Martha replied to the topic 'For those with focus issues' in the forum. 1 year ago

Hi guys,

I'd like to refresh the thread. Recently I bought a better class Altair triplet and the first impression says that it has a higher quality focuser. I tried to compare the refractor with another one that I have, it's optically superb Altair Starwave 102ED F/11 equipped with an ATM motorised focuser controlled manually via Arduino. I tried to capture Mars using the two refractors subsequently. I found that the focuser of the triplet keeps the planet perfectly in one place (excellent PA), while the focuser in the cheaper F/11 doublet is lifting the planet up or dropping down depending on moving the rocuser inwards or outwards. Did you take this problem of cheaper focusers into account?

Very soon after purchasing the triplet I decided to buy the ZWO EAF. I already performed three motorised focusers, one is driven straight by OnStep/Ekos, while two other are driven by the Arduino and my code. In the case of the triplet I expect excellent quality and comfort of focusing, that's why I ordered the EAF. It's installed now and waits for the clear sky. I'm going to perform a test of the backlash by moving it outwards for a number of steps and inwards.

My EAF is attached straight to the main axis of the focuser instead of the single knob and here I have a question about the number of initial steps. Some of you write that they set initial steps of 200-500 ticks, while others write that they use 20-50 ticks. What does it depend on? I have read all the thread and nowhere is mentioned whether the stepper motor should be attached to the main axis or to the 10:1 reducer axis. The ratio of 200-500:20-50 suggests that the bigger values are related to the reducer axis. I found (using a Bahtinov mask) that I get focus around 7600 ticks, which is around 27mm from its absolute zero position. Is it OK?

My second question is about the bin2 mode. I use a Canon 6D that has a quite big 6.54um pixel. The cameras mentioned within the thread usually have something about 3.7um pixels. I understand that my pixel works like 4 others, so should I consider the bin2 mode (if the Canon 6D offers it)?

I use a 3" x1.0 field flattener, so my stars are slightly elongated on edges of the full frame and of course in the corners. I think the 75-80% as the second value of the annulus should be OK.

I consider configuration like below:
- full field (should the "autoselect star" be selected then?),
- annulus 25% - 75(80)%
- SEP,
- linear algorithm,
- 5% tolerance,
- initial step 50.

Please let me know if I should think about something else. The good weather in the UK is so rare that I have no time for long experimentation. I appreciate your common experience and knowledge.

I will test with pleasure whether the linear algorithm brings the same focus point and the HFR like the polynomial one. I have experience with 10" budget Newtonian OTA equipped with Hexafoc 2.5" RnP focuser and the polynomial algorithm working with the ATM geared stepper motor (OnStep) and I was quite happy with the results, if the process was successful. Unfortunately, most often it fails. I hope the linear algorithm with the EAF attached to the high class focuser will surprise me positively.

Best regards,
Martha

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