Focusing with a filterwheel

[darrellx]

Guys
Sometime ago, I asked a few questions about imaging with a DMK21 and getting it into focus with a filterwheel, etc. Well, I purchased the filter wheel and I thought I would share some stats with you. Maybe someone will find it interesting. Then I will have some questions.

I have an 8 inch Skywatcher, with a Crayford focuser
There is a total of 40mm draw on the focuser

Now with the DMK at prime focus I have 10mm of inward travel remaining on the focuser.
with a 2x barlow - 19mm of inward travel remaining
with 3x barlow - 34mm of inward travel remaining
with 5x powermate - 35mm of inward travel remaining

When I include the filterwheel (the Orion advertised as requiring an additional 20mm of focus travel), things don't work out as expected.
With the 2x barlow, I move the focuser in by only 6mm, leaving 13mm of inward travel
With the 3x barlow, I don't need to move it much at all - in by about 1 or 2 mm.
I would have thought that in each case I would have had to move the focuser in by 20mm at least. I don't understand.

The equipment was configured as
telescope; barlow; filterwheel; dmk

Darrell

[Terry B]

You are changing the distance between the focal plane and the barlow. This will change the amount of magnification slightly and the focus position. If you used:
scope--- filter wheel-- barlow--camera
then the distances would add up as you expected.

[darrellx]

The distance from the focal plane to the barlow?? I thought the critical measurement was from the focal plane to the "last item in the line" - the camera in this case.

So is the distance from the focal plane to the barlow the important measurement because it is simply a barlow in there (and therefore I should always pay more attention to where the barlow is), or is it because it is the first "lensed" item (if thats the right word) in the line?

[gbeal]

Terry is correct. The issue is the distance from the Barlow to the chip in your case, and more so with the 5x Powermate which for some reason you didn't report back on?
Try this link http://www.televue.com/engine/page.asp?ID=42#MagChart
It shows the 5x Powermate increasing it's power with the greater distance Powermate to Chip. The 2.5x Powermate actually decreases slightly.
Gary

[darrellx]

Well, I actually ran out of time. I fogged up before I got to try the PowerMate, and the viewing was quite poor. I have since tried the 5x and the movement required to bring it into focus when I add the filterwheel is very small - like 1 mm.

I have tried changing the configuration to scope-filterwheel-barlow-camera and you are right; I cannot get the smaller magnification barlows to focus.

Darrell