6" f4

[alacant]

Thanks vlaiv. Your patience is amazing.

The cc is the GPU sometimes called Aplanatic. It screws into a canon t adaptor giving the 55mm required. With my pn208 f3.9, it focuses with the focuser racked out about 2cm:) With the 6" f4 I used, It would not come to focus without an extension. 

??? and TIA

 

[vlaiv]

1 minute ago, alacant said:

Thanks vlaiv. Your patience is amazing.

The cc is the GPU sometimes called Aplanatic. It screws into a canon t adaptor giving the 55mm required. With my pn208 f3.9, it focuses with the focuser racked out about 2cm:) With the 6" f4 I used, It would not come to focus without an extension. 

??? and TIA

 

Ok, you have everything you need to measure distances, provided that you still have access to that 6" F/4 OTA, or if you can contact the original owner to do a simple measurement for you.

First for GPU CC - you can use your pn208 to do measurement. Just measure focus position without CC in relation to fully racked in focuser. You can use Moon and a piece of paper to do this - point the scope to the moon, take piece of white paper and place it in front of focuser until you have sharp(ish) image of the Moon on the paper. You can either move paper by hand, or place it across focuser opening and rack out focuser (you might need to add extension tube for this to work).

Equation will be: ~2cm + total length of CC + 55 = measured focus position.

From that you can see total optical path of CC.

Now, you can check with mentioned OTA where it's focus position is, and how much focuser travel there is, to confirm that it would not reach focus with it. So you just need focus position on that OTA - do it the same way with piece of paper and Moon. Also focuser travel is easy - fully racked in vs fully racked out - just measure how much draw tube sticks out between the two.

Then, if everything checks out, provided that 93mm stated on TS website is correct for GSO 6", you can calculate if you will be able to reach focus with it.

One more thing to note, according to TS website GPU CC has 55mm sensor distance on 8" F/5 scope (1000mm), for 600mm scope it will need ~52mm, here are listed values:

F=600mm, working distance = 51.66mm
F=800mm, working distance = 53.66mm
F=1000mm, working distance = 55.0mm

So this might not be the best choice of CC, not sure if Canon eos T2 adapter can be made to have 52mm distance, or if improper distance will degrade image quality much.

[alacant]

34 minutes ago, vlaiv said:

6" F/4 OTA, or if you can contact the original owner

Yep. Have done so.

34 minutes ago, vlaiv said:

you can use your pn208 to do measurement. Just measure focus position without CC in relation to fully racked in focuser. You can use Moon and a piece of paper to do this - point the scope to the moon, take piece of white paper and place it in front of focuser until you have sharp(ish) image of the Moon on the paper

7cm from the fully racked in focuser, the moon is sharp.

34 minutes ago, vlaiv said:

Equation will be: ~2cm + total length of CC + 55 = measured focus position.

The physical length of the corrector metal m48 thread to the other extreme is 106mm. The length of the 2" tube which goes into the focuser is 100mm, then there's a 2mm flange then the 4mm m48 male thread. Details here. So, 20 + 106 + 55 = 181mm Doesn't seem right. Is 'total length of CC' its optical length or its physical length?

The 55mm is produced by screwing the cc directly to an 11mm eos t ring which along with the Canon flange to sensor of 44mm gives the required 55mm. So the optical length of the cc must be zero. 

Phew. Confused...

**Edit. Not sure if this is relevant: with or without the cc, the camera focuses at the same position, with the focuser racked out around 20mm.

[vlaiv]

So if 7cm from fully racked in focuser, that would mean that effectively total path is close to 0 because

~2cm + 0 + 55mm = 7cm (which it is, if we take ~2cm to be 1.5cm or 15mm)

Now total length of CC is not important because it ends up inside focuser tube. M48 thread is also not important because it will screw in something, only part that adds mechanical length is rim

I just realized that I used total length to mean something else in previous post. So probably I should have used some other term, what I was referring to was: optical length due to mechanical design + optical length due to optical design - first part one can measure easily, for second part one must measure shift in focal plane position and it can vary with focal length, and it is usually there if optical element acts as barlow or reducer.

And from image I guess it is about 3-4mm (now I've re read your post, so this is actually 2mm), so of no real importance (unless we are close to max of min values, then few mm can make difference) - this would be first part from above - optical length due to mechanical design. We will assume that optical length due to optical design is 0 (because it fits first equation well, or can be - few mm).

So calculation for 6" GSO would be 92 - 2 - 55 = 35mm

so you will need 35mm of focus travel to get to focus (should be easy with 50mm draw tube).

You can use 2" parfocalizing rings to further reduce needed focuser travel instead of using extension tube (this way you will increase optical path due to mechanical design from 2mm to whatever you like, depending on where on CC you put parfocalizing ring).

That OTA you had used, might have low profile focuser with travel of only 30mm, even if it had 92mm back focus it would not be able to reach focus, or maybe focus position was even further out, (like more than 100mm, in that case even 50mm focuser travel would not help).

 

[alacant]

12 hours ago, vlaiv said:

So calculation for 6" GSO would be 92 - 2 - 55 = 35mm

OK. Getting there. The figure we're missing is 14.5cm; the distance from the racked in focuser to the moon projected on paper on the 6" f4.

So I think that's 145 -0 -55 = 90mm 

Which, if you confirm to be correct, is not workable without significant tube modification. I notice the TS custom models have a more sensible -longer- primary to secondary separation, smaller secondary and therefore workable solution with the dslr within the focuser travel. As it is, trying to hold a dslr rigidly, 20cm from what is already a flimsy rolled steel tube is impossible.  Yeah, it works -and the speed is impressive when compared to my 130pds- but only for the first few frames; a tiny movement at f4 and you're knackered!

Thanks again vlaiv. You helped me understand all this.

[vlaiv]

57 minutes ago, alacant said:

OK. Getting there. The figure we're missing is 14.5cm; the distance from the racked in focuser to the moon projected on paper on the 6" f4.

So I think that's 145 -0 -55 = 90mm 

Which, if you confirm to be correct, is not workable without significant tube modification. I notice the TS custom models have a more sensible -longer- primary to secondary separation, smaller secondary and therefore workable solution with the dslr within the focuser travel. As it is, trying to hold a dslr rigidly, 20cm from what is already a flimsy rolled steel tube is impossible.  Yeah, it works -and the speed is impressive when compared to my 130pds- but only for the first few frames; a tiny movement at f4 and you're knackered!

Thanks again vlaiv. You helped me understand all this.

You are welcome.

Yes, 14.5cm is quite a long back focus. I was thinking, although not as cheap as 6" F/4, have you considered good CC with focal reduction? Something like x0.73 CC will bring your PN208 to ~592mm focal length. Just pay attention to corrected field and vignetting (for your particular sensor). If you can get good definition across the field it will be better solution than 6" F/4 for ~600mm focal length and certainly much faster than 130pds (8" vs 5" on same resolution, much more light captured).

[alacant]

6 hours ago, vlaiv said:

x0.73 CC will bring your PN208 to ~592mm focal length

That would de be nice but the only one I can find is over budget at around €700. 

vlaiv, is it possible that the sample I tested had a f5 mirror? I'd still find it faster than my 130pds but it may explain the focus position (?)

[vlaiv]

40 minutes ago, alacant said:

That would de be nice but the only one I can find is over budget at around €700. 

vlaiv, is it possible that the sample I tested had a f5 mirror? I'd still find it faster than my 130pds but it may explain the focus position (?)

Doubt it. Focus position is closer to tube with slower scopes, also secondary tents to be smaller. F/5 mirror would be "equally" fast in both instruments if you used the same camera. But you can easily check it out, just look at images you took, are they the same resolution as those taken with 130pds (close to being same) or are they zoomed in a bit? If they are the almost the same (600mm vs 650mm FL) then it is F/4 instrument. If images are more zoomed in that would mean 750mm vs 650mm, or 6" being F/5 instrument. If you don't have the same object in images, just plate solve, or measure distance between known stars (measure angle in Stellarium) and see the actual resolution in arc seconds per pixel - this will give you focal length that was used to shoot images, and hence F/ratio.