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Diagonal size

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#26 Jeff Morgan

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Posted 15 April 2019 - 10:02 AM

I ran into the same problem with my Definitive Newtonian Reflector  (see below).  I could get away with a .75' diagonal but collimation was a nightmare and I changed to a 1" diagonal.  There really wasn't any discernible degradation of the image.

 

It seems of all the Newtonian design trade-offs, the downsides of this one are the least painful.



#27 andreww71

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Posted 15 April 2019 - 11:07 AM

I don't need to butt into a thread with the fancy people, but since the SIPS makes the focal plane/apex inaccessible to the eye, why not use a collimation technique that doesn't depend on seeing the primary edge in the secondary? (Barlowed) laser, cross-haired sight tube, defocused star, etc.

I've never depended on the sight of the primary in the secondary when I collimate and I don't THINK that I'm doing it wrong. If you can afford a 22 and a SIPS, then maybe you can also afford a Glatter laser and tublug?

But, as Mel Bartels also says, it's probably not worth worrying about diagonal size too much.

drneilmb,

 

I have both the Glatter Laser and TuBlug and use them every time I collimate. I do, however, find it very useful to see the entire primary reflection in the secondary. As others have mentioned, collimation is somewhat of an art and I'm surprised how often that even with the Laser tools that I use (and they are nice!) when I place one of my Tectron tools in the focuser I am often able to see things that aren't right and make corrections.

 

I will add that I am far more diligent about collimation now than I was in my younger years and the images that I get from the 22" show it.

 

Andrew


Edited by andreww71, 15 April 2019 - 11:35 AM.


#28 andreww71

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Posted 15 April 2019 - 11:22 AM

Starman1,

 

In the case of the SIPS, the focal plane position that is used to size the secondary is inside of the SIPS assembly itself. The focuser mechanism is connected to the top of the SIPS assembly. In my scope, the top of the focuser is about 1.75" further away from the focal plane when racked in all the way. This is why when using the 4" secondary I could not see the whole primary mirror. The 4" secondary is pretty close to the smallest sized secondary that could be used in the optical system.

 

Andrew

 

 

 

I was referring to tools you look through.

Andrew mentioned the Tectron collimation tools, which are sight tube, Cheshire, and autocollimator.

But, technically, a laser should have its source at the focal plane in use..



#29 andreww71

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Posted 15 April 2019 - 11:28 AM

Jeff,

 

I agree. In my situation I went from an 18% obstruction to 20% - nothing I would ever notice. It does become somewhat substantial however when you get into the really big secondary mirrors and their associated prices.

 

Andrew

 

 

It seems of all the Newtonian design trade-offs, the downsides of this one are the least painful.



#30 Starman1

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Posted 15 April 2019 - 01:55 PM

Starman1,

 

In the case of the SIPS, the focal plane position that is used to size the secondary is inside of the SIPS assembly itself. The focuser mechanism is connected to the top of the SIPS assembly. In my scope, the top of the focuser is about 1.75" further away from the focal plane when racked in all the way. This is why when using the 4" secondary I could not see the whole primary mirror. The 4" secondary is pretty close to the smallest sized secondary that could be used in the optical system.

 

Andrew

If it functions as the insertion-style Paracorr, and I think it does, the original (mirror's) focal plane is about 9mm above the outer-most lens of the SIPS when the SIPS is properly positioned.

The New focal plane is, of course, moved outward quite a ways or eyepieces would never come to focus.

 

Like the insertion-style Paracorr, that puts the original focal plane well down inside the focuser drawtube.

If you remove the SIPS for collimation, that would make it impossible to have the pupil of your collimation tools at the focal plane of the mirror.

That could account for not being able to see the edge of the primary.

But, if you collimate with the SIPS in place, the new focal plane should be far enough out to do so.



#31 hakann

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Posted 15 April 2019 - 04:51 PM

Andrew,

As Don say your focus pt is 9 mm outside lens, so what you do when you rack the focuser is only get your EP in focus.
Your newer really change the L-distance here.

What that mean in collimation I can’t say but, but if I guess the laser beam will go out from where you has the telescopes std pt.

#32 Vic Menard

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Posted 15 April 2019 - 05:07 PM

...What that mean in collimation I can’t say but, but if I guess the laser beam will go out from where you has the telescopes std pt.

The laser beam is axial alignment--that's always on the axis.

A better question is, "How much do you offset the secondary mirror?" Do you offset to balance the illumination at the Paracorr field lens, the unused Newtonian focal plane, or the new focal plane above the Paracorr?


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#33 andreww71

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Posted 15 April 2019 - 07:05 PM

If it functions as the insertion-style Paracorr, and I think it does, the original (mirror's) focal plane is about 9mm above the outer-most lens of the SIPS when the SIPS is properly positioned.

The New focal plane is, of course, moved outward quite a ways or eyepieces would never come to focus.

 

Like the insertion-style Paracorr, that puts the original focal plane well down inside the focuser drawtube.

If you remove the SIPS for collimation, that would make it impossible to have the pupil of your collimation tools at the focal plane of the mirror.

That could account for not being able to see the edge of the primary.

But, if you collimate with the SIPS in place, the new focal plane should be far enough out to do so.

 

The installation instructions for the SIPS specifically state that you can ignore the barlow factor and the effect on the light cone that the paracorr lens system itself has and simply place the focal plane 65mm from the surface of the focuser mounting board. I would estimate that in my set up, the focal plane is about 70mm from the front lens in the SIPS lens unit.

 

To use the visual collimation tools you need to remove the SIPS lens assembly anyway so the effect of the lens system is a non-issue. You wouldn't have the P2 in while using the Tectron tools. I can leave the lens system in when using the Glatter Laser and TuBlug though.


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#34 Starman1

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Posted 16 April 2019 - 12:16 AM

The installation instructions for the SIPS specifically state that you can ignore the barlow factor and the effect on the light cone that the paracorr lens system itself has and simply place the focal plane 65mm from the surface of the focuser mounting board. I would estimate that in my set up, the focal plane is about 70mm from the front lens in the SIPS lens unit.

To use the visual collimation tools you need to remove the SIPS lens assembly anyway so the effect of the lens system is a non-issue. You wouldn't have the P2 in while using the Tectron tools. I can leave the lens system in when using the Glatter Laser and TuBlug though.

My point was that if you remove the lens to collimate you won't use your tools at the focal plane. If you use a tool with its pupil well outside the focal plane, you won't see the whole primary which might lead you to believe the secondary is too small when it is not.

Edited by Starman1, 16 April 2019 - 12:17 AM.

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#35 andreww71

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Posted 16 April 2019 - 05:02 AM

My point was that if you remove the lens to collimate you won't use your tools at the focal plane. If you use a tool with its pupil well outside the focal plane, you won't see the whole primary which might lead you to believe the secondary is too small when it is not.

I agree with everything you state – it’s basically what I wrote in the second to last paragraph in my post 19. The location of the top of the focuser in the SIPS unit is not at the focal plane – it’s another 1.75” further away from the secondary mirror than the focal plane is. This created the problem for me in not being able to see the primary mirror. Although a 4" secondary did meet the minimum requirements, in my opinion, it's of little value if I can't collimate the telescope properly using the tools I have available.

 

Current discussions by some members here show they are relying heavily on the Bartels calculator and its results to size their secondary. For those who are also incorporating the SIPS focuser I chimed in with my own personal experience with the SIPS to provide some additional information to assist in choosing their secondary.

 

All this has been great discussion and I think that the point I was trying to make is understood.

 

This is a question for those following this thread: how important to you is it to “see” the entire primary mirror reflected in the secondary mirror when collimating?

 

For the record and I’ve stated this before, I’m very impressed with the SIPS and would recommend it to anyone. It’s by far the nicest focuser I have used.


Edited by andreww71, 16 April 2019 - 11:46 AM.


#36 hakann

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Posted 16 April 2019 - 01:12 PM

Andrew,

I has the same 'focuser' from Starlight, but the PII.
-All insights is good insights.

I had a longer thread that actually got stopped on my question as some say the PII had a advantage over the Sips ( optically wise )
But in reality it’s the same lens system.
Why I got the indicate that the PII was 'better' - was never cleared out here.

Some like the PII for a easy slip in and use, and some just love the Sips as one adjust direct focus by adjust the focuser to each EP.
Some say the Sips will dew more easy and the deal to aply it in vs drop it of greasy fingers.
I kept my PII as the only disadvantage I see is the bad rolled top design, but if one accept the huge sag ( run-out from centre ) it is rather simple to use.
But both this system will be long & tall and very heavy in end.
Allof of weight is at end of the PII thread and that deal is really losy.

Asa full adopted PII can be from mirrors edge and out near half a meter long ,-)"

Edited by hakann, 16 April 2019 - 05:16 PM.


#37 mark cowan

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Posted 16 April 2019 - 01:45 PM

There's no difference either optically or in the geometry of the unit in operation. SIPS is a PII in a focusing holder.



#38 hakann

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Posted 16 April 2019 - 05:17 PM

Mark
Yes, that is the fact here.


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