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Need Help Achieving Focus for SCT w/Crayford Focuser

cassegrain equipment Meade optics SCT
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#1 adamchicago

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Posted 16 September 2020 - 02:06 PM

Can anyone provide any good tips as to how to achieve focus, literally ANY FOCUS, with this setup?  I struggled for hours last night and couldn't get the main camera nor the OAG camera to achieve any sort of focus.  If it helps at all, the OAG image had a large bright circle in it that I'm guessing means the back-focus is out of whack one way or the other.  I tried adding tube extensions to the OAG and sliding its camera in and out and could never get past this...is it in the right location in this optical train?

 

A couple questions I have...noting I am new to SCTs and only a few months into this hobby all together:

- Do I have the back focus distances correct? ...note: Optec Reducer lens is 100mm to ZWO 1600 camera sensor

- Is the order of the optical train incorrect?

- How should I set the SCT's focuser...should it be turned all the to the right, or to the left (which direction focuses in vs out)?

- What is a good tried and true process for achieving focus?  On my refractor I usually start with the focus all the way in and slowly edge it out, taking a new photo with each new increment

 

Setup:

- Meade 10" F10 LX200 SCT

- Starlight 20-2013 SCT to Feather Touch crayford adapter

- Feather Touch crayford focuser

- Optec 0.62 reducer (no flattening)*

- Optec reducer adapter

- 16mm tube extension

- ZWO OAG...with ZWO 174 mini camera

- ZWO 7 place filterwheel

- ZWO 1600mm camera*

 

* The ZWO 1600mm camera sensor is almost exactly 100mm from the Optec reducer main lens

 

Thank you for any guidance!!!!!

Attached Thumbnails

  • SCT Optical Train SML.jpg

Edited by adamchicago, 16 September 2020 - 02:13 PM.


#2 KTAZ

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Posted 16 September 2020 - 02:33 PM

I would set both the SCT focuser and the Feather Touch focuser at roughly the middle of their travel. The feather touch is for "trim" focusing, so use the SCT focuser and run it all the way in, and then all the way out, until you get to a focus point.

 

Turning the SCT focuser counterclockwise will push the mirror away from you (in towards the corrector) while turning it clockwise will pull the mirror back towards you (out from the corrector).



#3 adamchicago

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Posted 16 September 2020 - 02:52 PM

Great, that definitely gives me a good starting point.

 

I would set both the SCT focuser and the Feather Touch focuser at roughly the middle of their travel. The feather touch is for "trim" focusing, so use the SCT focuser and run it all the way in, and then all the way out, until you get to a focus point.

 

Turning the SCT focuser counterclockwise will push the mirror away from you (in towards the corrector) while turning it clockwise will pull the mirror back towards you (out from the corrector).

 



#4 rgsalinger

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Posted 16 September 2020 - 03:09 PM

I do the following. Move the auxiliary focuser all the way in with the mirror at roughly the mid point and locked. Command the focuser to move all the way out while taking continuous 2x2 binned 1 second exposures and watch my screen. At some point you will almost certainly see stars flash up on the screen. Then stop the focuser movement and refine the focus.

 

If that does not work, move the mirror with the SCT focuser all the way in or out and try it again. I did this just 3 weeks ago and it worked really well for me.

 

Rgrds-Ross



#5 adamchicago

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Posted 16 September 2020 - 03:23 PM

Ahhh, nice technique.  Are you using Astrophotography Tool?  How are you doing the continuous 2x2 binned 1 second exposures...via live view?  If using APT are also doing the image stretch?

 

Thank you!

I do the following. Move the auxiliary focuser all the way in with the mirror at roughly the mid point and locked. Command the focuser to move all the way out while taking continuous 2x2 binned 1 second exposures and watch my screen. At some point you will almost certainly see stars flash up on the screen. Then stop the focuser movement and refine the focus.

 

If that does not work, move the mirror with the SCT focuser all the way in or out and try it again. I did this just 3 weeks ago and it worked really well for me.

 

Rgrds-Ross

 



#6 rgsalinger

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Posted 16 September 2020 - 05:37 PM

Actually I was using MaximDL, but pretty much any imaging software will do continuous exposures to support focusing a camera. I don't know much about APT, sorry. It's would be weird if your software couldn't take continuous exposures to get things focused - it's a common technique when you don't have an automated focusing system. From what I saw you are using a mono ASI1600 camera so downloads should be really fast. If they are not for some reason try to just use a subframe. Again, I don't know how to do this things using APT.

 

Rgrds-Ross



#7 adamchicago

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Posted 16 September 2020 - 06:55 PM

Thanks, yep there's ways to do it in APT but being relatively new to this I didn't know if there was a better method.

Actually I was using MaximDL, but pretty much any imaging software will do continuous exposures to support focusing a camera. I don't know much about APT, sorry. It's would be weird if your software couldn't take continuous exposures to get things focused - it's a common technique when you don't have an automated focusing system. From what I saw you are using a mono ASI1600 camera so downloads should be really fast. If they are not for some reason try to just use a subframe. Again, I don't know how to do this things using APT.

 

Rgrds-Ross

 



#8 michael8554

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Posted 17 September 2020 - 01:18 PM

Initially try finding focus on the Moon, much easier than stars.

 

Although SCTs have a very large range of focus, your setup looks extremely lengthy, so I hope there's enough range.

 

Crayfords are usually mounted much closer to the OTA, without that long adapter.

 

As for setting up the OAG, get the main camera in focus.

 

Eyeball the distance the chip is from the prism.

 

Initially set the guidecam chip the same distance from the prism.

 

Now focus main camera on moon

 

Then fine focus the guidecam, and finally for best HFD or FWHM on a star.

 

Once both are accurately focused do not alter the guidecam, both should come to focus with just the Crayford.



#9 Tapio

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Posted 17 September 2020 - 01:30 PM

Just wanted to ask why did you get this, Optec 0.62 reducer (no flattening), for normal SC scope ?



#10 speedster

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Posted 17 September 2020 - 09:55 PM

The 100mm from corrector to sensor assumes the corrector is attached to the back of the OTA.  It doesn't look like you can physically reach focus with as much length as you have.  I can tell you how I do it with a C8 and you can convert to your Meade.  Changing focus changes the focal length and the scope is designed for minimal aberrations at its design FL.  To find that primary mirror location, put a camera at the designed back focus.  For a C8 that is 127mm from sensor to the back of the OTA threads.  Then focus and you have the primary in the sweet spot.  Leaving that alone, now work on your image train.  Screw on the reducer with "stuff" between the reducer and sensor to get 100mm spacing.  You should be pretty close to focus.  The "stuff" is filter wheel, OAG, and a spacer.  If your focuser will also fit in that "stuff" space, you have it made.  If it's close but not quite, you simply have to focus the primary out of its sweet spot.  Once you have the train as you like it and focused, focus the OAG camera.  Its distance from sensor to prism must be the same as the distance from prism to imaging camera sensor.  I think you'll find it all works well with the focuser and its adapter out of the equation.  The penalty for not sticking to the backfocus distances is increased aberrations and you'll be operating at a different focal length.  The C8 will focus range will take you from f/8.7 to f/14 but the farther you are from f/10, the worse the aberrations, vignetting, etc.



#11 adamchicago

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Posted 18 September 2020 - 11:44 AM

I thought the F10 on the Meade 10" SCT was a bit too slow.  And applying a 0.62 reduction to a 2,500 focal length scope would still result in about 1,550 focal length which is double what my 130mm refractor was produces with a 0.80 reducer/flattener so wasn't worried about cutting that back to 62% of total.  And the Meade LX200 SCTs are already coma-corrected so don't require flattening...as a matter of fact if you try to use a flattener it will stretch out the stars along the edges per what I read.

 

And I FINALLY achieved focus last night and it was nice clean image edge to edge:)

 

 

Just wanted to ask why did you get this, Optec 0.62 reducer (no flattening), for normal SC scope ?


Edited by adamchicago, 18 September 2020 - 12:00 PM.


#12 adamchicago

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Posted 18 September 2020 - 11:58 AM

Just want to thank everyone for their help with this!!  Last night I finally achieved focus with the current setup and it was a nice clean image all the way to the edge.  I'd say the greatest advice given on this was the following:

 

1. Ensure back focus requirements were met via manufacturer recommendations (primarily the 100mms from reducer to sensor)

2. Set the crayford focuser to about midway (this will be used as a fine-tuner on top of the SCT focuser)

3. Start at either end of the SCT focuser and slowly work your in or out in small increments as your camera takes 5 second shots.  I made 3 turns of the SCT focuser between camera shots.  Using APT I put it in LiveView mode with a 5 second exposure and 5 second pause and turned the SCT focuser 3 times on each pause.

4. Once I was able to see at least blurry stars, I fine tuned the SCT focuser manually just a bit and then locked the focuser (Meade has a locking knob)

5. Used APT Auto Focuser tool (with ZWO EAF electronic focuser) to get the focus perfect...I can't say enough good things about APT in terms of focusing, finding, guiding and programming a photo session but that's a discussion for another thread

 

HOWEVER I still have to get the OAG focused but that should be the much easier after the SCTs finally tuned.



#13 Tapio

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Posted 18 September 2020 - 01:11 PM

I thought the F10 on the Meade 10" SCT was a bit too slow.  And applying a 0.62 reduction to a 2,500 focal length scope would still result in about 1,550 focal length which is double what my 130mm refractor was produces with a 0.80 reducer/flattener so wasn't worried about cutting that back to 62% of total.  And the Meade LX200 SCTs are already coma-corrected so don't require flattening...as a matter of fact if you try to use a flattener it will stretch out the stars along the edges per what I read.

 

And I FINALLY achieved focus last night and it was nice clean image edge to edge:)

Original Meade SC scopes are not 'flattened', ACF models are. That's why I asked.




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