A Question Of Magnification When Using A Filter Wheel

[Dennis G]

Page 3 of the Iceman’s article Planetary Imaging with a Mono Camera, the pictures describes the setup. The added distance that the DMK imager is held off the 5x power mate by the insertion of the filter wheel, raises the question in my mind, does this added distance change the magnification?
The article speaks of the F number as 10m . How is this figure arrived at?
One further question arising from this article on page 6, Focus Flutter. This isn’t something I’ve experienced and would appreciate a better understanding of what this is or looks like. It is possible that it is hidden by poor atmospherics?
And finally: The Toucam imager requires the AVI recording to be saved before capture. Is the software for the DMK 21 similar or saved after capture, or is it automatically saved?

[monoxide]

Hi Dennis,

when imaging people tend to talk more about focal length than magnification, if your scope has a focal length of 1500mm for example and you used a 2x barlow, your focal length then becomes 3000mm.
so a 12" f/5 scope with a 5x powermate would be somewhere around the 7.5m range.

once you add a filter wheel and other bits n pieces it gets a bit harder to work out but i think there are programs that help you work out exactly what focal length your imaging at.

cant really help you with the other questions, im sure someone else will jump in though

Cheers,
Tj

[Dennis G]

Thanks Tj. I'm sure others will fill me in. Here is another question. Powermate and a Barlow. Is the difference between the two only quality?
Have a 2x Barlow which at a guess would be rated as cheap.($21) Purchasing a DMK 21 mono to improve the quality of recording and a weak link ruins the chain! Like wise there must be a trade off with atmospheric conditions and magnification when choosing a Barlow or Powermate? Being an old beginner I appreciate any advice available.

[monoxide]

Hi Dennis,

barlows and powermates perform the same function but use slightly different ways of going about it.
powermates tend to have the reputation though and a cheap chinese barlow wouldnt have the same optical quality.

this page here might explain a few things:
http://www.televue.com/engine/page.asp?ID=42
it also has a graph that will let you work out roughly what your focal length would be with a filter wheel etc in place, just remember though that those are the guys that are selling the product.

for planetary imaging i think the norm is to go with the 5x powermate BUT you cant get out there every night and use it, in poor seeing though i dont think youd get sharp images at any focal length but it would still make for good practice. one problem is mistaking tube currents for bad seeing, if the scope isnt cooled down properly first it will look like the seeing is poor when it could be excellent

[iceman (Mike)]

I'll reply in depth when I get some more time, sorry. But so far great job TJ!

[Dennis G]

Increased magnification and tube currents

may be my question has been misinterpreted. I wasn't after the actual value. Just wanted to have an understanding of what occures when the camera is moved away from the Powermate by the thicknes of the filter wheel, does it increase the magnification. It is similar to a long tube 2x Barlow. Although I haven't tried it, I have wondered if by moving the tube further out of the focuser it would increase the magnification without distorting the optics. Reference to tube currents. My 10 inch GS has been converted into a truss Dob. see 250 Light Bucket IIS. The lower tube section is about 500 mm in length. The suplied fan draws the air from the tube. I improved this with a cover and edge seal preventing the air flow from being drawn other than from around the mirror and tube. When deciding on a viewing evening, the scope is taken outside located in shade and the fan switched on and left running while recording. Would Tube currents still be a concern?

[monoxide]

Hi Dennis,

with barlows and powermates if you move the camera or eyepiece further away from the lens the focal length will increase, the graph in the link i posted before shows this (sorry if i didnt explain it better) but with some powermates you dont get this increase, it actually drops off. the 5x powermate with a filter wheel + camera would actually be more like 7x than 5x.
if you have a good quality barlow lens you can use very long extension tubes, i remember seeing one website where a guy used a 30cm extension tube!
i pointed out that graph though since with the 2.5x powermate if you put an extension in, the focal length will be reduced also if you take a look at the others theres no real relationship between them, they all do different things with different distances to cameras/eyepieces

regarding the tube currents, not so much of a problem with truss scopes id think but you will still get a 'boundary layer' of warmer air sitting right on the face of the mirror, the cooling fan will help with this but its still best to put the fan on and leave the mirror to cool as long as you can before you start imaging, its not so much the problem of making the warmer air 'go away' as getting the mirror to get somewhere close to the ambient temperature.

hope im not confusing you Dennis!

btw thanks Mike, im trying but im not the most knowledgable person when it comes to planetary imaging since i aim my camera elsewhere. just echoing what i've seen on here before

Cheers
Tj