Can someone check my SCT calculations and thinking please?

[Adrian]

Apologies for the long post that follows…

I was all set to buy an 8” SCT the other day but I was still persuading the Mrs that I did need to drive 150 miles to pick it up when it sold on me. That meant there was nothing for it but to buy the 10” Meade that was a bit closer

I have bought it for the purpose of planetary webcam imaging and splitting doubles, which it should be excellent at (although a mount upgrade may have just moved up the list of requirements ).

I have been doing some reading & calculating and I was wondering if I could check my thinking/numbers with the experts regarding extracting the maximum field of view from this scope, should I wish to go scanning for fuzzies with a 10” scope

Despite the 2” thread, the rear opening of the OTA is 1.5” so the maximum possible FOV, in degrees, of the scope in it’s current form is (57.3*38.1mm)/2500 = 0.873252* (formula from Wiki)

Going to a 2” visual back and say a 38mm panaview or 56mm plossl won’t give me more field due to this 1.5” opening effectively becoming the eyepiece field stop and restricting the view.

Working backwards using the above formula, the widest angle eyepiece I can use with no/little restriction is the 32mm Panaview or 31mm Baader aspheric. these will extract the maximum FOV from the scope as it is (If I fit a 2" visual back and diagonal).

Now, if I want to go wider (and not get a different scope) there seem to be 2 options:

1) Buy an “eyeopener” type adaptor such as this (2 ClickLock SCL Eyepiece Clamp for C11-C14 SC Telescopes [baa-9a-2956233] - £69.00 : 365Astronomy: Discovery for every day!).

2) Get an f6.3 focal reducer.

Option 1 gives me a full 2” field stop so the maximum FOV would be (57.3*50.8mm)/2500 = 1.164* with a 56mm plossl (or 1.064 with the 38mm panaview – are you guessing my budget yet? )

Option 2 gives me 0.873252*/0.63 = 1.386*, still with the 32mm Panaview or 31mm Aspheric, but puts more glass in the system.

So my questions are whether my thinking/calculations are reasonable (i.e. what factors have I forgotten ) and which option would you take for wider views (given that the Mrs will not have another scope in the house)?

And a gold star to anyone who made it through that

Cheers

Adrian

[beamish]

just spotted this

"......... The C14 was raised by holding the eyepiece. We expressly point out that the imitation of this test setup is not recommended and we cannot take responsibility for an (otherwise unlikely) event of damage."

too bl**dy right !! wouldn't recommend doing that with any scope -Baader have you gone off your collective rockers ???

[Adrian]

Yeah, You won't catch me trying that!

[John]

Interesting stuff Adrian and the way my mind worked when I was calculating the widest true field that I could squeeze from an SCT. Trouble is I then found that folks were using eyepieces with field stops wider than the rear aperture of the scope, apparently quite successfully

I've never got a fully convincing answer as to why this might be the case but what I think happens is that vignetting does occur when the eyepiece field stop exceeds the field stop diameter, but the light lost is spread evenly (ie: not a sudden cut off at the field edges) and so is not noticeable to the eye unless the observer is able to compare the views directly with a scope of equal aperture that can fully illuminate the whole field of view of the eyepiece.

Your approach appears to ensure that the field of view you can achieve will be fully illuminated

[Adrian]

I suppose it also depends on how the plane of focus forms - if the cone of light goes to a point and the plane of focus is beyond that node, then it could be possible to get that node in the 1.5" aperture so the light cone is not restricted and opens out again in a diagonal to fully illuminate the 2" field stop if you see what I mean.

My knowledge of optics is not good enough yet...

[Mr Spock]

I don't know about the 10" but my C9.25 will take a 42mm LVW for a field of 1.287° with no vignetting.

A reducer won't alter the size of the rear aperture, will introduce more glass into the path and have a restricted lens size. You may not find any advantage to using one.

[ollypenrice]

As above, I think the reducer will add nothing to the FOV available in the widest of 2 inch EPs. Isn't the baffle tube the final arbiter here? I used a 35 Panoptic in my 10 inch Meade and, more recently, a 26 Nagler.

Olly

[Merlin66]

I used a 10" Lx for many years....great as you say for double star etc the maximim FOV was with a 32mm eyepiece.

Yes, you can use the reducer visually, but I think you'll be disappointed.

[riklaunim]

I got the 3" SCT ClickLock recently for C11 to hold the 2" Optec NextGen imaging reducer/corrector - http://oi51.tinypic.com/2hevzow.jpg

It uses the 3" SCT thread that shows up in C11 and bigger SCT. It's easy to use, nice and strong grip.

For visual you can stick with the standard SCT thread and use for example 1,25" 20 mm Erfle (70 deg fov) with or without the f/6.3 reducer/corrector as the widest EP. It works, and it's cheap

[Adrian]

Thanks for the responses so far...

I thought the reducer would cause problems, especially for wider views (too good to be true option). Presumably you can't create FOV from nowhere?

I've been thinking about the telescope system and 'nodes' and my thinking was off, wasn't it? The telescope produces a cone of light down to a node. The eyepiece does the same. When you place the eyepiece so that the nodes coincide you have a telescope system with magnification of the FL ratio.

I can also see from this that the resriction in the system is likely to be the secondary end of the baffle tube as this is the point most likely to intrude into the light cone, as it doesn't seem to open up much - although it may be wide enough. I therefore can't see much benefit from the eyeopener type adaptor over a simple 2" VB.

I will check when I get home (i see telescope pointed at the sky with a piece of paper to see the size of the light cone as it passes through the aperture).

[Adrian]

Right, so ignore the above post (what? you already have )

I started reading & thinking about why SCTs have this narrow opening - I figured that there must be a reason or the manufacturers would have opened them out. The baffling of the system is already tight, so if I open up the back to 2" will I be allowing light straight through the corrector, past the secondary, into the eyepiece? (There is already the merest hint of this at the edge of the 1.5" opening)

Zoltan at 365astronomy has kindly said I can try an adaptor to make sure it fits on Meade scopes (may have a slightly different thread size). His place is about 10 minutes walk from my work, so that's great. If I can see through the 2" aperture past the secondary (a light ring round the secondary) is that a big problem? will I lose contrast on bright objects?

Maybe I should stick to a 2" VB on the standard backplate...

[riklaunim]

In case od Celestron starting from 8" they have full standard SCT diameter at the end (almost 2") and they can handle most if not all 2" EP. Starting from 11" (Meade 10"?) you also get the 3" SCT thread that will illuminate even bigger things - but when racing to very big eyepieces note that SCT doesn't have big corrected fields. SCT threads are the same among vendors.

The opening diameter at the visualback is related to secondary mirror size. It can't be to big for given SCT/Mak scope size (and design). So either use SCT diagonal with not to big 2" EP or 1,25" EP with the f/6.3 reducer/corrector.

[Merlin66]

Adrian,

No. The baffle doesn't change.. so the relationship of the diameter and length of the baffle to the secondary doesn't change.

If you were physically changing the baffle inside diameter or its length, then there would be an impact...

HTH

[Adrian]

I'm not sure I was clear: I'm worrying that if I get the 3.25" to 2" baader adaptor to replace the factory adaptor and open out the SCT thread (1.5") constriction then the existing baffling (which as you say, doesn't change) will not be sufficient to prevent light coming straight past the secondary into my eyepiece/diagonal - As with Astrogeordie's RCT in this thread: http://stargazerslounge.com/equipment-discussions-help-telescopes-whole-setups/155361-gso-ritchey-cretien-254mm-f-8-first-impressions.html

I therefore am wondering if the constriction that these sort of adaptor (Baader, peterson eyeopener etc) are trying to remove actually forms part of the baffling and so removing the constriction would have a detrimental effect

If I look through the telescope without an eyepiece fitted and can see daylight past the secondary - what kind of impact could this have on my viewing?

[riklaunim]

The bigger thread is the to use - safely. There won't be any light problems.