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Focus options with mono ccd & filters

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#1 Gray

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Posted 29 January 2013 - 04:50 PM

Hello again,

I have decided to revisit the option of auto-focus while imaging with my mono ccd + filters. All filters are supposed to be parfocal. I'm trying to use my AT 72ED, since it is easier to setup on my mount. If anyone knows any other options that would be cheaper than autofocus; please let me know. I'd like to automate the imaging with Main Sequence software.

My problem is once I start out, I find a bright star and use a focus mask with my first filter. Then I slew to the desired object, make sure it is centered in the f.o.v. and take an exposure. Now, when I'm through taking exposures in that filter, I must switch filters and repeat. But the focus will be off unless I slew back out again. I can do it with a LXD75 using high precision mode and wish Synscan had that option. But the point is what can I expect from automated focus.

If I automate focus, can I remain on target throughout the entire imaging sequence?
Is there a way to use a mask while on target if dim? (Somehow, I think no.)
Should I just order the auto-focus option for my scope and be done with it? Is it worth it??

Also, does anyone know where I can outfit my stock focuser with what I need. I'm assuming an encoder based stepper, that will fit the Astro-Tech 72ED refractor.

Thanks for following along. Any help will be much appreciated. :bow:

This post should self destruct if no answer back.

#2 oo_void

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Posted 29 January 2013 - 10:11 PM

First, yes, you can use a filter on a somewhat dim star, but you have to up your exposure times. You can check with rigelsys.com if you're set on getting a stepper. He might be able to rig something up for you.

As for the procedure with a stepper, this is where automation software and MaximDL come in handy. Typically, the software will move to a focus star, refocus with the new filter on that star, then reslew back to the target. If you use Maxim, it'll also plate-solve and get you pretty darn close to where you started.

#3 microstar

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Posted 30 January 2013 - 01:56 AM

MainSequenceSoftware's Sequence Generator Pro will also do autofocus and no need to slew to a bright star - just make sure your autofocus exposure duration for each filter is long enough. You do need an absolute positioning focuser however (stepper motor/encoder). Mine is a Moonlite. Nice but not the cheapest solution. I'm assuming your FW is also automated?
...Keith

#4 Alex McConahay

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Posted 30 January 2013 - 09:26 AM

>>>>But the focus will be off unless I slew back out again.
Your slewing should not be changing your focus. I am assuming you mean you must slew to a brighter star. Not necessarily. If you have truly parfocal filters (and a reflector or truly apo refractor system) it should be within the critical focus zone. Furthermore, as has been pointed out, you can focus on a dimmer star, you just need longer exposures.

See below about focuser offsets.

>>>>>>If I automate focus, can I remain on target throughout the entire imaging sequence?

You can, although fully automated systems slew around to find and center ideal focusing target stars, and then automatically slew back to the target.

>>>>>>Is there a way to use a mask while on target if dim? (Somehow, I think no.)

Not that I know of, but when you automate, you don't use a mask.

>>>>>Should I just order the auto-focus option for my scope and be done with it?
Yes, eventually, you will want an automated focuser. Be aware that it will cost $400 plus dollars for a robofocus, and you will want only a decent focuser to use it on.

>>>>>>Is it worth it??
That is a personal question. Depends on what money and the satisfaction of automated imaging is to you. But, as I said....it is what the big boys use.

>>>>Also, does anyone know where I can outfit my stock focuser with what I need. I'm assuming an encoder based stepper, that will fit the Astro-Tech 72ED refractor.

The Robofocus and other focusing systems come with the necessary parts. But, make sure you have a quality focuser first.

About focuser offsets: Once you have automated, you will get a numerical readout of your focuser position. You can use that to calculate offsets for your system. You get a reading for where each of the filters focus compared to one of the filters. Say your Luminance is the standard, and you consider that at "0." Your red focuses at +12 steps, blue at -8, and Green at +4. You can go to a star, focus using your Luminance filter, and then when it comes time to focus the others, simply tell the focuser to move 12 steps out from that Luminance focus for the red filter, 8 steps back for the blue, and 4 steps out for the green.

Or, you could take an hour worth of one filter, then an hour of another, etc. to minimize the need to focus so much. Of course you should focus regularly anyway to compensate for atmospheric refraction and changing focal length due to temperature.

Alex

#5 Gray

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Posted 30 January 2013 - 03:14 PM

Thanks guys

oo void, I have submitted my measurements to Rigelsys. He was quick to help me. Although I poised a problem, because the dimensions of my stock focuser are not the same as an AT66. I measured the best I could to give him some decent numbers to work with. I haven't heard from him in a couple days now. Maxim software is nice, but too much for me at this point. Main Sequence was suggested at first by Rigelsys. Much appreciation for that.

microstar, Yes my filter-wheel is motorized and will be part of the automation. I was thinking of that when I made the decision to buy :) It's kinda looking like I will have to buy another focuser to boot. Then buy the stepper config. Looks like Moonlite will be the way to go, unless someone else offers a better idea.

Thanks for your input Alex. I'll have to check into Robofocus. I believe I can get by with Moonlite's offer too. I'll have to call. I need to make sure that I do not need a manual control box, just to get started with computer automated focus and imaging sequences with my filter-wheel. I checked Moonlite's prices, and it is $465 for a new CR refractor single speed focuser with the 72 Flange and a "Stand Alone" high res stepper motor attached. Hopefully this is all I will need.

Thank you all for the help. It is expensive but fascinating to combine all of the technologies together and make them work like they were designed to do. It should make my imaging a whole lot easier. If I go with M.L., I'll have to take the TV 3x barlow & ES 18mm 82 off my upgrade list :)

#6 Gray

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Posted 30 January 2013 - 10:11 PM

Scratch that. Moonlite says I need a focus motor control board too. So grand total is now $805; (Moonlite $715) + ($90 Software).=OUCH

#7 microstar

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Posted 30 January 2013 - 11:40 PM

Scratch that. Moonlite says I need a focus motor control board too. So grand total is now $805; (Moonlite $715) + ($90 Software).=OUCH


True, automation is expensive. The Rigel Systems stepper and controller seems to be the least expensive option since it uses your existing focuser. I'd be inclined to wait and see if Leon can make it work. You might also want to post to the MainSequenceSoftware Yahoo group to see if others are auto focusing with the Rigel Systems USB-nStep and if they have had any problems with it (in fact, I'd be interested in the responses to that question myself as I'm looking at the Rigel system for my second scope).
...Keith

#8 microstar

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Posted 31 January 2013 - 12:11 AM

Scratch that. Moonlite says I need a focus motor control board too. So grand total is now $805; (Moonlite $715) + ($90 Software).=OUCH


True, automation is expensive. The Rigel Systems stepper and controller seems to be the least expensive option since it uses your existing focuser. I'd be inclined to wait and see if Leon can make it work. You might also want to post to the MainSequenceSoftware Yahoo group to see if others are auto focusing with the Rigel Systems USB-nStep and if they have had any problems with it (in fact, I'd be interested in the responses to that question myself as I'm looking at the Rigel system for my second scope).
...Keith


In fact, I did ask the question myself on the MainSequenceSoftware Yahoo group. I keep this thread posted on responses.
...Keith

#9 jerryyyyy

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Posted 31 January 2013 - 12:18 AM

Can I slip a question in here???

I am about ready to go with an autofocus as I now have Maxim DL/CCDComander working all night.

I have a C8 and the options seem to be custom for the C8, about $500... Is there any solution that would allow me to keep the autofocuser and maybe move up to a C11 or similar scope? Or, do you really have to go custom with the tube you have?

#10 Gray

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Posted 31 January 2013 - 12:23 AM

Thanks Keith. Yes, rolling an $805 figure around in my head is getting less attractive as time wears on. I have yet to receive a reply from Leon. I'll wait. I still have about a week before I start pulling triggers so to speak.

I have a detailed list of other goodies I was going to focus on but I am re-visiting this idea, since I'll have the funds to do it shortly, but I must get a grasp of the benefits entirely. Frankly, that's a lot of money for one scope specific part. :grin: It would make imaging life a whole lot easier though.

On a side note, I even called the Mighty Kong's store and the fella told me to shoot them an email regarding the AT 72's focuser because he didn't know. Must've been a new salesman? IDK. Seems like the Kong would know though. Of course I get it, the stock focuser would have to go. Again, that's a lot of money for a small ED scope. Sorry to ramble on. Thanks again & CS.

#11 SL63 AMG

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Posted 31 January 2013 - 01:50 AM

I'll post my 2 cents worth here regarding my recent experience with learning to autofocus.

I have two systems, an ASA N10 10" Astrograph with a high end ASA OK3 focuser with a .85 micron step size and I have an AstroTech AT12RC with an FLI Atlas focuser also with a .85 micron step size.

Both focusers perform well. It took me a lot of time and patience to setup FocusMax with each of these systems. I measured my filters using CCD Autopilot to determine my focus offsets between my Astrodon LRGB filters.

What is most interesting is the behavior of focusing during an image run. Particularly, with the AT12RC which is an aluminum OTA and is more susceptable to temperature changes than my carbon fiber OTA of the ASA Astrograph.

I have noticed that when trying to get temperature compensation working properly that not only does the temperature change the focus, but also the angle of the object in the sky changes the focus for the same filter across a set of images.

in other words, while temperature is dropping which would force me to refocus, imaging from east to west as the image rises and is susceptable to less atmosphere, that changes the focus in the opposite direction of the temperature drop.

So as I image and temperature drops, to remain in focus I would need to extend the focuser, however, because the image is rising to less atmosphere, the offest becomes less.

The opposite is true when imaging from meridian to west. The offest becomes greater as the object moves west and the temperature continues to drop.

I must say that my time spent learning to autofocus using temperature compensation and filter offsets has been challenging.

I still have much experimentation to perform as I am studying autofocusing. Needless to say, the challenge is both exciting and frustrating.

One thing for certain. If you like the technical aspect of this hobby, the time you will spend learning to autofucs properly will keep you busy and entertained for weeks, perhaps months.

I am now imaging most of my red data while the object is lower in the sky. I image my green data midway between the horizon and the meridian and as the object nears the meridian I capture my blue and luminance images.

I try to capture all my luminance images near the meridian, on either side, as I obtain the sharpest focus through less atmosphere.

I also break the image run to refocus when I can. I have some difficulties doing this because of the type of mount I have and the fact I am imaging unguided. This requires me to run a specialized software that doesn't play well with leaving the image sequence to go find a focus star, return and plate solve.

I have had pretty good success with autofocusing, filter offsets and temperature compensated focusing.

Have fun!

#12 Alex McConahay

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Posted 31 January 2013 - 09:52 AM

>>>>>>Or, do you really have to go custom with the tube you have?

I would say once you have bought the stuff, you will rarely have to buy more.

Most automated focuser solutions I know about do not depend on the tube so much as the focuser itself.

In the first place, for many of us using SCT type tubes, the focusing is done with an add-on focuser. You may do a gross focus with the main tube-moving mirror setup, but the fine focus is done with a crayford hanging off the back. Take that off one scope and pop it on another--nothing changes as far as your investment in equipment, brackets, etc..... You will find that many of thes add-on focusers can also be used in a refractor. You have less portability with a newt, but it is still an option.

Another point: RObofocus and others are sold as a controller with a separate motor and connectors and brackets. One takes the knob off the current focuser, connects the shaft of that with one of those connectors, and bends the bracket as needed to hold the motor from turning. In other words, as long as you have a connector to fit the shaft of your focuser, and the imagination to bend the bracket the right way, you can swap motors around.

I have three scopes I use regularly. I have two Robofocus motors. I keep one motor permanently installed on one of the tubes, and the other motor moves back and forth to the other two tubes. I swap the electronics back and forth. The whole changeover takes ten minutes or so.

You do need to reconfigure software with the different arrangements, but that is part of the startup procedures whenever you get a new focuser.

Alex

#13 Alex McConahay

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Posted 31 January 2013 - 09:54 AM

I agree with everything Dave just said.......

Alex

#14 Gray

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Posted 31 January 2013 - 10:23 AM

Lots of help here, and I for one sure do appreciate it!
I got a reply back from Leon, he wants me to get more measurements of my focuser and says he can put something together. That's awesome. I'm wondering if buying another focuser would be a better idea though. I know my experience with it; having to tighten the tension screws quite a bit to push/pull all that weight. Might be easier on the focus motor if I were to replace the focuser. Not sure on that one, just a guess. I think I've read enough on here to understand what I will be getting into. :bow:

That's an interesting imaging sequence you have there Dave.
Hopefully I won't have to worry too much about temp. changes with a small focal length :confused:. Thanks for your perspectives. :bow:

I'll keep rolling this around in my thoughts until next Wednesday, then I should have a decision made and will decide what to order and from who. If you all think it would be a good idea to replace the focuser, please let me know. Or if you fine folks think I can get by without it for now, then that would save a lot of cash and Leon will get my business. Just not sure about those things yet. No experience, only common sense so to speak. CS

#15 microstar

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Posted 31 January 2013 - 10:25 AM

In fact, I did ask the question myself on the MainSequenceSoftware Yahoo group. I keep this thread posted on responses.
...Keith


According to one of the Sequence Generator Pro developers the Rigel USB-nStep system works well for autofocus with SGP. So that doesn't seem to be an obstacle.
...Keith

#16 Gray

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Posted 31 January 2013 - 10:29 AM

That's good to know. Thanks Keith. What do you think I should do? Buy another focuser or keep what I've got? It'll work but you gotta tighten the hello out of the tension screws.

#17 shams42

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Posted 31 January 2013 - 11:28 AM

If your old focuser sags at all under the weight of your imaging train you are going to regret spending the money to motorize it.

#18 microstar

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Posted 31 January 2013 - 12:11 PM

That's good to know. Thanks Keith. What do you think I should do? Buy another focuser or keep what I've got? It'll work but you gotta tighten the hello out of the tension screws.


I think shams42 said it well. Only motorize your focuser if you are confident that it will do the job (in my case the StellarVue 2.5" SV25 rack & pinion seems more than up to the job from other's reports so I'm seriously looking at the Rigel system) but if the AT focuser can't handle the weight then you could end up motorizing then having to buy a Moonlite to correct the problems in the AT at much greater cost than switching in the first place. I image with a similar scope, the Megrez 72FD, with a JMI DC motofocus unit, but I have the beefier DDG focuser on it; I've heard reports that the crayford on the non-DDG version had problems with slippage and sag that could have made this unfeasible. So if you decide to motorize your AT focuser be sure it's worth the investment!
...Keith






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