Reducer/ field flattened distance to ccd chip calculation

[Geordie mc]

I have an Explore scientific 102 mm f7 refractor and a QSI 660 wsg8 ccd camera. I've just bought a William Optics ff IV field flattened reducer. How do I calculate the optimum distance between the reducer and the chip in the ccd? The wo IV is adjustable between 66 and 86 mm. Is it trial and error or can it be calculated? Any advice much appreciated. Thank you.

[brantuk]

The optimum distance between reducer and chip is usually stated in the camera documentation - it's different for each camera. Then you have to make the distance correct using spacers. Some reducer/flatteners are designed for correct spacing (eg dslr's), but ccd cameras usually require working out. There's sometimes a little tolerance 1 or 2mm either way - sometimes you have to be more precise and even make correction for filter thickness. Hth

[ollypenrice]

The optimum distance between reducer and chip is usually stated in the camera documentation - it's different for each camera. Then you have to make the distance correct using spacers. Some reducer/flatteners are designed for correct spacing (eg dslr's), but ccd cameras usually require working out. There's sometimes a little tolerance 1 or 2mm either way - sometimes you have to be more precise and even make correction for filter thickness. Hth

Hmmm, it's different for each flattener, in my view. What varies with the cameras is the distance from the front to the chip, and with the QSI there are various options like whether or not you have the OAG.

So, firstly, what do WO say is the chip distance for their flattener at your scope's F ratio?

Then what is the front to chip distance of your variant of the QSI? The difference is the length of spacer you need, though the WO has some adustment.

Some experimenting is needed unless you can find someone who has it sussed for your combination.

Olly

[Alien 13]

As stated above the FF will normally describe the ideal FF to chip distance but this is usually only a guide the larger the chip the more accurate the distance will need to be with a dslr this will require sub millimeter accuracy.

Alan

[Geordie mc]

Unfortunately no documentation came with the ff IV and there is nothing on the WO website either. The QSI site gives the ccd to t thread distance as 50.17 mm for the 660 Wsg-8 which has a filter wheel and OAG. It also states a back focus adjustment of -0.7 mm for Baader filters. I saw another post on here with a way of doing the distance by looking at star shapes that might be worth a try but not sure that works for all reducers.

[steppenwolf]

I would recommend that you assume that the WO FR IV should be set at 73.5 for your f7 instrument. This *should* be the correct setting when using a DSLR camera  which (with a 'T' adaptor in place) has a mounting face to sensor distance of nominally 55mm. Now, you are not using a DSLR camera and your CCD's mounting face to sensor distance is an effective 50.77mm which is 4.23mm less than a DSLR. So you will need to add a 4mm T extender to get the correct spacing.

[perfrej]

The FF IV is a tricky beast. WO gives figures for their FLT series and some more, so I suggest you start by getting the F-numbers for these scopes and see if there is any relationship between F and distance:

FLT 98 - 76.8 mm

FLT 110 - 73.5 mm

FLT 132 - 71.5 mm

GT 102 - 76 mm

M90 - 77.3mm

They usually respond very quickly to e-mails as well.

/per

[Geordie mc]

Thanks everyone. WO just suggested taking test shots and adjusting. Having a starting point of around 73.5 is a real help. As soon as I can actually see some stars I'll get out and give it a go. Your help is very much appreciated.

[perfrej]

Trial and error is a pain... Here's more:

FLT 98 (f/6.3) - 76.8 mm

FLT 110 (f/7.0) - 73.5 mm

FLT 132 (f/7.0) - 71.5 mm

GT 102 (f/6.9) - 76 mm

M90 - 77.3mm

 

No easy maths there

 

/per

[Geordie mc]

Thanks Per. Looks like somewhere between 73 and 76mm will be good. As 76 is marked on the reducer might start there and work back in half mm increments taking test shots and then compare them. I'll only need 6 shots that way and can then try and narrow it down a bit further based on the best result.

[orion69]

Frankly, I would avoid FF/FR without stated backfocus distance for scope I'm buying it for.

[Davey-T]

I would recommend that you assume that the WO FR IV should be set at 73.5 for your f7 instrument. This *should* be the correct setting when using a DSLR camera  which (with a 'T' adaptor in place) has a mounting face to sensor distance of nominally 55mm. Now, you are not using a DSLR camera and your CCD's mounting face to sensor distance is an effective 50.77mm which is 4.23mm less than a DSLR. So you will need to add a 4mm T extender to get the correct spacing.

Sorry Steve, I'm confused if the thing is adjustable can you not just adjust it 4mm ?

Dave

[Geordie mc]

This gets worse. Now discover the Wd IV is so big it hits the c thread adapter for my guide camera. The QSI has a built in OAG. To clear this I'll need a 10 mm spacer. This looks too much distance will be put between the reducer and chip. Going to have to get in touch with Ian king and sort this out. May try the gt 102 flattener/reducer instead. I'll see what Ian says.

[perfrej]

TS flattener is supposed to be good!

[steppenwolf]

Sorry Steve, I'm confused if the thing is adjustable can you not just adjust it 4mm ?

Yes and no! The adjustment moves the lens cell within the FR so you can dial in the extra extension required BUT in doing so, you move the lens cell closer to the primary lens which will move it out of its optimal position as defined by the manufacturer. Experimentation helps enormously here and it could indeed be that there is sufficient latitude for achieving a flat field.

This gets worse. Now discover the Wd IV is so big it hits the c thread adapter for my guide camera. The QSI has a built in OAG. To clear this I'll need a 10 mm spacer.

I use a WO FR IV with my QSI 683 WSG-8 and I use just a 6mm spacer and it clears just fine.

[Geordie mc]

Thanks Steve. good to know a 6mm spacer will do the job. I'll get one ordered. No real rush now as it's not getting properly dark here in the north east. Up until 1am this morning and could only see a few bright stars even though it was quite clear. Looks like at least a couple of months now before it will be worth trying any dso imaging. Might try some lunar imaging with webcam or maybe the qyhv might be worth a go. Never tried either before. Thanks again.

[steppenwolf]

Sadly, the nights are indeed getting short but then I love the summer - just spent a fantastic day at a 'Hounds for Heros' fund-raising event running a cake stall with my wife and it was warm, sunny and very British with Union Flags, bike motorcades and a great family day out - you can forgive the lack of darkness on a day like this! [emoji4][emoji4]

Sent from my iPhone from somewhere dark .....