Motorized focuser with temperature compensation


#1

Hello together,

in the next time I want to go for a apochromatic refractor for astrophotography. Apart from the optic itself, one big topic is the focuser and I hope to get some advice here.

For sure I want to have an ASCOM compatible motorized focuser with absolute positioning, so I can focus with help of SGPRO. Another feature I would like to have would be a sort of temperature compensation DURING imaging, because I think if doing narrow band imaging with 30 to 60 min integration time this might be very usefull.

For example: I found a thread in another forum where a Takahashi FSQ106 owner said that his telescope needs to be refocussed at least every 0.2 degree, if he wants to have really perfect focussed images. Although I think I will not purchase a Takahashi, I believe also other (cheaper ones) apochromatic refractors suffer more or less from the same problem.

If doing narrow band imaging with very long single exposures, my plan was to use such a temperature compensation during imaging AND to refocus between each image (just to be sure because the model used for compesation might not be 100% correct and allways suffers from the fact that there is always a lag between the measured temperature of the probe and the temperature of the optical relevant parts of the scope).

As far I have seen to do a temperature comensation DURING imaging is not possible with SGPRO. Therefore initial I wanted to post a corresponding feature request. But this request already exists and was to my regret rejected by the developers, because

(from: TempComp control)

Therefore my questions:

  1. Please can anybody - who has expierence with focuser/motor sets, where the advertising is compromising “temparture compensating during compensation” - tell me, which system will actual work, resp. which will not work well (as they are too rough and destroying the image by its vibrations). I am currently thinking of the USB_Focus V3 system (prefered as a little bit cheaper) and the Focus Boss II Kit for FeatherTouch focusers. (I know that is not directly SGPRO related, but would be happy if I get an advice anyway).

  2. If I use one of the systems above: Do you thing it will be possible to use them together with SGPRO? What will the USB_Focus or Focus Boss software do, if the SGPRO focusing routine starts? Will it be necessary to manually close this 3rd party software before and to restart after SGPro focusing?

Thanks a lot for your advice and best regards

Reinhard


#2

Richard…just a very brief reply.

I have a Tak FSQ 106 ED and use a RoboFocus on this. I also do mostly
Narrow Band - 20 minute to 30 minute subs. Rather than trying to set up
temp triggered auto-focus, I just refocus between each sub. The autofocus
routine does not take up too much time - and no sub is out of focus. Pretty
simple really.

One thing to do is get the filter focus offsets set up before hand (just
once) - focus each time with a LUM or Clear filter and the NB filter focus
point will be set for each particular filter as you use it.

Hope that helps,

Brendan.


#3

Optec makes a line of excellent, temperature compensated (TC) focusers. If you get an APO with a Starlight focuser, Optec has a motorized upgrade for it. I use an Optec Gemini focusing rotator on my 12" Meade LX850 OTA and use the automatic TC built into the Gemini control software. This software runs the TC routines continuously while allowing SGP to do overall focuser control. The focuser makes multiple adjustments during the imaging with no bad side effects.

This image:

http://www.astrobin.com/full/208335/0/

was four hours of imaging time. Initially focused at the start of the imaging run and then only the automatic TC of the Gemini software was used after that. My Astrodon LRGB filters require no focuser adjustments when changing filters

While the Gemini may be overkill for an APO, Optec’s other TC focuser products should solve your problem nicely.

Charlie


#4

Hello Brendan, hello Charlie,

thank you very much for feedback!

@Brendan:
My plan was to do focusing between each sub as you described. But I also wanted to have a temperature compensation DURING imaging. My consideration is, that if a refocus is required after an image was taken, the focus has already been lost (more or less). The temperature compensation during imaging should serve as mitigation. And the focusing between the imaginges should serve as a sort of fallback, if for some reasons the temperature compensation works not 100%.
But perhaps I am too worried about the problem. To be honest: I didn’t thought about it until I inspected (one of my dream telescopes) the Tak FSQ106 at an astronomy fair. There the ICS Focus Wizard system was installed and it was strongly recommended to purchase one, if you want to take very long subs. The Tak FSQ106 will currently stay a dream, but I came to the conclusion that temperature drift during imaging might be a general issue for apochromatic refractors.

@Charlie:
Nice to hear that the temperature compensation does not interfere with SGPro and that your Optec device does not cause side effects.

Your input helped me a lot. I did not own too much telescopes, but with every one I was not happy with the focuser. For that I decided not to be mean with it - independent of temperature compensation considerations. There may be other good/very good focuser around, but now I think I will go for the FeatherTouch - together with the FocusLynx/Focus Boss system. If the temperature compensation during imaging might not be required for the most images - even better. But I will not be annoyed about myself, that I have saved money, in the case I have a night with many useless images.

Clear skies and best regards

Reinhard


#5

Hi Reinhard,

When I read your post a second time - I realised that it was during capture
that you wanted the ongoing focusing. That does indeed sound very demanding
and I would not have the experience to say it was or was not entirely
necessary.
Although I can do longer subs, there is always a fear of losing one for
some stupid reason or other…so in general I stick with 20 minute subs.
Maybe where I am in Spain I do not experience huge temperature changes
during the night and 20 minutes is sufficiently short to catch the the
gradual change…or maybe my eyes are not good enough to notice.

I let SGP AF just do its thing between subs and that appears sufficient for
me. I must pay more attention to the numbers next time out…I might
learn something. :slight_smile:

Brendan.


#6

I am using the Focuser Boss (I) and a FeatherTouch focuser on both my FSQ and my TOA. You do not need any other SW for these to work with SGP, just the SW that comes with the system. I would suggest the HSM motor instead of the standard motor since it has half as large a step size (finer control) as well as having a built in temp sensor so will work with temp comp if you want to do that. I used to do that but I feel it is better to characterize your system so that you know how much temp change you can tolerate before refocus is required and then just set that up in SGP and it will focus when needed.

The system has been virtually bulletproof and I highly recommend it.

Hope that helps


#7

That is true of some, but not all, systems. I have seen quite a few systems where folks assumed that because the filters (which are great and I use as well) are parfocal, therefore one does not have to use offsets or change focus with filters.

The truth is that unless you have verified this by actual test on your system, if the system has any refractive elements, it may not be truly parfocal at different wavelegnths. This is not the filters, it is the lack of perfect achromaticity in the refractive elements. It can range from minor to huge. With my Taks it is minor but significant. I would suggest that one test this rather than assume it. It may well be true but I would bet most folks will be in for a surprise once they test.


#8

Hello CCDMan,

as far I have seen the HSM motor is already included in the Focus Boss II kit (as offered at my local dealers). It is also possibile to upgrade - without much costs - to the “Posi Drive Motor System”. This motor has a resolution of 11400 steps per inch. It has a built in temperature sensor too.

In the meantime I got an answer from OPTEC. They suggest an external temperature probe. So it might be possible (with some testing) to find a place somewhere on the scope which is representative for the thermal behaviour of the optical system. I am also thinking about to put the temperature probe in a sort of thermos bottle to simulate the cool down/warm up behaviour of the system. (It seems it becomes a dissertation :slight_smile: )

Thank you for sharing your experiences!

Best regards

Reinhard


#9

I’m working on this very issue right now. I’m using an FSQ106 for NB imaging and some of my subs are 60 minutes long. This can be too long to remain in focus considering FSQ’s CFZ is 25 microns at f/5. I have a MSM30 focuser motor and a MicroTouch controller. Last week I trained the system and this coming weekend, if weather permits, I will try live focus during exposure. Hope to report back here next week.


#10

Hi Kinch, I live in Spain as well, on the southern part around Estepona. I have as well the Tak106ed and installed a Robofocuser, but having issues in the setting up the data, not sure what to put. Given that you have similar set-up, would it too much to ask if you can provide me with the figures that you put so that I could at lease start with the autofocusing, I am currently new with the SGpro, so a bit of everything at this moment.

Thanks

Charles


#11

Sent you a message Charles…you can email me directly and we’ll see how to go forward.


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