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Focus problem with f/6.3 reducer/corrector

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#1 rflinn68

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Posted 30 January 2013 - 08:37 AM

Hello. I recently bought a Celestron f/6.3 reducer/corrector. I got it mainly for astrophotography but I'd like to use it for visual as well. It works fine with my 1.25" diagonal and eyepieces but I cant reach focus with my 2" stuff, it runs out of focus travel just before reaching a good sharp image.

I believe the problem is my diagonal. It is a Meade Series 5000 refractor style with an adapter. If I buy an SCT style 2" diagonal will I be able to reach focus? I've been looking at the Celestron #CEL-93527 and I'm wondering if it would work for me. I just hate to buy it and it not work and thought there would be quite a few here that has tried this combo. I use the corrector with my C8 and 10" Meade and would like it to work on them both. Its so close now its very frustrating. I'm also thinking about machining the bevel off on my adapter. Any other ideas or adapters that might work for a cheaper fix? Thanks

#2 Ira

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Posted 30 January 2013 - 09:16 AM

I have the same problem with many, but not all of my 2" eyepieces. Kind of defeats the purpose of using the reducer optically. Wish I had known before I bought it.

/Ira

#3 JamesL

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Posted 30 January 2013 - 09:20 AM

my sct style gso 2" diagonal threads onto the focal reducer on my c8 and it focuses ok.

#4 Eddgie

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Posted 30 January 2013 - 09:42 AM

Just a word of warning... Using a focal reducer with a 2" diagonal is likley cutting your aperture to maybe 5 inches or less.

That being said, my assumptoin is that the mirror is running out of forward travel? You turn the knob counter-clockwise and it stops before reaching focus?

Most SCTs will reach focus with a reducer and a 2" diagonal, but sometimes they don't.

The problem is that unless you know exactly how much more travel you need, you won't know if changing this or that will buy you what you need.

First, estimate how much short of focus you are. If you have a long focal lenght plossl, put it in the diagonal and see if you can reach focus. Leave the focuser turned fully counter-clockwise and just by slipping in the plossl, see how deep you need to slide it for the target to come into focus. This will let you determing exactly how deep inside the eyepeice barrel the focal plane is with your mirror fully forward.

I say to use this because the field stop in these 32mm and 40mm Plossls is usually much further down in the eyepiece barrel than with many shorter focal lenght eyepieces.

For example, many 32mm Plossls will use the very end of the barrel for the field stop, or the field stop will be a very small ridge just inside the end.

See if you can reach focus with one of these by sliding it down into the diagonal, using the very shortest 2"/1.25" adapter. Again, with the know fully CCW, try to reach focus by sliding the eyepeice in so you can see how far into the diagonal the field stop goes.

If you can reach focus, now see how much distance between this and the field stop of your 2" eyepeies when inserted into the diagonal.

For example, suppose you have a 32mm Plossl and the field stop goes down into the eyepeice holder 15mm.

But now you put in a 20mm eyepeice and the field stop is near the top of the barrel, buy you can't reach focus. You now know that if you can shorten your light path by the difference of the positino of the two field stops, you will be ablet to reach focus with at least that eyepeice.

You can then ask people about the lenght of the differnt components you are considering. For example, the SCT Clicklock that Baader sells is about 42mm. If you were using one of these, you would want to search for a adapter.

Of course if you need more than about 10mm, you now have to start looking for diagonals with the shortets possible nose too, or the nose of the diagongal will bottom in the adapter against the port at the back of the SCT. Normally it is the SCT/2" adapter that defines the light path length becuase usually the diagonal does not bottom. But you can't get shorter than the lenght of the diagonal nose.

SCT diagonals (the real, true SCT diagonals) usually have a very short back focus.

They usually however are limited to a 38mm nose. But they often offer the shortest possible light path in a 2" diagonal.

The good news is that you can sometimes pick up used SCT diagnals cheap. I bought a Meade UHTC 2" SCT diagonal for $45. IT had maybe 25mm of light path at the nose. I think this is about as short as you can get.


Also this: It is unusual for SCTs to run out of focus travel in this configuration... You may want to ensure that no screws holding on things like your finder are sticking down in front of the mirror. If so, this can limit the forward travel of the mirror. It happens sometimes and people don't realize it until they do what you are doing, which is driving the mirror forward to get more back focus. Make sure you check this.

Again, my bet is that with the 2" diagonal and focal reducer, you are probably turning your C6 into a C4.5, but that is not what you asked about. If you can't reach focus, you need to determine that there is not obstruction to your mirror travel, and, and you need to determine exactly how much you need to shorten the light path.

Otherwise, you are rolling the dice by buying this or that component to see if it will work.

Better to try to determing exactly how much you need, then take it from there.

#5 rflinn68

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Posted 30 January 2013 - 09:53 AM

Focal reducer and 2" diagonal is cutting the aperture of my 10" Meade and C8 to 5" or less!!?? Wow

Well I've done some more looking at my components. Machining the adapter wont work. Its already pushing my diagonal back about 1/8" (about what I'm needing to reach focus). The barrel on the diagonal is too long.

Eddgie...nothing is in the way of my mirror. I measured my adapter and it is 1.625" long. I thought about buying the TeleVue 1.5" adapter for $50. When I screwed the reducer into my adapter I noticed it pushed my diagonal back about 1/8" because its hitting on the barrel of the diagonal so the TeleVue adapter wont help any. I only need that 1/8" to reach focus so it looks like (aside from cutting down the barrel on the diagonal) that I will need either an SCT style diagonal or a refractor style diagonal with a shorter barrel.....Real bummer here. Looks like I will be searching the classifieds :(

EDIT: Just noticed at the bottom of your post you said "cutting the aperture of my C6..." Dont know why you assumed I had a C6. I stated I use the reducer with my C8 and 10" Meade. What would it be cutting them down to and why is this? Thanks

#6 Eddgie

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Posted 30 January 2013 - 10:33 AM

I am so sorry, Somehow I thought you were using a C6. My bad.

For the C8, aperture is probably being reduced to about 7".

It is likely you are loosing an inch or two of aperture for the 10", but I don't have an analysis for the Meade scopes.

Again, my apology. For some reason I though you were using a C6.



#7 GlennLeDrew  Happy Birthday!

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Posted 30 January 2013 - 10:34 AM

That reducer is optimized for a working distance of something like 105mm. If the interposed 2" diagonal is putting the eyepiece field stop farther behind the reducer than this, the reduction factor increases, and the focus must be pushed farther outside the OTA's back end. Fir each incremental increase in reducer-to-focus distance, the required in-focus--hence moving rearward of the focus--becomes ever more significant. That is, as the working distance approaches the reducer's own focal length, focus adjustment converges (probably something like) logarithmically toward the telescope's own focal length.

Can you determine the distance from the reducer's rear lens to the location of your typical eyepiece's field stop? If it's much larger than 105mm, you have your reason for inability to achieve focus.

#8 rflinn68

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Posted 30 January 2013 - 10:39 AM

I am so sorry, Somehow I thought you were using a C6. My bad.

For the C8, aperture is probably being reduced to about 7".

It is likely you are loosing an inch or two of aperture for the 10", but I don't have an analysis for the Meade scopes.

Again, my apology. For some reason I though you were using a C6.


No problem. I was just wondering if you could explain to me why it reduces the aperture of the scope. You just scared me for a minute thinking it would cut the aperture of my 10" Meade in half! :lol:

#9 rmollise

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Posted 30 January 2013 - 12:07 PM

Just a word of warning... Using a focal reducer with a 2" diagonal is likley cutting your aperture to maybe 5 inches or less.


2-inch diagonals and eyepieces work very well indeed with the reducer corrector. Or have for me over the last twenty years, anyway. :lol:

#10 EdZ

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Posted 30 January 2013 - 12:08 PM

The reducer expands the converging light cone and part of the light cone hits the baffles before it reaches the eyepiece field stop, hence clipping aperture.

from my analysis of the C8 aperture

In my C8, with a 0.63x reducer in place and my Astro Tech 2" SCT diagonal screwed onto the reducer, aperture measured 190mm. So, a 2" diagonal attached to the 0.63x reducer on the C8 reduces aperture, but only by 13mm.


On the C6, the same configuration resulted in aperture = 140mm.

edz

#11 rflinn68

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Posted 30 January 2013 - 12:26 PM

So EdZ...from what I see here, C6 reduced by 12mm and the C8 reduced by 13mm, I guess its safe to assume I'm only losing 14mm on my 10"? If so, thats no biggie...having a flatter and wider field of view is worth losing 1/2" to me.

#12 Eddgie

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Posted 30 January 2013 - 01:09 PM

It really depends on the exact configuration.

Notice that Edz was using a 2" SCT diagonal, not a refractor diagonal.

This is important because the SCT diagonal often has a much shorter light path than most SCT to 2" adapter/refractor diagonal combinations.

So, Edz has kept his total back focus very short. This is the key to minimizing aperture loss when using a focal reducer in front of a 2" diagonal.

For example, suppose Edz went to a configuration that was 20mm longer (about how much longer many SCT to 2" adapters add to the back focus), this could increase the aperture loss from 13mm to more than 25mm.

Once the baffle starts cutting into the light cone, every millimater of back focus you add takes more and more aperture away. It is very aggresive once you go past about 210mm in all of the Celstron SCTs.

The exact nature of the apeture loss is dependend on how tight the baffling is. For example, the C14 has very tight baffles and will loose over 2" of apeture when used with a 2" diagonal behind a focal reducer.

You can measure it directly though. It is easy to do.

But it only matters if it matters to you. If you don't mind that your scopes is working at a smaller aperture with a bigger obstruction, then enjoy it.

If you do decide to measure it though, please let me know. I am kind of keeping mental track of this.

So far, I have not seen one SCT being used with a focal reducer and 2" diagonal that did not loose some apetrure and in some cases it has been a pretty big loss. The difference was glaring in my C14. I would never use a focal reducer on a C14 for visual use. Better to use a 41mm Pan to get the widest field. The focal reducer/2" configuratino is catastrophic on a C14.

And if one wants to do this, the best approach is the one Edz uses, which is to use a standard SCT diagonal becuase these usually have the shortest light path.

The Televue Shorth 2" adapter and shorty 2" diagonal would be another good choice.

But rember, not only does the apeture get reduced, you also loose contrast because the secondary obstruction becomes a bigger percentage of aperture.

For this reason, it should always be removed, or you should go to a 1.25" diagonal behind the reducer for planetary viewing.


Unless you don't mind the loss of contrast there either.

Again, if it concerns you, it is easy to measure the apeture loss, and that way you would know for sure.

It may also be useful for people in the binoviewer forum to know. Many SCTs are loosing effective apetrure when used with binoveiwers, so having this info for a Meade 10" SCT will help someone down the line with their bying decision.

I have ray traces for the C8, C9, C11, and C14, but I don't have any data for the Meade scopes at all...

#13 EdZ

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Posted 30 January 2013 - 01:13 PM

This is important because the SCT diagonal often has a much shorter light path than most SCT to 2" adapter/refractor diagonal combinations.


The difference is +/- 10mm, so doesn't make much difference.. the SCT diagonal is 130mm light path length, much longer than a refractor diagonal. The refractor diagonal goes into a visual back. The VB + ref diag is 10mm longer.

#14 rmollise

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Posted 30 January 2013 - 01:14 PM

It really depends on the exact configuration.

Notice that Edz was using a 2" SCT diagonal, not a refractor diagonal.

This is important because the SCT diagonal often has a much shorter light path than most SCT to 2" adapter/refractor diagonal combinations.


More like: "maybe sometimes." Most 2-inch visual backs place the diagonal about the same distance back as an SCT model.

#15 EdZ

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Posted 30 January 2013 - 01:14 PM

So EdZ...from what I see here, C6 reduced by 12mm and the C8 reduced by 13mm, I guess its safe to assume I'm only losing 14mm on my 10"? If so, thats no biggie...having a flatter and wider field of view is worth losing 1/2" to me.


same conclusion I came to regarding the C8.

edz

#16 rflinn68

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Posted 30 January 2013 - 01:55 PM

It really depends on the exact configuration.

Notice that Edz was using a 2" SCT diagonal, not a refractor diagonal.

This is important because the SCT diagonal often has a much shorter light path than most SCT to 2" adapter/refractor diagonal combinations.


Well, I cant reach focus anyway with my adapter/refractor diagonal so it really doesnt matter. I just really need to find someone with the SCT 2" so I can try it out. I'll probably just end up buying the Celestron unit. I'm sure all will be good then.

Regarding measuring my Meade for aperture loss...I will do that someday just so I will know. I also use binoviewers. Wonder if that'll present another problem :question: :lol:

#17 Eddgie

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Posted 30 January 2013 - 04:15 PM

Some SCTs will loose a little aperture with binoviewers, and a lot of aperture if the binoviers are used with a focal reducer or low power switch.

My binoviwer is a Mark V with a Baader T2 diagonal. The total light path is about 203mm. My C14 appears to be working at full apeture, but my C5 only works at 122mm with the binoviewer.

Just keep the light path as short as possible. 200mm and under and you are probably OK.

#18 ensign

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Posted 30 January 2013 - 09:01 PM

I just got an SCT nosepiece for my refractor-style 2" diagonal. Cost about $40 shipped. Then reaching focus with all my EPs was a non-issue.

I noticed no obvious issues using the .63 reducer with a C8. However my C9.25 Edge gives much better views.

#19 rflinn68

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Posted 31 January 2013 - 08:24 AM

I just got an SCT nosepiece for my refractor-style 2" diagonal. Cost about $40 shipped. Then reaching focus with all my EPs was a non-issue.

I noticed no obvious issues using the .63 reducer with a C8. However my C9.25 Edge gives much better views.


I'm not sure that will help me. The focal reducer is actually pushing my diagonal out of my adapter about 1/8" (about what I need to reach focus!). Will this work with the Meade diagonal? The ends of mine are glued in and not threaded. At least the eyepiece end is. It broke on me once so I'm afraid the twist the other end to find out if its threaded or not. I havent seen one of these. Do you have a link?

EDIT: Just seen a William Optics adapter. Wouldnt this just make my diagonal even longer than it already is?

#20 rflinn68

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Posted 10 February 2013 - 07:58 PM

Well, I finally got to try this with my Meade scope. It works just fine with it. Just needs a tiny bit more travel on the C8 though to work with my 2" eyepieces.






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