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Old 19-10-2013, 11:04 AM
stevous67 (Steve M)
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Filter Offsets

Hi all,

I've been using filter offsets for a long time, and I guess many others have been too. But do you ever question - are these right?

I've previously worked these out by running focusmax consecutively (manually), recording position and temp per filter and averaged the results; but did focusmax do a good focus run to get the position per every sample? Did momentary atmospheric conditions alter a sampling result? I've also run the CCDAP5 wizard, it gave a result, it seemed to be okay?

Ive noticed that if you have a low F ratio setup, focusmax can provide a fast and repeatable result (with good supporting V curves). But, if you image at the higher F Ratio (say F8), then results can wander and larger sampling is needed to accurately predict the focus position. This I think is largely due to the broad CFZ of the higher F ratio system.

Last night, under less than favourable conditions (Melbourne ), I tried something new; not sure if its been discussed here before, sorry if its a repeat. Selecting a brighter star, I set to manually do my offsets once gain. Checking old data to confirm results. This time using a bahtinov mask and the bahtinov grabber, I quickly and consecutively mapped results. Inputting my equipment details, the grabber somewhat assists by predicting how many microns youre out. Considering you can make many samples per minute, the samples flow in quickly, and I feel reasonably accurately should you monitor a sample of say 5 results at the focus position for consistency.

In an hour to two hours, you can have a lot of data recorded, and this was the first time I had very consistent results. Comparing to my CCDAP5 results, I found most offsets to be within 15-20microns (my CFZ is up around 40-50 depending on which filter) of these results. One filter was very different, Blue. CCDAP established a +229 micron move from Green, but last night confirmed the difference to only +123.

Anyway, just thought to share the idea and positive results.

Cheers,

Steve
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Old 19-10-2013, 09:11 PM
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RickS (Rick)
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Hi Steve,

I find the offset calculation a bit of a crapshoot at slow focal ratios as you suggest. I do FMax focus runs on each filter in turn and toss any result that finishes with a HFD that looks out of whack. I do this several times and put this all in a spreadsheet and calculate the offsets for each run, delete any outliers, then average the offsets. Then I use a Baht mask and Bahtgrabber to check and tweak the results. This process takes a while but fortunately I don't have to do it often

Cheers,
Rick.
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Old 20-10-2013, 11:08 AM
stevous67 (Steve M)
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Hi Rick,

Thanks for your confirming feedback. I have the same feeling, and a "crapshoot" description being very appropriate. So the reality is that for slower F stop users, close scrutiny of your offset data is absolutely needed. The baht mask+baht grabber works well to collect the data, then CCDIS to test the results and do some fine tuning. Last night I had the opportunity to test my offset calculations, and spent 3 hours checking and fine tuning it, using CCDInspector to measure full rate FWHM. Ideally, you need the best seeing conditions to do both tasks [otherwise you have a very sloppy CFZ], so I will repeat all this again once an ideal night comes along.

Thanks Rick.

Steve
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