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Edge hd Focuser

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#1 Ken82

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Posted 22 November 2019 - 12:58 PM

So what options do I have if I want to add a Focuser to my 11”edge Hd for imaging ?

I would like to use it at f10 and with the focal reducer at f7.
Of course spacing would need to make 146mm so it would have to be less than 91mm long to accommodate my current camera/filter wheel.

Thanks

#2 smccully

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Posted 22 November 2019 - 01:20 PM

Moonlite has a version of the NiteCrawler called the LiteCrawler that takes 2.7" (68.58mm) of Backfocus. Its not available on the EStore, but if you contact them you can find out more. Unfortunately, with Camera, Filter-wheel, OAG and soon Adaptive Optics. I am trying to figure out if I can give up that much back focus to accommodate it.

 

QHY600 Camera - 23.5mm (but maybe able to bring that down with QHY's custom adapter, not sure how much it actually free's up). 

QHYCFW3-L - 21.5mm

QHYOAG-L - 10mm

SXV-AO-LF - 44mm 

-----------

Total 99mm 

 

Thats without any adapters. So trying to decide if Focuser or Adaptive Optics is the better add-on. 



#3 Janco

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Posted 22 November 2019 - 01:22 PM

Moonlite Focuser works fine for me.

Sent from my SM-G975F using Tapatalk

#4 Crashcourse

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Posted 22 November 2019 - 01:25 PM

I use this 
http://starlightinst...9&product_id=58

and this 
http://starlightinst...&product_id=107
on my Edge 11. 
Holds the OAG, 7 position wheel and camera no problem.


Edited by Crashcourse, 22 November 2019 - 01:27 PM.


#5 Benni123456

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Posted 22 November 2019 - 02:04 PM

I do not really understand why one would want to add a focuser to an sct.

Why not simply add an ascom focus motor to that knob?

 

Celestron now has a focus motor for its scts and there are many third party systems that give motorfocus. JMI motorfocus has some. And they can be used via usb by a shoestring adapter.

 

It is true that the knob at the sct has a bit "backslash" but the drivers of these motors can take care of that, 

 

What I know is that the smallest changes I can make to the focus knob via motor seem not to affect the sharpness and image.

 

So I guess the motor suffices. 

 

There appears not much need to add a focuser and then a focus motor, if one can motorize the knob itself. 


Edited by Benni123456, 22 November 2019 - 02:04 PM.


#6 Janco

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Posted 22 November 2019 - 02:11 PM

I do not really understand why one would want to add a focuser to an sct.
Why not simply add an ascom focus motor to that knob?

Celestron now has a focus motor for its scts and there are many third party systems that give motorfocus. JMI motorfocus has some. And they can be used via usb by a shoestring adapter.

It is true that the knob at the sct has a bit "backslash" but the drivers of these motors can take care of that,

What I know is that the smallest changes I can make to the focus knob via motor seem not to affect the sharpness and image.

So I guess the motor suffices.

There appears not much need to add a focuser and then a focus motor, if one can motorize the knob itself.

Well there is a few reasons actually:

1) Mirror Flop while focusing and you can safely engage mirror locks and focus with the crayford focuser

2) Easily be able to rotate the camera sensor using the focuser

3) Able to achieve critical focusing for planetary work without image shift.

4) Be able to set the camera at variaty of backfocus distance to ensure optimal performance (as can be seen on other posts, variations fron the 146mm back focus distance can occur).

And I am sure some other reasons will come up

Sent from my SM-G975F using Tapatalk

Edited by Janco, 22 November 2019 - 02:13 PM.

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#7 freestar8n

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Posted 22 November 2019 - 05:42 PM

If you have backfocus room to add a separate focuser to EdgeHD then there would be some advantages since you can lock the mirror locks and there would be less backlash.  But at the same time you trade off a rigidly attached image train for a system that has all that weight sliding in and out on a tube - and that by itself could introduce some flexure.

 

I think most image trains would have an element that allows rotation, such as the filter wheel or oag, so I don't think you need a focuser for that.

 

For planetary work I agree that a separate focuser is preferred - but in that case you don't usually have OAG so you have more room for a focuser.

 

In my case I have a Pyxis rotator plus OAG and filterwheel, so I don't really have room for a separate focuser.  But leaving the mirror unlocked and focusing with a simple stepper on the primary focus knob works extremely well and allows me to get repeatable focus curves and small fwhm.  The only real disadvantage is the added time needed to unwind backlash.  But there is no issue with mirror flop since the only 'flop' that happens is a slight shift of the object as the mirror goes back to unwind backlash.  But once it moves forward into focus - the object is back where it started.  And for me the shift is only about 15-30".

 

So - I recommend just a stepper motor on the primary focus knob - combined with autofocus software.  You don't need to mess with the mirror locks - and it keeps the imaging train rigid and simple.

 

Frank



#8 Benni123456

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Posted 22 November 2019 - 07:05 PM

well said frank. There may be some almost defect edges that unattentive celestron people may have sold which have had large mirror flop. But usually, this should move the star a few pixels at 100%. so you can usually well use an apo Barlow or something and still focus with the primary and a motor.

 

I guess the main disadvantage of a focuser and a motor is that it costs more than just a motor.

 

For rotation, there are adapters. But i think rotation can be quite problematic.

At first you think rotation is good because it frames your subject. But it isnt. few subjects are so large that they fit in the edgees view only diagonally.

 

A rotation adapter that can be used manually guarantees that every imaging session, you will have a different rotation angle. This only creates problems during stacking. A few degrees with a different angle and you will see that large portions of the images become unusable if you stack these raws together. Pixinsight can do mosaics well, but with different rotation angles you have 50% less images at certain regions, and this means your edges become noisy...

 

As for a focusser, the only benefit seems to be to roam through different backfocus positions fastly.

 

But i think that too is a risk with the edge. The sharpness of the edge reacts to differences of 0.1mm in the backfocus.

 

Do you reliably know at which position, up to 0.1mm, your crayford was at the last observing session? If not, have fun to find the correct backfocus position exactly again...

 

So for the edge, i really do not see any use of a focuser. This is a scope that should be used with one fixed backfocus only. You should not change that....


Edited by Benni123456, 22 November 2019 - 07:06 PM.


#9 Hobby Astronomer

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Posted 22 November 2019 - 07:05 PM

I acquired a nightcrawler for the 8 edge hd. Very nice. Brings focus to the limit of seeing when you run the v-curves. Amazing....

 

Worth the price waytogo.gif waytogo.gif

 

HA


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#10 smccully

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Posted 22 November 2019 - 07:45 PM

4) Be able to set the camera at variaty of backfocus distance to ensure optimal performance (as can be seen on other posts, variations fron the 146mm back focus distance can occur).
 

 

Can somebody explain this to me? or point to some other thread on this subject. How does adding the focuser change the back focus?



#11 Benni123456

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Posted 22 November 2019 - 08:07 PM

He means that, unfortunately, the celestron specs are sometimes wrong.

 

For example, in some other thread, i think i have shown extensively that, while the edge 800 with celestron 0.7 reducer has an advertised backfocus of 105mm, i get better, more precisely, much better! images at 136mm with the reducer, and it may even be better a bit further out...

 

 

In that case, a crayford enables you to change the backfocus quickly by moving out, then focus the primary with the knob and make some images. Then repeat until you have found the correct distance which should deliver the most pinpoint and smallest stars.....

 

 

but actually, this is a problematic argument. It will get easier to find that distance, but then you always have to remember this distance and set your focuser up to that before imagin... i guess if one has found the correct backfocus, it is better to switch for a fixed tubelength



#12 freestar8n

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Posted 22 November 2019 - 08:26 PM

Can somebody explain this to me? or point to some other thread on this subject. How does adding the focuser change the back focus?

When you have a separate focuser you can change the primary focus knob and then re-find focus with your separate focuser - and as you make those changes you should find a sweet spot where the overall field is sharpest and you don't see astigmatism at the edges.  The system focal length will also change as you do that.

 

You can also dial this in by adding separate spacers if you don't have a separate focuser - but having a separate focuser makes it easier.

 

The specs for EdgeHD 9.25 and above suggest an optimal backfocus of around 146mm but I don't think anyone knows the variance in that value - and it is probably worth trying different spacings to see where the field looks best.  A separate focuser makes that easier - but it isn't essential.

 

Frank



#13 smccully

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Posted 22 November 2019 - 09:31 PM

When you have a separate focuser you can change the primary focus knob and then re-find focus with your separate focuser - and as you make those changes you should find a sweet spot where the overall field is sharpest and you don't see astigmatism at the edges.  The system focal length will also change as you do that.

 

You can also dial this in by adding separate spacers if you don't have a separate focuser - but having a separate focuser makes it easier.

 

The specs for EdgeHD 9.25 and above suggest an optimal backfocus of around 146mm but I don't think anyone knows the variance in that value - and it is probably worth trying different spacings to see where the field looks best.  A separate focuser makes that easier - but it isn't essential.

 

Frank

 

Thanks Frank, Benni for that explanation. Ive seen this mentioned and had me wondering if the Crayford changed the back focus somehow.  




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