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Focal planes, lengths and focusers

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#1 25585

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Posted 12 October 2019 - 01:07 PM

I have been advised that changing focuser from low profile to higher, will need shortening of the primary to secondary distance in an OTA. This is for a 1700mm F8 scope with a current low profile focuser and small secondary.

 

I want to fit a Baader Steeltrack https://www.firstlig...newtonians.html

 

What are the margins within which focuser height can be changed? I think for the scope in question, perhaps a low profile focuser was used in order to get focus, rather than choosing a focuser first, chicken & egg.

 

Are there any other solutions? Collimating mirrors is bad enough!

  



#2 astro744

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Posted 12 October 2019 - 06:46 PM

If you move the primary up the tube the secondary intercept becomes larger so to maintain off axis illumination you may (likely) require a larger secondary.

 

See Mel Bartels off axis calculator at https://www.bbastrod...om/diagonal.htm

 

It comes down to how much off axis illumination or magnitude drop you are prepared to accept.  Mel used to talk in % illumination (now magnitude drop) and 70% off axis illumination for your widest field stop eyepiece was a good compromise (100% if variable star observer), less if high power planetary observer.  Another rule of thumb was to aim for 100% illumination over 1/2” (12.7mm field diameter).

 

The other thing you should consider is draw tube length and mid point positioning.  You want to make sure when fully rack in your focuser will accomodate every eyepiece you own and ideally too when fully racked out without the need for extension tubes, (the former more critical).  Also need to take into account the extra distance needed for a Paracorr if in use.


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#3 Jon Isaacs

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Posted 12 October 2019 - 07:28 PM

What are the margins within which focuser height can be changed? I think for the scope in question, perhaps a low profile focuser was used in order to get focus, rather than choosing a focuser first, chicken & egg.

 

 

Generally a low profile focuser is preferred because it allows one to use a smaller secondary.  Most likely, the focuser was chosen first and the scope designed around the focuser and tube diameter. 

 

Why not replace the current focuser with another low profile focuser?

 

Jon


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#4 MitchAlsup

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Posted 12 October 2019 - 08:11 PM

I have been advised that changing focuser from low profile to higher, will need shortening of the primary to secondary distance in an OTA. 

Are there any other solutions? Collimating mirrors is bad enough!

The center of the primary to the focal plane is constant.

So, if you move the focal plane outward, the primary will need to move upwards by the same amount.

 

Who is advising you that this is necessary.

The only advantage to a longer focuser (focal plane farther outward) is that the image plane may (MAY) be darker.

There are other ways to make the image plane darker:: 

a) stray light shield

b) Wood glue, sawdust, and flat black paint

c) focuser baffle

...

 

The question you did not address is WHY ?


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#5 25585

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Posted 13 October 2019 - 01:12 PM

The scope's seller has a low profile helical focuser fitted, probably inadequate for big heavy loads. I want a Baader Diamond Steeltrack instead. The seller said the scope's truss poles will need shortening, it was news to me and complicates everything. 

 

Sky-Watcher make a low profile Crayford, but again it may not be strong enough. The Baader also looks stronger, a poor man's Feather Touch maybe.

 

If a focuser's height is so determinate for primary placement, how are different height adapters for eyepiece fit, or switching design - Clicklocks are chunky - accomodated? I know a Clicklock on top of a Paracorr 2 messes up calibrations.



#6 dan_h

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Posted 13 October 2019 - 03:10 PM

The Baader with the flat base option is only 50mm minimum height.   You didn't say how low the current focuser is at focus but it can't be very much lower than the Baader.  The impact on secondary sizing would be very little and I wouldn't worry about that at all.  

 

Depending on just where the current focus position is you may find that the change needed to the truss poles to accomodate the Baader is very little, maybe none.

 

Typically, the focus position is set so that the focuser has enough free in travel to allow for the use of a Barlow or a coma corrector.  The Baader has fairly short travel so setting focus to the centre may be best. This may not be sufficient for all accessories but only you know what pieces you have or expect to have. 

 

 

dan



#7 Jon Isaacs

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Posted 13 October 2019 - 10:01 PM

The scope's seller has a low profile helical focuser fitted, probably inadequate for big heavy loads. I want a Baader Diamond Steeltrack instead. The seller said the scope's truss poles will need shortening, it was news to me and complicates everything. 

 

Sky-Watcher make a low profile Crayford, but again it may not be strong enough. The Baader also looks stronger, a poor man's Feather Touch maybe.

 

If a focuser's height is so determinate for primary placement, how are different height adapters for eyepiece fit, or switching design - Clicklocks are chunky - accomodated? I know a Clicklock on top of a Paracorr 2 messes up calibrations.

 

Is this the 8.5 inch F/8? Are you planning on using a Paracorr?

 

A helical focuser could have a 1 inch height. 

 

The Steel track looks to be a low profile but the link had so specifications I could find.

 

You'll probably have to shorten the tubes but your probably OK with your current secondary. If you cut 2 inches off the poles, at F/8 the effect would the same as reducing the secondary by 1/4 inch.

 

Jon



#8 25585

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Posted 14 October 2019 - 05:27 PM

This is the telescope http://www.dobsonian....5-inch f8.html as is.

 

The low profile Crayford is this one https://www.firstlig...rd-focuser.html but could it take a 3.2lb load, my heaviest eyepiece. I won't use a Paracorr for F8. 



#9 dan_h

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Posted 14 October 2019 - 07:19 PM

This is the telescope http://www.dobsonian....5-inch f8.html as is.

 

The low profile Crayford is this one https://www.firstlig...rd-focuser.html but could it take a 3.2lb load, my heaviest eyepiece. I won't use a Paracorr for F8. 

According to the specs I could find for the Skywatcher, it is not any lower than the Baader Steeltrack with the flat base.

 

https://www.astrosho...ab_bar_0_select.

 

dan


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#10 Jon Isaacs

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Posted Yesterday, 03:58 AM

This is the telescope http://www.dobsonian....5-inch f8.html as is.

 

The low profile Crayford is this one https://www.firstlig...rd-focuser.html but could it take a 3.2lb load, my heaviest eyepiece. I won't use a Paracorr for F8. 

 

The current focuser may just need to have the draw tube and pinion cleaned. Often a Crayford slips because the draw tube flat and pinion are contaminated..

 

That focuser is available with a rack and pinion gear drive.

 

https://agenaastro.c...dual-speed.html

 

Jon


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