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CraigT82

Heritage 130p focuser mod

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Hi all, been busy this afternoon adding a 2" Crayford to my heritage 130p.

My idea is to turn it into wide field DSO travel scope for airline transport to very dark skies (I'm visiting Tasmania I'm January - very excited about super dark southern skies there).

I've always thought it's a great little scope which punches about its weight, just let down by its primitive  1.25" focuser. I bought a TS 2" single speed Crayford to fit to the OTA, this particular focuser is relatively lightweight (500g) compared with other aftermarket crayfords which are closer to 1kg. The weight matters as the trusses will only take so much added before it begins to twist and flex (about 1.3kg by my testing before the collimation is affected). 

I can now use weighty 2" widefield EPs (up to 750g) with this scope, and with a bit more fettling it will be a superb travel scope. 

 

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Edited by CraigT82
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Nice one! I was considering this scope for wide field / travel / grab and go, but the lack of 2" focuser put me off. Looks like a really neat upgrade and useful comments about the weight too which is something I wondered about. All the best for your trip!

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Thanks!  There are still some issues with the mod that needs sorting out, the focuser drawtube is quite long as needs cutting down a bit as it intrudes into the lightpath when my EPs are at focus (the other option is to collapse the trusses an inch or two to shift the focal plane further out - but then I'd be worried about light loss from the secondary being too far forward).

Also as the drawtube protudes into the OTA it prevents the trusses from collapsing fully, the last photo shows how much the focuser has to be wound out to allow the bottom of the drawtube to clear the OTA.  I think for airline travel I'd remove the focuser and store it within the OTA with suitable foam padding and protection for the mirrors.

Once I'm happy that the whole thing works, I'm going to cut off the original focuser and move the new focuser around to take up the original position, that's simply a matter of drilling 4 new holes for the ends of the steel strips to attach to the UTA.

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Very interesting mod Craig, quite ingenious!

Do you know whether you get full illumination of a 2" eyepiece? That would be my only concern as I assume the secondary was optimized for 1.25" to be as light and cheap as possible. You might get some vignetting on longer focal length eps?

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Thanks Stu, I did worry about that before starting out with the mod, but as far as I can tell it should be fine for use with longer FL EPs.

The secondary is actually oversized by some margin, it's 40mm (31% CO).  The image below show that with a 28mm EP there will be a 0.3 magnitude drop at the edges, which I think is acceptable (however I've only guessed at the 'diagonal to focal plane distance', I need to measure it and re-run the chart).

I think with this F5 scope 28mm EPs are about as long as I'd go, my current 32mm panaview gives a 6.4mm exit pupil which is a bit too much.

Off axis illumination.JPG

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Sounds good! You've clearly done lots of thinking about this :). Does the eyepiece field diameter mean the focal length or the field stop?

I'm sure the end result will be good, I've used systems which I know are vignetting but in practice it is barely noticeable.

I do like the way you've found a system which allows you to mount the new focuser but still keep the ability to collapse the tube. I look forward to hearing how you get on with it under the stars! I love widefield scopes :) 

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Yes I wondered about that, I think I read somewhere that the field stop diameter is broadly similar to the focal length but I might be wrong.  Actually thinking about it some more that's not quite right, as a 32mm plossl will have a much smaller field stop than a 31mm nagler.  I do have a plastic vernier caliper at home which I could use to measure the actual diameter without risking scratching the glass :)

I will report back once I've used it in anger!

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54 minutes ago, CraigT82 said:

Yes I wondered about that, I think I read somewhere that the field stop diameter is broadly similar to the focal length but I might be wrong.  Actually thinking about it some more that's not quite right, as a 32mm plossl will have a much smaller field stop than a 31mm nagler.  I do have a plastic vernier caliper at home which I could use to measure the actual diameter without risking scratching the glass :)

I will report back once I've used it in anger!

Yep, the 31mm Nag is 42mm, I guess a 28mm 82 degree is mid 30's perhaps, given that a 27mm 68 degree is 30.5mm

All good experimental stuff :) 

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Brilliant!!! I have wreaked my head thinking of ways to improve the focuser on this scope but they all involved modifying the existing one never occurred to me to bypass it altogether like you have. Have you considered some sort of clamping device for the tube end of your struts, may help improve rigidity. Did you have any problems realigning the secondary?

Edited by Warped Drive
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Thanks!  I think the tube struts as they are are just about stiff enough to be used with my current EP's.  If I wanted to use something like a 30mm ES82 I'd have some problems as they are weighty beasts, though saying that the struts can be stiffened up nicely by retracting them a couple of inches.  The problem then is that you may get some light loss from the secondary as it could be too far forward to intercept all the light from the primary, plus you'd be bringing the edges of the secondary into play which may not be as flat as the center of the mirror.  I haven't done any testing to see exactly at what point of retraction the light loss starts but I'm planning to do it.

The secondary mirror was easy to realign, just loosen the central screw then turn it around ad re-collimate.

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Nice!  When I get a chance to have a go at it, I will probably stick with just a 1.25" rack and Pinion or Crayford if I can find one at the right price, had a quick look earlier when it was quite in the office and most of them seem to cost a lot more than the scope. I currently only have one 2" eyepiece that came with my XT8g. I have a full set of BSTs plus all my filters are 1.25" as well, so no real point in going the 2" rout

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Quick update on this mod...

Have swapped out the 2mm steel flat for some 2mm ally angle (asymmetric 20mm x 5mm). Much lighter and stiffer than the steel (which had started to rust before I got around to painting it).

Also secured the 'sliding' ends of the ally to the OTA with a couple of screws for better rigidity but now I have to remove the two screws in order to collapse the tube, which to be fair I'll only really do when packing it up for holiday. 

I've had this setup out under the stars a couple of times now and I'm really pleased with it :) 

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Edited by CraigT82
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Posted (edited)
On 11/08/2016 at 19:23, CraigT82 said:

Hi all, been busy this afternoon adding a 2" Crayford to my heritage 130p.

My idea is to turn it into wide field DSO travel scope for airline transport to very dark skies (I'm visiting Tasmania I'm January - very excited about super dark southern skies there).

I've always thought it's a great little scope which punches about its weight, just let down by its primitive  1.25" focuser. I bought a TS 2" single speed Crayford to fit to the OTA, this particular focuser is relatively lightweight (500g) compared with other aftermarket crayfords which are closer to 1kg. The weight matters as the trusses will only take so much added before it begins to twist and flex (about 1.3kg by my testing before the collimation is affected). 

I can now use weighty 2" widefield EPs (up to 750g) with this scope, and with a bit more fettling it will be a superb travel scope. 

 

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I like that. Pretty genius. Wouldn't suit me though being in a wheelchair. Although it wouldn't be too difficult to do it where the focuser is on the Heritage.

We'll done.

Edited by LukeSkywatcher

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