Can a F8 refractor become a F10 ??

[Craney]

Afternoon everybody.   Since it is cloudy and there are no sunspots for my new PST  I was musing about mods ( stage 1  etc...).    A good native F10 refractor is needed apparently to make the light cone optimal inside the tube as it hits the etalon.

Not a great deal of donor scopes out there for this purpose.   Then I had a thought,  which may be optical non-sense !!!

I was thinking along the lines of,     take an  100mm  F6   (Fl=600mm)  but put an aperture mask over the objective to limit it to 60mm.  ( I suppose you could incoporate a 60mm ERF into this accessory).

  Since the FL is still 600mm, this effectively becomes an F10.  (??)

Is this correct or am I breaking a fundamental  law of optical physics  ?

Thanks in advance for any info.

Cheers,

Sean.

 

[xtreemchaos]

theres the bresser AR127L at F9.5 which is near, but there a big and heavy scope. charl.

[Craney]

Thanks Charl,    I'm not sure how critical the F10 bit is for the donor scope.  When looking recently I found more scopes available  that were  F6 or F8 rather than  F10.

The F10's  tended to be at the more budget end of the market as well.   This is fine as I think a bit of cosmetic surgery is need on the tube to fit the etalon into the right place.

I would hate to take the hack-saw  to my Takahashi  Fluorite  

 

[Stu]

TAL 100R or Vixen 102mm are both good f10 donors. The f10 part is important as it matches the focal ratio of the PST (40mm aperture, 400mm focal length) and ensure that the mod will work at an optimum.

Your theory is not wrong as far as I'm aware, but it doesn't make sense to use a much bigger scope if you are only effectively using 60mm of it. There are quite a few 70mm f10 scopes around on AstroBoot which would be much smaller and lighter. Like this one...

 

[andrew s]

47 minutes ago, Craney said:

Afternoon everybody.   Since it is cloudy and there are no sunspots for my new PST  I was musing about mods ( stage 1  etc...).    A good native F10 refractor is needed apparently to make the light cone optimal inside the tube as it hits the etalon.

Not a great deal of donor scopes out there for this purpose.   Then I had a thought,  which may be optical non-sense !!!

I was thinking along the lines of,     take an  100mm  F6   (Fl=600mm)  but put an aperture mask over the objective to limit it to 60mm.  ( I suppose you could incoporate a 60mm ERF into this accessory).

  Since the FL is still 600mm, this effectively becomes an F10.  (??)

Is this correct or am I breaking a fundamental  law of optical physics  ?

Thanks in advance for any info.

Cheers,

Sean.

 

Yes stopping it down makes it f10 

 

Regards Andrew 

[nightfisher]

There is a TAL 100 rs cheap on astro buy sell 

[Craney]

Thanks Andrew.   That is a good piece of info.

Cheers Nightfisher, I will have a peek.   A SGL member had a TAL  mod for sale on here only this week.  He said that he did not need to cut the tube down to accomodate the etalon, which is real bonus for somebody as kak=handed as me.

 

 

[Craney]

Thanks Stu,

Yes I was just musing over the possibility of using a different aperture, in case the pool of available ( reasonable quality)  F10's ever dried up.    The one you have pictured is a Skywatcher and should be of a half decent quality.   I supoose it is an advantage having the original focuser removed as you would probably want an upgrade with the replacement having 1:10 fine adjustment capability.

That one doesnt look too expensive either.   Many thanks,

 

Sean.

 

[Stu]

I think @Moonshane uses a 60mm Meade for eaxactly the same purpose, sure there is a thread around somewhere.

[nightfisher]

1 hour ago, Craney said:

Thanks Andrew.   That is a good piece of info.

Cheers Nightfisher, I will have a peek.   A SGL member had a TAL  mod for sale on here only this week.  He said that he did not need to cut the tube down to accomodate the etalon, which is real bonus for somebody as kak=handed as me.

 

 

Sorry, seems it must have sold as i cant find it now, was there just a couple of days back

[Astrobits]

Stopping down the aperture as you suggest is exactly the same as in camera lenses. Varying aperture with fixed focal length gives variable focal ratio although camera lenses do this internally it is perfectly acceptable to do it externally as you suggest.

Nigel

[Craney]

No worries Nightfisher,  somebody has given me a new lead in the quest.

Thanks Astrobits.  Yes, I could not see an obvious flaw in the logic but there is always seems to be an unknown factor to be taken into accounrt.  Aberrration goes as the cube root of the effective aperture minus the refractive index.....or something like that.

 

S.

[xtreemchaos]

me too ,  id need a gold hacksaw to do it, steel wouldn't be worthy. charl.

[Craney]

1 minute ago, xtreemchaos said:

me too ,  id need a gold hacksaw to do it, steel wouldn't be worthy. charl.

...finished off with Fabergé's personal diamond encrusted emery paper as well......

[Peter Drew]

One advantage of using an oversize donor telescope for masking down to F10 is that you can still use it at full aperture for white light and other non solar observations. 

[Peter Drew]

A Bresser AR 102L would make a good 100mm PST mod at a reasonable price.   

[Craney]

The Bresser looks tidy.    So is it possible to use a scope like this as a Stage 1 mod without butchering the tube length ??

...and then again there is the ERF to consider. They become a tad expensive when the aperture gets to 102mm.....   

Sean.

[Astrobits]

The main problem when going to ever larger apertures for solar is the extra energy collected by the larger objective.  Your ERF has to handle more than 50% extra energy when going from 80mm to 102mm aperture. Hence the increased cost.

Nigel

[Merlin66]

The PST mod will always function as an f10 "aperture stop".

You don't really need a focuser between the OTA and the PST etalon. Once in final position (-200mm inside prime focus) - it never moves!

For the stage 1 mod - the existing PST focuser in the Black Box is used, for the Stage 2, yes a focuser (a Borg helical works well) is needed after the PST etalon to allow more cameras/ eyepieces to be accommodated.

Ken

 

[Moonshane]

https://stargazerslounge.com/topic/268088-60mm-pst-mod/