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Final apeture reduction measurment for EdgeHD 8"

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#1 Eddgie

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Posted 11 October 2013 - 06:03 PM

If you have been following my previous posts, you know that I have discovered that the EdgeHD 8" suffers severe aperture reduction if you use to long a light path.

The first time I used binoviewers on the EdgeHD 8", I was appalled at how dim the view was.

I quickly realized that the EdgeHD 8" has a more restrictive back focus than the standard C8.

this was with a 2" diagonal based system, and the aperture was reduced to about 6.2 inches or so.

The first change I made was to go to the Maxbright/T2 Prism, and I noticed a tremendous improvement but with the Baader Clicklock visual back, I was still working at less than 7.6 inches.

I then went to the Mark V and was under 7.5 inches.

This might not seem like much of a big deal, but it constitutes a considerable brightness falloff. Do the math and see for yourself.

Anyway, I then went to a Televue Short visual back, and this got me back over 7.5". to about 7.6" actually.

Next to the Baader SCT to T2 15mm Visual back, and I picked up a little over .1 inch so 7.75 or so.

Last configuration. It doesn't get shorter than this except by going to the Maxbrights, but I wanted the wider true field of the Mark V.

I an now using the Baader 10mm SCT to T2 thread, the T2 Prism, and the Mark v.

I just got around to measuring it today, and here is the final number.

7.85".


I even moved to a NexStar 8SE mount so that I did not have to worry about rotating the diagonal because the 10mm adapter really has only one position and can't be easily rotated.

This is as good as it gets then on the EdgeHD 8" in a large prism.

My estimation is that the 10mm/T2 Prism/Maxbright will be the only binoviewer that will work at full aperture (maybe a millimeter or two short).

Otherwise, 7.85" is about as good as it gets with a Mark V, and no other binoviewer out there will beat the Mark V except the Maxbright, so if working at or near full aperture is important and you don't have the money for a Mark V, I recommend the Maxbright/T2 for the EdgeHD 8".

C5 is going to be about the same. Have not measured it with the 10mm back yet, but I have another T2 Prism on order for that one and will check it when I have it.

I figured it is easier to use the Quick Connect to move the Mark V back and forth than moving the diagonals, so I am putting Quick connect rings on my two SCTs and my refractor so I can easily move the BVs back and forth.

Hope someone has found this info useful.

#2 Hothersale

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Posted 11 October 2013 - 06:10 PM

Do you think you would get 100% aperture with the Maxbright bino, Ed?

#3 BWAZ

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Posted 11 October 2013 - 06:25 PM

Ed, do you use glasspath in all your configurations? I believe the 1.2x GP may save you at least 25mm back-focus, which would definitely alleviate the aperture loss, of coz at the cost of slight true FOV

#4 Eddgie

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Posted 11 October 2013 - 07:10 PM

The Maxbright with the 10mm connector and T2 Prism should come in right at or very very slightly below full aperture in the EdgeHD 8", but it is the only configuration that will do this.

#5 Eddgie

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Posted 11 October 2013 - 07:21 PM

7.85 is enough that it would be impossible to tell the difference on planets, buf I were wanting to do serious planetary, I would put in the 1.25" GPC and I am confident that this would get me full aperture with the Mark V.

I really have only been talking about the aperture reduction, but the focal length is much shorter now than when I started too.

My initial shock the first time I BVed the EdgeHd was I am sure very strongly compounded by the huge increase in focal length.

In the current configuration, I would estimate the focal length to be about 150mm more than the spec for the EdgeHD 8" says.

Spec says that the focal length for 133mm of back focus is 2125mm. Every 10mm of additional back focus will ad about 31mm of focal length, so I estimate that current focal length with the 10mm/T2/Mark V is about 171mm, or 38mm more than spec, which works out to an additional 120mm of focal length so the total for the EdgeHD 8" in this configuration is about 2250mm.

From all of this, I have become far more anal about back focus in SCTs. It is a real negative and people with SCTs should make every effort to trim every millimeter possible off of their light path even in Mono-vision.

If I ever planned to use the EdgeHD 8" with 2" eyepeices, I would get the baader Cliclock that allows you go screw the mirror box directly to the rear port. I am convinced that this is the best possible diagonal for the EdgeHD 8" and all of the SCTs with 38mm rear threads.

But I don't intend to use the EdgeHD 8" with anything but binoviewers, so not and issue for me.

And this solidifies my position that no other diagonal should be used for binoviewing with the EdgeHD 8". There is no other configuration made that will work at full aperture, and every 10mm or so costs 2.5mm of aperture.

Again, people will think I am being crazy, but do the math for light collection and you get to 20% light loss pretty quickly..

#6 REC

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Posted 12 October 2013 - 08:36 AM

Wow, that is scary...6.2" on the HD! I have been following your posts on all these discussions regarding back focus and aperture loss in SCT scopes. There has been a few solutions posted and if I had 20/20 hindsight I would have bought the Maxbrights for my three types of scopes I'm using. Seems their OCS adapters are better overall than other fixes.

Right now I'm using my WO BV in my refreactors and the Denk2 PS in my 10" Dob and my Meade LS8 scope.

So Ed, do you think I'm getting a similar amount of aperture reduction in my SCT to? The scope is not the traditional design, but the ACF method? I think they use a different secondary mirror to reduce the coma in SCT scopes. Would that be similar to the C HD version scopes?

I'm using a Meade OTA that is 32mm long, then the Meade 2" dialectric diagonal and then insert the Denk PS BV. If I am getting well below 7" of aperture, I definitely want to fix that to get it back into the mid sevens inches.

Bob

#7 Eddgie

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Posted 12 October 2013 - 11:37 AM

I cannot say. I have ray traces or have measured many Celstron f/10 scopes and the C14, and all have lost apeture with back focus starting at between 160mm (EdgeHD 8" and C5) and up to 200mm (C14).

Your scope though is f/8. My bet is that whenever you cross the threshold of apeture reduction, you are loosing more apterture per millimeter of back focus increase than an f/10 scope because of the steeper light cone.

For examlple, if th scope is using a focal reducer, the aperture loss is far more agressive for each 10mm of back focus than at f/10 and there is not a standard SCT that I am aware of that will work withoug loosing aperture wtih a focal reducer past about 100mm of back focus.

But the only way to know (and this is not the first time I have suggested this to you) is to measure it.

Also, jsut look through the darn thing in mono and compare it to bino. If the view does not seem so much dimmer as to make you feel like you are loosing to much brightness, then ignore it.

Different people will of course have different threasholds. I can see a 20% transmission loss with no difficulty and don't want to accept it.

Some people might be happy with a 40% transmission loss.

Only you can say what is acceptable.

If it it doesn't bother your enjoyment of the scope, ignore me because what I am saying is not important.

If you fell like it could be better though, then measure the apeture and calculate how much you can gain by going to a shoreter light path. You may find that you are not willing to make enough compromise to make it worth it.

#8 HowardK

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Posted 13 October 2013 - 07:01 AM

Edd...

Measured aperture at 178mm with

8" Edge
Baader 2" cliklok visual back
Denk 2" WO diagonal with filter switch and power switch attached
MK V binos attached to the above with Baader T2 quickchange adapter
Power switch in reduction mode (.7x reduction approx)

Will try soon with straight thru mode on the power switch.

So am operating at approx 7" aperture in this configuration.

Seriously thinking of going the BAADER prism route and losing the 2" diagonal with power switch setup....
But...do love the convenience of the power switch

What to do....i guess i will test with my eyes on some doubles, the moon and jupiter to see if the loss of 1" aperture really makes a difference to me.

Any comments?

#9 REC

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Posted 13 October 2013 - 08:57 AM

Howard, that's just about what my configuration is with my 8" SCT, so it looks like I'm about 7" as well.

Ed, BTW the scope is an f/10 not f/8 in this ACF design. I think you once told me that with my 2" backend configuration I was working at something around f/13 now.

Bob

#10 Eddgie

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Posted 13 October 2013 - 09:51 AM

Is your setup the WO diagonl with a dovetail that allows you to attach the Powerswitch directly to the top of the diagonal?

Every 10mm of back focus you can cut out will buy you about .1" of apeture.

Now restoring .5" of apeture sounds like a little thing, but you have to consider that the outside of the light cone is where a disproportinate amount of the energy is because the outside 10mm of mirror has more are than the next 10mm of mirror and so on.

Here is the math for an EdgeHD 8".

The collection area with full apeture is about 50 sqaure inches, but you loose about 5 square inches to the seconary mirror shading so you have a total of about 45 squre inches of light collection.

If you cut one inch of the mirror off, you reduce the lighht collection area of the primary to about 38.5", but you still have to take the same 5 square inces off for the secondary shading so this puts you at 33.48 square inches.

That is a 33% light loss.

It gets worse though becaues that 34% obstruction when the scope is working at 8" becomes a 38.75 obstruction.

And bad news.. I bet you did not make the measurement with the low power arm in. When you put the low power arm in, you have to move the mirror even further forward, and this takes another .4" of aperture, so with the low power arm, you were likely working a about 168mm.

This would give you 34.4 square inches of light collection for the primary minus the 5 sqaure inches for the secondary, for grand total of 29.5 square inches. Now you are bascially into 6" refractor area.

None of these figures take in to account the normal 10" transmission loss of JUST the SCT, much less the diagonal, splitter, and prisms in the binoviewer, but since these are common to all configurations, we can ignore it. Anyway, with 29.5 square inches of light collection, you are not working at 52% of the brightness of a full apeture scope.

Compare this to a Mark V working at 7.85" and using a pair of 35mm Ultimas. Hmmmm.

Your diagonal is likelly a light path of about 3" or 75mm or so from the front of the mirror box to the mirror face and on to the top of the mirror box.

going to a T2 Prism would cut about 37mm and going from the Clicklock to the 15mm SCT to T2 would buy you another 20mm or so.

This would give you 57mm of light path savings, and this would buy you back about .5" to .6" of aperure and lower the focal lenght of the scope by about 180mm.

Let's be optimistic and say that it is .6 apeture improvement.

Now the scope is working at 7.6" in straight though mode and about 7.2" in reducer mode.

In straight though, you would be getting about 45.3 square inches of light collection at the prmiary minus the 5 swuare inches of the secondary, so 41.36" total.

This is only an 11.3% loss when going straight though.

Loss when using the low power arm will be about a 26% loss, so a huge improvement over the 50% loss in the current configuration.

I have been ranting on this for a year now.

Most people simply look at a little apeture ruduction as no big deal, but they dismiss the amount of energy that is contributed to the image by the outside of the light cone.

My own configuration the first time I binoviewed an EdgeHD 8" was a 2" visual back with 40mm of light path, a 2" diagonal with eyepeice holder (about 100mm of light path), a power switch (18mm of light path) and a binoviewer with 128mm of light path.

Even in straight though, the scope was working at less than 7" of aperture (about 6.6" by my calculations and 6.2 with a low power arm but at the time I did not know how to measure this). I knew immediatly by looking though that something was seriously wrong.

When I changed to the Maxbright T/2 the improvment was (to me) dramatic.

I would rather work at 7.85"" with a pair of 35mm Ultimas for low power work than use any kind of reducer will will at the very best case reduce the aperture between .4 and .6 inches. Even the Glee telecompressor costs .5" and it is designed to work with binoviewers.

Only you can decide the compromises you are willing to accept.

But that is the way you do the math on it. Expect about .1" loss or gain for every 10mm of light path you add or take away.

Loosing the Diagonal by itself may not buy you much, but combined with loosing the Baader Clicklock, it could make a big differnece.

And that has been my message. A little change doesn't make a difference that is easy enough to see and may not be worth it.

Once you do the math though, it can become apparent that making a big change will pay off.

There is no diagonal I would use an an SCT with binoviewers other than the standard T2 prism, and as inconvenient as it might be, the only way I would connect it would be with the 10mm SCT to T2 connector. If the scope is GEM mounted, I would still use the 10mm connector and I would add a Baader SCT lock ring. This adds about 5mm, but it is far easier to use than the standard 15mm connector's SCT type collar, which is not a good idea for a binoviewr. The SCT lock ring with 10mm connector is the right way to go.

If you did not make the measurement with the low power arm in, I would be intrested to see what you get. I have been conservative by saying .4" of aperture, but it is as I recall actuallly a bit closer to .5".

And this is simply math. You can calculate the image brightness change pretty easily once you have the back focus difference.

I know that people will probably think I am ranting about it, but I am only trying to make people aware of the reality here. It is impossible to get any SCT to work at full apeture when using a binoviewer and any kind of reducer, even with the T2 prism.

By keepng the lgiht path as short as possible though, you can greatly minimize the impact of the apeture reduction.

I hope this helps, and I encourage you to measure with the low power arm in and give us the figures...

#11 HowardK

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Posted 13 October 2013 - 11:41 AM

This was with the low power arm in
In reduction mode
178mm

#12 Eddgie

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Posted 13 October 2013 - 12:43 PM

That is excellent then.. It proves that your efforts to keep the light path short have been effective.

But this is with the Mark V and that is a shorter light path than the Denk, so that accounts for some of the difference.

Did you measure the apeture straight though with no power arms?

The first coniguration I used where I had such bad result was a Clicklock, Televue 2", and Denk Powerswitch.

Aperture reduction was pretty horrid.

In your case though, you may be working at full apeture with the high power arm in, and that is where it is most important because that is when you are observin planets.

And some of the lower power apeture loss is offset by the bigger exit pupil.

But working with a reducer at 7" is pretty good.

I was able to get the C14 working at 13.5" with the Binotron by goiing to the T2.

With a standard 2", apeture was something like 12.6"

So now you know that for about every 10mm you cut, you can improve the apeture by about .1 inch.

If you cut 57mm, you can get to about 7.5" or 7.6" and that is still a meaningul increase in brighness.

One of my key points has been that you really don't know what you are working at until you measure it.

And your improvement over what someone using a standard 2" diagonal with eyepeice holder and a longer light path binoviwer unit is probably about .3 to .4 inches because your bino light path is shorter and you have eliminted the eyepeice holder from the light path.

But 7" is excellent for reducer mode. As I said earlier, best I have heard of with a reducer was with a Glee in front of a Maxbright in a C8, where I think the measured apeture was only .5" doan.

Would love to know your apeture though straight through. and your total light path. Have you measured it? Mark V is 123mm.

#13 HowardK

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Posted 13 October 2013 - 01:49 PM

Not measured aperture straight thru.

Will post a photo soon for your comments

#14 HowardK

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Posted 13 October 2013 - 02:08 PM

photo...

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#15 HowardK

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Posted 13 October 2013 - 02:09 PM

and....

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#16 HowardK

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Posted 13 October 2013 - 02:20 PM

Do not know how to measure my total light path..

Pls tell me, Edd

#17 Eddgie

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Posted 13 October 2013 - 02:33 PM

If you want to do it, you would start by using a ruler or caliper to measure from the flat at the rear of the SCT port (inside the Clicklock back)to the end of the visual back.

This will usually be about 30mm or 40mm. Not the outside, but the inside from the rear of the port to the end.

It is hard to measure the diagonal.

Best you can do is approximate.

Measure along the side from the front of the mirror box to where you think the center of the mirror would be.

Then again, from the center of the side of where you think the front of the mirror would be to the top of the mirror box.

For most 2" diagonals this is about 1.5" front to center, and about 1.5" center to top.

Now, just measure to the top of the mirror box to the top of the Powerswitch to get the total light path to the top of the part where the T2 thread is.

Now, add 123mm for the light path of the Mark V.

Total it all up.

#18 HowardK

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Posted 13 October 2013 - 02:41 PM

How does it all look to you Edd?

Long light path or what?

#19 Eddgie

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Posted 13 October 2013 - 02:43 PM

I have to say that I am surprised that you would be getting as much aperture as you report.

I estimate that you are at 250mm of light path.

Are you sure that you were at infinity focus when you made this measurement?

Based on a 250mm light path, your aperture should be 7" when straight though.

If you focused without the powerswitch, then slid it in to make the aperture measurement, then you are getting the spacing for straight through.

Any time you measure, you have to ensure that you are at infinity focus for that specific configuration.

If your result with over 230mm of back focus was this good, then there must be some difference between our telescopes. My EdgeHD 8" starts to loose aperture right at 160mm of back focus.

I have measured it in a half dozen configurations and it always works out about the same, with each 10mm past 160mm taking about .1" of aperture without the reducer, and about .13" with the reducer.

So, not sure where the discrepancy is coming from, but with 171mm of back focus on my scope, I am at 7.85".

But that is why I keep telling people to measure for themselves.

#20 HowardK

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Posted 13 October 2013 - 02:55 PM

Edd...

I am fairly sure i was focused at infinity with the left reducer arm in.

Least the arm was in from the last session .....usually when i bring the scope in doors i leave the focus and powerswitch where it was when i turned the off switch.

Will check though

#21 A6Q6

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Posted 13 October 2013 - 03:00 PM

Eddgie, tell us you didn't get rid of your C14!

#22 Eddgie

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Posted 13 October 2013 - 04:28 PM

That is a pretty big difference between my scope and your scope.

I was under 6.5" with a baader Clicklock and standard 2" diagonal, but no filter switch, and also using a scope with a longer light path than the Mark V.

Again, perhaps there is some difference in our scopes that is making mine work differently.

If you could make the light path measurements and double check that you did the measurement at low power at infinity focus, I would really appreciate it.

Perhaps there is something in my scope that could be changed to improve it.

Again, this is why I encourage people to measure though. You don't really know what you are getting until you measure.

7" is not bad for having a reducer. Still a 34% light loss though, and if you can shorten up by 50mm, that will put you at 7.5", which is only a 15% light loss, that still gives a 20% improvement.

20% is only very moderate improvement though, so if you are happy with it, it may not be worth dumbing the 2" diagonal. Only you could say.

You might though want to do the math to see how using the Mark V with 24mm Panoptics or 35mm Ultimas might work out.

You may find that the power is not all that different and that you can get almost as wide a field at a bigger exit pupil when you factor in the light loss.

But this is academic really. If you are happy with the way it is, there really isn't any need to do anything and you can simply enjoy it the way it is.

I am curious about the disparity though. I would have calculated well under 7" of aperture based on my own configurations.. If your measurements confirm your original readings, then there must be an issue with my scope.

#23 Eddgie

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Posted 13 October 2013 - 04:38 PM

C14 has passed to a new owner.

I love it and enjoyed it for many years, but had been waiting to buy a Go-To dob.

The CGE 1400 was simply getting to much to move in and out, and it took too much room to leave on my covered patio.

The new scope has a very small footprint and can be left on the patio in a sheltered corner and deployed in a couple of minutes with a hand truck. So, 12" stays outside 24/7. CGE on a cart would have been to big.

The other factor was that I simply could not get much true field using binoviewers and the 12" dob is far better in that respect.

The primary driver though was the desire to simplify the setup routine and the 12" has been simply fantastic from that standpoint.

So, was time.

The 12" is of course giving up a little light, but I get a much wider field of view, so I am happy with the trade.

Been revisiting a lot of targets. Globulars are not as good, but most other targets are still very satisfying.

Have not really had much chance to do planets or the moon but seems like it will be good enough.

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#24 REC

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Posted 14 October 2013 - 10:41 AM

Nice pic of your new scope! Where do you observe from, backyard area and if so, what is your LP level there, red, yellow zone type of sky?

#25 Eddgie

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Posted 14 October 2013 - 11:56 AM

I am in Austin Tx about 4 miles north of downtown, but central neighborhood.

Lot of tree canopy, but still most nights VLM is 3 or 4 if I am lucky.

But I do an enormous amount of observing in terms of catalog objects. Brighter galaxies only, but tons of planetary nebula, Globulars, and clusters and doubles and most of these are still really excellent.

Of course better under dark skies, but the nearest dark sky observing site (where the skys are really black and you won't get shot at for being on the edge of someones private property... Yeah, this is Texas and they $**T like that here) is 70 miles away.

But the scope has been working great. It is a heavy sucker, but I can move it on a hand truck and the alignment is really fast because you don't need to do any kind of home position. I really like that.

And if you only want tracking, you really don't even need to use a star!

Sweet.

Once I got past the weight, I was really happy with the telescope and Go-To, and it is great with the Binoviewer and 1.7x coma corrector. I can't use to low a power anyway because the sky washes out, so is working fine.






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