm for the EdgeHD 8" which makes the Edge HD 2125mm of focal length by the way, rather than the stated 2030mm).

My goal here is not to make you feel like you are making a mistake. Use what you enjoy. Rather, my intent is only to make you aware of the various characteristics of how the SCT design functions in different configurations.

#19 dpippel

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Posted 20 October 2013 - 06:00 PM

Thanks VERY much for the detailed information Eddgie! You've explained things thoroughly and clearly, and I really appreciate the time you took to do so. I believe that I get it
now.

:bow:

One of the things I really love about this hobby, and about Cloudy Nights in particular, is how willing most people are to share their knowledge and experience with others.

#20 UncleMossy

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Posted 22 October 2013 - 04:44 PM

I did have some trouble getting Sky Safari to control the mount properly via my iPad and Skyfi, but that can wait.


If I remember correctly this could be the issue.

I had to select Nexstar GPS from the scope list to gain control of my scope, not Nexstar 6/8.
The iPad works as the gps and sends the location.
If you can connect to skyfi but not your scope then this should fix things, hopefully.
I am assuming that you are plugged into the hand controller and not the aux port on the base.

Hope this helps.
I wouldn't see a quarter of the stuff without SkySafari and SkyFi.

Cheers
Jim

#21 dpippel

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Posted 22 October 2013 - 04:57 PM

I did have some trouble getting Sky Safari to control the mount properly via my iPad and Skyfi, but that can wait.


If I remember correctly this could be the issue.

I had to select Nexstar GPS from the scope list to gain control of my scope, not Nexstar 6/8.
The iPad works as the gps and sends the location.
If you can connect to skyfi but not your scope then this should fix things, hopefully.
I am assuming that you are plugged into the hand controller and not the aux port on the base.


Thanks for the tip Jim, but that's the mount I've selected - "Celestron NexStar GPS/SLT/SE". Yes, I'm plugged into the controller. I use the SkyFi/iPad/Sky Safari combo to control my CGEM DX all the time and am familiar with its operation.

The problem I'm having is VERY strange too. Once I level the mount and go through a SkyAlign, I connect to the scope via SkyFi. Everything is just dandy unless I try to slew to something in the *southern* sky. For example if I use Almach, Vega, and Altair as my three alignment objects and then try to slew to Venus or the Lagoon Nebula, the scope slews NORTH towards Polaris. I can go to practically any star around zenith or north of the celestial equator, but if I select Fomalhut, for example, she slews north again. WEIRD! I'm selecting Phoenix, AZ as my location from the controller database, so the software knows where I am on Earth. If I use the hand controller to select these same objects the gotos are just fine, so I suspect the problem is some setting in Sky Safari.

One evening when I have the time to sit down and really dig into it I'll find out what's going on and report back here.






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