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Edge HD Mirror clutches and autofocus

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#1 Alfredo Beltran

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Posted 18 January 2013 - 01:01 AM

Hi

This might be a simple question: when mirror clutches of a Celestron HD telescope are engaged, can the autofocuser still do minor adjustments required by filter change on the filter wheel?

Thanks for your help

Alfredo

#2 bilgebay

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Posted 18 January 2013 - 04:40 AM

Nope. You will need a FT crayford focuser for that.

When you try to move the mirror with locks engaged the scope goes out of collimation.

Hope this helps

#3 Alfredo Beltran

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Posted 18 January 2013 - 06:37 AM

Hi Sedat

Then , when using an automated system as the feathertouch and Microtouch for the HD series, the minor adjustments couldn't be done.

If filters are not completely parfocal (you would need just mirrors in your optical train), this means that mirror should be left with no clutches at the risk of mirror flop, and that's exactly what wants to be avoided when you have the clutches.

Even the crayford should have to be locked in any other telescope design. So I guess that it needs to be chosen between the risk of mirror flop or not to use autofocus adjustments on the fly? :question:

Best regards

Alfredo

#4 freestar8n

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Posted 18 January 2013 - 06:51 AM

For best results I would use OAG and a focuser on the primary focus knob - and no separate focuser. Then you can autofocus, use filter offsets, have no problem with any mirror flex, and you don't need to use the mirror locks. You also have the full backfocus available.

But if you do want to use the mirror locks and are worried about filter offsets - I would measure how big they are in Edge in the first place, because the filters may be more parfocal than, for example, a normal sct with reducer.

Frank

#5 Alfredo Beltran

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Posted 18 January 2013 - 08:00 AM

You make a very good point. The HD should be more parfocal because stars are focuser all through the FOV.

I guess that shouldn't be an issue.

Best regards

Alfredo

#6 bilgebay

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Posted 18 January 2013 - 12:15 PM

Hi Alfredo,

Unless I am shooting in the Hyperstar configuration at f/2, my workflow is as follows:

1 - Focus the camera by using the focuser at the back of the scope (where the focal plane of my camera is at 146mm) . This way the optimal distance between the primary and secondary is established.

2 - Lock the primary mirror locks and forget about the focuser at the back of the scope.

3 - Check the collimation. If it is not good redo it.

4 - Use the FTF2015BCR Feather Touch Crayford focuser for any minute adjustments.

@f/6.7 (with AstroPhysics' CCDT67) and f/10 I always use Lacerta OAG which takes up 11mm of back focus real estate. The remaining 135mm is a lot of back focus for many applications. None of my imaging trains had any problem with that so far.

#7 Alfredo Beltran

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Posted 18 January 2013 - 01:39 PM

Hi Sedat

This is a good procedure. I have a couple of questions:

1. What is your procedure for hyperstar use? I'm interested in using a C9.25 HD with hyperstar and the Canon T1i DSLR.

3 - Check the collimation. If it is not good redo it.


Why is collimation afected?

Thanks for your help

Alfredo

#8 bilgebay

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Posted 18 January 2013 - 02:41 PM

Hi Alfredo,

With Hyperstar, there is no other option, so I use the primary mirror focuser. But then, till now, I have been using it with a DSLR.

With a Mono CCD, focus change between the filters can be a problem. However, for Hyperstar, using filters is a big problem anyway. We are only able to use filter drawers. Filter wheel is not option here...just too big.

A DSLR might be a bit on the large side for the C9. You may want to talk to Dean at Starizona. I am using a 160mm donut mask to hide the silhouette of T2i. Otherwise, you could see the silhouette in all the bright stars. Now, if you deduct 160mm from 235mm the annulus will be a very narrow one(for C11 this means going up from a 100mm central obstruction to 160mm. I don't know the diameter of the secondary housing on C9.25 but it will a bigger increase for sure).

I am waiting to see how this EOS M story will develop. It is quite small and mirrorless. All I needed for the Hyperstar (mirror creates a lot of vignetting). AFAIK, none of the image capture programs are supporting it or there is no remote capture possibility yet. I will wait for a while to see what people can do with this camera. Maybe the second generation will be usable for astroimaging.

#9 bilgebay

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Posted 18 January 2013 - 02:57 PM

Why is collimation afected?


Collimation is affected by the mirror shift, though it is a tiny bit. When shooting at 2800mm FL this can be important. Most of the time I am just checking, the collimation is usually good. For imaging purposes, I collimate the scope after moving the primary inward and engaging mirror locks, all the time. I also come to focus by moving the mirror inward. So, most of the time things go well but once in a while I have to touch up the collimation alittle bit. This has to do with the mirror lock mechanism.

When shooting with the Hyperstar, I prefer not to use the mirror locks. They disturb the focus.

Clear skies

#10 Alfredo Beltran

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Posted 18 January 2013 - 03:23 PM

Hi Sedat

Very useful information about collimation. Something to be considered indeed.

The C9.25 hyperstar is compatible with DSLRs:

http://starizona.com...-P3208C773.aspx

Best regards

Alfredo

#11 wolfman_4_ever

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Posted 18 January 2013 - 06:00 PM

You can use an FLI focuser. It sits on the rear image train and eats up very little backspacing. .35" minimum travel for focus, just perfect and designed for filter changes. Very little backfocus used. Expensive.

#12 ewave

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Posted 18 January 2013 - 06:42 PM

I am waiting to see how this EOS M story will develop. It is quite small and mirrorless. All I needed for the Hyperstar (mirror creates a lot of vignetting). AFAIK, none of the image capture programs are supporting it or there is no remote capture possibility yet. I will wait for a while to see what people can do with this camera. Maybe the second generation will be usable for astroimaging.

Thanks for posting about EOS M, Sedat, but still interesting enough for hyperstar use with up to 60 sec exposure and up to 10 sec timer delay. Still a bit cumbersome. But I agree, a filter wheel would be out of the question for hyperstar use for ccds. I still thinks locking the miror locks a good idea for the Edges no matter which end of the OTA you are shooting from.

#13 wolfman_4_ever

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Posted 18 January 2013 - 08:08 PM

Optec also sells a new secondary focusing system... Moves the secondary in and out..






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