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C11 edge focuser options advice.

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#1 XSubmariner

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Posted 05 January 2019 - 09:24 AM

Hi, I have recently acquired a C11 edge with 0.7 reducer for Astrophotography. It is fitted with a Starzona touch light focuser, is it ok to use this focuser with SGPro and its auto focus routine or will I need an additional Crayford Focuser for fine adjustments under autofocus. 

Secondly am I likely to experience unacceptable primary mirror flop if I stick with just the starzona unit?

New to long FL sct’s any advice is greatly appreciated.

Thanks in advance, Martin


Edited by XSubmariner, 06 January 2019 - 05:01 AM.


#2 Crashcourse

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Posted 06 January 2019 - 12:35 PM

I have the EdgeHD 11 with the .7X Celestron Reducer (and a Hyperstar III). For focusing I use a Starizona Microtouch/Feathertouch Focuser on the mirror and a Feathertouch FTF2008BCR (just .8" travel) Crayford focuser with the "Shorty" adapter, also with a Microtouch Focuser controller. It was pretty tight getting the correct backspacing but we got it after trying several adapters.
I'll make sure my Crayford is racked exactly halfway (so the sensor is at 146.5mm backfocus) then get the best focus I can with the mirror, lock the mirror, then switch over to the Crayford and let SGP do all the work. :)



#3 f430

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Posted 06 January 2019 - 03:05 PM

I've been using an Optec fastFocus unit for over a year now, and it's outstanding! This focuser adjusts the secondary mirror, so you just lock down the primary mirror and forget about it. About the only drawback is that it obviously won't work with a Hyperstar unit.

 

Cost is a bit more than others, but is well worth the bucks, I think.

 

https://www.optecinc...focus/index.htm

 

As for the long Focal Length imaging, don't let it scare you. With a good mount, good guiding, its really no big deal. True it takes a bit of practice and learning, so start at f7 and pick easy targets.

Mirror flop is something I've seldom seen, and once collimated the scope seems to stay collimated,,, at least mine does.

 

Enjoy!


Edited by f430, 06 January 2019 - 03:14 PM.


#4 XSubmariner

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Posted 07 January 2019 - 04:27 PM

Thanks crashcourse and F430 for your comments. Having reviewed the Optec unit it is well outside my affordability at this time.



#5 gcardona

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Posted 07 January 2019 - 04:40 PM

I have motorized both the stock focuser (for use with Hyperstar) and my Clement focuser (for use at F/10 and F/7). The focus with the Hyperstar is much more critical due to the fast focal ratio, and I find the stock focuser works very well with SGP's auto-focus routine. You have to set the back-lash compensation to have it finish every focusing movement in a CCW direction to minimize mirror shift. Once this is done, it holds focus quite well. I re-focus every 1/2 hour or so.



#6 gomezdb

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Posted 07 January 2019 - 04:52 PM

As mentioned by f430 above, Optec FastFocus is the way to go with the Edge HDs. It's expensive but it will allow you to do continuous focus while keeping you primary mirror locked. On the other hand, it doesn't eat anything of the backfocus available.

 

Unfortunately, it is not available for the 8" or 9.25 Edge HD series.

 

Cheers,

Diego



#7 chadrian84

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Posted 08 January 2019 - 11:51 PM

Moonlite Crayford Edge 2.5" focuser is a great option for the Edge 11 and .7x reducer.  I purchased mine a couple months ago and I'm happy with it.  Just set the 146mm backfocus approx. in the middle of the 30mm? tube travel, focus the primary by hand, then lock the mirrors down.  

 

I'm considering selling it and purchasing a "Litecrawler" (not shown on their site) in order to have a motorized rotator.  The litecrawler is about $2100, the 2.5" Crayford is$ 800 for the focuser + motor + controller.

 

I used to use a non-crayford feathertouch with starizona micro-touch on my Edge 8.  It required the mirrors be unlocked.  I didn't have too many problems with mirror flop, but if you can afford a crayford it's a better choice.



#8 orthogonal

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Posted 10 January 2019 - 09:32 AM

Celestron has announced its new Focusing Motor for SCT's and RASA telescopes. Comes with ASCOM and native support. Relatively cheap in price at $199.95 USD. Available soon. Stay tuned.

 

https://www.adorama....ASABEgJ3avD_BwE


Edited by orthogonal, 10 January 2019 - 09:37 AM.

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#9 freestar8n

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Posted 10 January 2019 - 05:17 PM

Celestron has announced its new Focusing Motor for SCT's and RASA telescopes. Comes with ASCOM and native support. Relatively cheap in price at $199.95 USD. Available soon. Stay tuned.

 

https://www.adorama....ASABEgJ3avD_BwE

Hi-

 

This is great to see - and I didn't know about it until you posted the link.  Here is a direct link to the celestron page:

 

https://www.celestro...-sct-and-edgehd

 

I have my own stepper motor connected to the primary focusing knob and have been focusing this way with EdgeHD11, EdgeHD8, and C11 for many years - and it works very well despite its simplicity and low cost.  I have posted focus curves and more recently temperature dependent plots showing how well it works and how reproducible it is - along with deep sky images with low fwhm.

 

It works very well with autofocusing routines as long as you include about 1/4 to 1/2 turn of the knob for backlash compensation - and focus by turning ccw.  There is no need to apply mirror locks.

 

There is some confusion about the difference between f/10 focusing and f/2 (hyperstar).  If you focus by moving the primary mirror then f/2 is no different from f/10.  In fact, if the min fwhm you get is larger at f/2, in arc-seconds, then hyperstar is actually less critical than focusing at f/10.  I have numerous examples at f/10, f/7, and f/2.

 

Finally - there have been exaggerated reports of temperature drift with SCT's and a need to refocus rapidly to track it.  I have posted multiple studies of focus drift while imaging that show there is little need to focus more than about every 40 minutes - unless the scope is somehow far from equilibrium at the start.

 

So - this is a fortunate example where the simplest and lowest cost solution ends up also having many advantages - including no loss of backfocus - and a fully rigid and lightweight imaging train.   Now that celestron offers a low cost focusing motor that attaches directly - it is even better.

 

Frank




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