WO diagonal mod for some SCT's

[bomberbaz]

This can apply to any diagonal owner but being an SCT owner whose mount is loaded to the max I needed to look at my options to make it more balanced.

I have added to the tube a Telrad although I have tried to get this as near to the centre of balance as possible,. Out of my control regarding balance is the RAVF which fits onto the rear casing of the OTA. So I wanted to relieve the rear end weight and try to get the whole set up more s=centrally balanced.

One of the problems is the WO diagonal. Great piece of kit, carbon fibre side plates to help reduce weight but about 3 to 4 times heavier than the supplied standard diagonal. Between the (forgive my terminology from here on) screw attachment to the SCT base and the actual diagonal is a 40mm (when tightened) tube. Two issues with this. 1 it makes the focal length of the scope longer as the basic diagonal is about 20mm shorter. 2 it upsets the balance by having this already extra weight set further back than the basic piece would have been.

I found a solution as yet untried in terms of viewing but that I am in no doubt will not be an issue in the Baader Hyperion FTRings (see link below) which in fitting size is exactly the same by design. Coated to the same standard as the WO supplied piece but the 14mm Baader FTR piece shortens the WO diagonal overall length to roughly the same as the supplied piece, see attached:

The two pieces in question side by side, the mod plus supplied sct attachment same size as the supplied connecting tube on its own!

The two diagonals side by side for comparison (the WO is the modded one).

The original as it came attached to sct;

Finally, the modded digonal attached to sct

So although untested I believe the mod will have the effect of bringing the scope plus new diagonal back to roughly the original SCT quoted focal length.

Another plus side of this is my two Baader Hyperion at 13mm and 10mm will have quicker fixes on the FTR's. My 13mm with the now redundant WO connector becomes a 8.2mm effective. The 10mm becomes a 6.2. The 28mm FTR which isnt involved in the mod still give me a 9.2mm and 7.1 on the same two EP's.

Hope this makes sense but if not this might help:

http://www.firstligh...uning-ring.html

see 13mm plus 14, 28 and 14+28. It doen=snt show the 10mm but trust me the stats are right.

Baz

[John]

The balance issue I can understand Steve, but I don't see how using a 2" diagonal alters the focal length of the scope ?.

Maybe I'm missing something that relates to SCT's though

[bomberbaz]

I was under the impression (maybe wrongly) that when you increase the effective length of any tube, you increase its FL. By using a diagonal that adds 20mm to the tube, you are in effect adding to the OTA FL.

You are in effect stretching the light according to Einstein to get to the same point.

[bomberbaz]

Details of the Hyperions tuning rings including the 10mm. : http://www.baader-planetarium.uk.com/en/pdf/Hyperion_Eyepiece_Series_Information_Sheet.pdf

[John]

I was under the impression (maybe wrongly) that when you increase the effective length of any tube, you increase its FL. By using a diagonal that adds 20mm to the tube, you are in effect adding to the OTA FL.

You are in effect stretching the light according to Einstein to get to the same point.

I don't think that is the case Steve. Not in my experience anyway.

[bomberbaz]

Not sure where I read that John,(think it was on here ) but I wont disagree. Your experience is well respected and I am happy to go with what you say with this.

ANyway leaving that out, the weight balance and Baader EP enhancements sstill apply.

The former being the main consideration.

Actually this raises a question about EP's and balance, new thread to follow..................

[John]

You can get weights to add to the top end of an SCT tube to balance heavy equipment at the back end. I used to use a 2" diagonal and a 38mm 2" eyepiece with my old C8 SCT and it seemed to handle it OK. The dew shield I used counter balanced them to some extent.

[Stu]

I think there maybe something in what Baz says. Just had a quick search and it does show that moving the focal point on an SCT or Mak affects the focal length because of the compound nature of the scopes and the fact that you use the primary to focus. Using a 2" diagonal will affect the focal length in this case.

Stu

Quote from an SGL thread from JamesF

Forget the diagonal. It just doesn't matter. The focal length is the same whether it's present or not.

Focal lengths of compound lens/mirror systems are not as straightforward as those for a newt or simple refractor. Let's say, for the sake of illustration, that you have a Mak with a 1.25" diagonal connected. We'll say that the diagonal has an optical path length of 75mm and that the image plane is flush with the top of the diagonal which should be about the right place for a standard 25mm plossl eyepiece. If you then swap the 1,25" diagonal for a 2" model with a 110mm optical path length, the focal plane is somewhere down inside the diagonal, 35mm below the top (110mm - 75mm). To return the image plane to the top of the diagonal you need to move the focal plane 35mm further "out". With a Mak (or SCT) you can do that by moving the primary mirror closer to the secondary. It's a bit like moving the mirror up the OTA of a newt instead of moving the focuser outwards.

But here's the thing... Unlike a newt, the secondary in a Mak or SCT is not flat. It forms part of a compound optical system together with the primary and the corrector. And the focal length of a system of optical components is dependent on the distance between them. So, by moving the primary you're partly causing the image plane to move because the focal length of the system is changing.

If you have a Mak or SCT with a diagonal and eyepiece where the image is in focus then you could remove the diagonal and replace it with an extension tube of exactly the same optical length, refit the eyepiece and the image would still be in focus. The diagonal does nothing do the focal length, but, if you remove or replace the diagonal with something of a different optical length then by moving the primary to get the image back in focus you are also unavoidably changing the effective focal length of the scope.

At least, that's the way I understand it

James

Edited by JamesF, 06 July 2012 - 12:00 AM.

Stu

[E621Keith]

Indeed, as JamesF and Stu said, a scope with moving mirror focusing system have variable focal lengths. The view through a 2" diagonal does appear more magnified than a 1.25". The situation is even worse when used with a binoviewer.

A SCT's true focal lengths is determined by the distance between the primary and the secondary. I didn't realised this until I started using binoviewer and objects appeared more magnified than mono-viewing.

[John]

Thanks Keith, Stu and James and apologies to Baz / Steve for doubting

Always something to learn

[bomberbaz]

Glad I didnt dream it and apology although not needed accepted. My understanding was slightly different to James explanation but I will defer to that one without question as I have no doubt his is the right one.

Anyway, forgot to mention the other reason that came to mind when thinking about this is, in case you hadn't noticed I had been thinking about buying a new 2" wide field EP , and this shortening of the FL will of course help increase the TFOV.

Thanks to Stu(James), Keith and John for the input.............

[Stu]

Glad we got sorted :-). To be perfectly honest, I have never properly got my head around the change in focal length in maks/SCTs, although I know it happens. Need to sit down with a diagram or two and work it out.

Stu

[bomberbaz]

I think I have it figured, well maybe. If you take the primary further from the secondary, there is a slight flattening out the light cone angles and in effect make the scope slower, see below:

What actually happens is the diagonal light cone extends due to longer connection tube and the mirror moves out to compensate as James points out, effectively changing the focal length and making the scope slower as per diagram. At least I think thats what happens.

[Stargazer_00]

I noticed this effect with binoviewers in a 127 mak as well. I thought moving the primary to focus would've decreased the focal length of the scope making the magnification lower but it seems to do the opposite. It wasn't my scope so I didn't put much more than a passing thought into it.

[Dunkster]

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Anyway, forgot to mention the other reason that came to mind when thinking about this is, in case you hadn't noticed I had been thinking about buying a new 2" wide field EP , and this shortening of the FL will of course help increase the TFOV.

Steve the 2" diagonal will have a longer optical path than the 1.25" diagonal, so some increase in focal length is inevitable. You'll be able to compare and see the difference, but it's probably only a couple of percent. I use the Revelation SCT diagonal and it gets closer to the rear of the ota than some alternatives I've seen, but since I've put it on I've not taken it off, so any difference in focal length is irrelevant the ability to add a longer focal length EP is much more valuable IMO

[bomberbaz]

Steve the 2" diagonal will have a longer optical path than the 1.25" diagonal, so some increase in focal length is inevitable. You'll be able to compare and see the difference, but it's probably only a couple of percent. I use the Revelation SCT diagonal and it gets closer to the rear of the ota than some alternatives I've seen, but since I've put it on I've not taken it off, so any difference in focal length is irrelevant the ability to add a longer focal length EP is much more valuable IMO

we shall see soon dunkster. My new EP's will be arriving by courier later frim FLO. Willl just stick em on the sct to see how it blalances and just have a few terrestrial views to get a feel for them. But what I really want are some clear skies. Oh well hehe