Focusers and Filters

[avandonk]

The RH200 has a ten position FLI filter wheel and Atlas focuser. The Atlas focuser's resolution is 85nm per step or 100 steps is 8.5 micron.

Did a bit of testing last night. I used a Bahtinov mask to focus all my different filters.

The temperature of the optical train started at 17.8C and fell to 17.3C over the course of the measurements.

This is a table of the results.

FILTER___ Absolute Steps__ Relative Steps____ Actual differential distance in Micron
RED -----------39600__________ -200_____________ - 17
GREEN ____39800 _____________0_______________ 0
BLUE ______39800_____________ 0 _______________0
LUM _______39800 ____________0_______________ 0
HA ________39900 __________100 _______________8.5
OIII________ 40600__________ 800 ______________68
SII _________40200__________ 400 ______________34
CONT ______39400__________ -400_____________ -34
NII ________40200___________ 400 ______________34




Astrodon filters have a Silica substrate and are 3mm plus or minus 0.025 mm in thickness. This 0.050 mm maximum thickness difference is actally 1.5x0.050 = 0.075mm or 75 micron in optical path difference as the refractive index of the silica is 1.5. As you can see from the measurements they are much better than the quoted maximum especially the LRGB set. The RH200 is of course not totally free of CA as it has lenses and is most probably contributing to slightly different focal positions at the IR and blue ends of the spectrum without any filter thickness difference.

I have wrapped six dew heater straps onto the RH200 and Atlas focuser. A PID (proportional integral differential) controller holds the temperature of the optic train within 0.2 C of a set temperature. This means focus should not change with ambient temperature.

Bert

[Mighty_oz]

Have u tried focusmax via ascom with the focuser ? Also can u post a pic of the dew strap setup for us ?

Thanks Marcus.

[avandonk]

No Marcus it is pointless chasing focus with such a fast system in Melbournes weather. Ambient can change as much as 25C from dusk to dawn.

I used to keep my 300mm lens at constant temperature 24/7 and it did not change focus at all over days or weeks or months if left undisturbed. Another benefit is that condensation just cannot occur on the optics when the weather is very cold and wet.

I am testing the current distribution of dew straps and it looks very promising. At the moment I have a fluffy towel as an overcoat for the RH200. On wednesday night at 1900hr with ambient at 17.4C and the RH200 temperature set at 17.0C (not heating yet) I measured the focus position of the NII filter. At 0200hr Thursday I checked it again and focus was still the same position of the Atlas focuser and ambient was at 8C!

Thursday night at 1700hr and an ambient of 15C focus had not changed. After battling clouds till about midnight last night checked focus again at 11C ambient and focus was still spot on!

The dew strap heaters are only working at a small percentage of their rated wattage. For example with the optics temperature set at 17.0C and ambient at 11C the PWM (pulse width modulated) power of the PID was only at 15% of maximum.

The aim is to have the whole optic at a constant temperature and at equilibrium so only very gentle heat is needed that is as widely and evenly distributed as possible. This will ensure no tube currents are generated due to slight temperature differentials that will lead to a degradation of image quality.

Bert

[Merlin66]

Bert,
I think this is wrong:
"difference is actally 1.5x0.050 = 0.075mm or 75 micron in optical path difference as the refractive index of the silica is 1.5."
The effect on the focus position "in a converging beam" would be 0.5 x 50 micron = 25 micron.
The focus "difference".......

[gregbradley]

[FILTER___ Absolute Steps__ Relative Steps____ Actual differential distance in Micron
RED -----------39600__________ -200_____________ - 17
GREEN ____39800 _____________0_______________ 0
BLUE ______39800_____________ 0 _______________0
LUM _______39800 ____________0_______________ 0
HA ________39900 __________100 _______________8.5
OIII________ 40600__________ 800 ______________68
SII _________40200__________ 400 ______________34
CONT ______39400__________ -400_____________ -34
NII ________40200___________ 400 ______________34


What filter is CONT? Is that contrast or something?

Greg.

[RickS]

I presume it's a continuum filter. From the Astrodon site:

[Moon]

Bert is this comment related to Focus max? If it is, I don't see why it would be 'pointless'. I'm guessing Marcus was suggesting focus max is more objective for these test, rather then the good old Bahtinov mask.
James

[avandonk]

It is pointless to waste imaging time refocusing no matter what method is used.

Yes it is a continuum filter and in theory can subtract any artefacts whether introduced by the filters themselves or the optic. It will be very useful when very bright stars are in the field or just outside as it will also eliminate any secondary reflections from specular reflecting bits in the image train.


Bert

[avandonk]

The speed of light is slower in a medium of higher refractive index than one ie a vacuum. The optical path length is then therefore longer and this factor is a measure of refractive index.

A 3mm filter that has a refractive index of 1.5 has an optical path that is 1.5 times longer than a vacuum or air to a first approximation. In other words the 3mm filter increases your optical path length by a factor of 1.5 that is by 1.5mm to 4.5mm.

Bert

[rally]

Bert,

It is never a waste of time accurately focussing your system !

Using FocusMax will take about 60 seconds and nail it every time and do it automatically. No human error.

At least do it all once so the focus offsets are accurately quantified and recorded in your software and then focus the first filter so you have an accurate relative starting position each night.
A night of bad focus will ruin the whole night, a season of relying on unreliable information could potentially ruin a season.

At least in terms of capturing the best FWHM that could have otherwise captured - meaning much better nebulosity detail, you would want to use a quantifiable focussing methodology.

You spent a small fortune getting a wonderful OTA and an electronic focus system that can focus your stars to a tight point and then to rely on a Bhatinov mask and leave it up to the gods that nothing will change throughout the night !
The CFZ on the RH200 is about 21 microns (in the blue) I think for your OTA, so you have a system that is quite sensitive to focus.
According to your figures, the focus position varies from 68µm to -34µm although at least LRGB is almost parfocal to within 17µm.
So this will be a much greater problem for narrowband than LRGB

So that is a maximum range of around 100µm with a system that only has a focus range of round 20ishµm.

That indicates to me that Focus is likely to be the weak link in your imaging systems fine tuning.

Given the CCD has 9µm photosites and the OTA can focus to 8µm (central area) any variation in focus is going to bloat your stars.

So I wouldn't discount the necessity to focus no matter how little time it might take - which just isnt much if you are using Focusmax

Just my 2c worth for what its worth.

Rally

[avandonk]

I may not have made my ideas clear. Of course accurate focus is important. I am not going to waste time chasing it due to ambient temperature fluctuations.

I established the offsets for each filter for just this reason. The focuser setting can be controlled with MaximDL for each filter or so I am told.

Just think for a short while. I am not leaving focus up to any vagaries of variation or fictititious gods. Constant temperature of the whole optical train will minimise any focus variation with ambient temperature change.

Focus once established by any means will then be immaterial as the whole optic train will not vary due to ambient temperature.

By the way the Bahtinov mask can easily resolve less than 100 steps of the Atlas focuser or in the real world 8.5 micron.

If I am wrong I will go through the tedious procedure of focusing by numbers.

Bert

[bmitchell82]

Bert I agree with everything you say apart from the focusing method. Im with rally on that one focusing with a bat mask while great for people who do not have electronic focus is reliant on your eyes picking a very very fine point is madness. I focus by numbers by hand and it takes me but a few minutes to do though maximdl.

I don't have as fast of a system at you with mine being F4.7 but my CFZ is only 42 ish micron. Sure the bat mask gets it close but when you have spent in the order of 10s of thousands of dollars (what you spent close to 5k on filters alone) why are you using a 20 dollar focusing tool?

You may just be surprised at how easy focus is once you have the right software!

A friend i image with focus's approximately every 30 minutes and it takes about a minute to do it. further more its written into the autosave so it doesn't even move off target. just click go it picks its own star, does its own focus then goes on its merry way of imaging.

[Moon]

Bert,
You are reporting remarkable focus stability: at an amazing level we all dream about. You are also correct in saying unnecessary focusing during the night wastes time.
However I still think there is merit in using focusmax to characterise your system. It's a very efficient way to collect many data points. I'm not sure how long it takes you to focus using the mask, but as a comparision, I find focusmax usally takes about 30 seconds.
James

[Merlin66]

Bert,
Correction....
The effect of a glass plate (ie filter) when placed in a converging beam is to extend the focus by:
delta = t*(n-1)/n

This is approx 1/3 the thickness.
If you apply snells law to the beam at the front surface and again as it exits the rear surface you'll see this change.

[gregbradley]

I have never been able to get FocusMax to work.

How do you work it with CCDsoft and an electronic focuser?

Greg.