Focal length

[saturn5]

Hi,
I tried posting this earlier but it seemed to fail so apologies if a slightly different version to this turns up!
When I first saw an advert for the Infinity in the Sky at Night magazine a few months ago, the wording in the advert said something like “the Infinity works best at a focal length of 1500mm”. I’m sure the wording wasn’t quite this and the focal length stated was probably different (I don’t have that magazine issue any more) but as I have a Meade LX90 with a focal length of 2000mm, I asked a question on another forum (or maybe the old version of this one?) as to whether the Infinity would be suitable for use with the LX90. I received a reply from someone at ATIK saying that it would work fine and that the advert wording was a little misleading.
I now notice that current adverts in the Sky at Night and the Sky and Telescope say the Infinity is designed to work from around 300mm – 1500mm. This is at odds with what I was told.
Looking on the ATIK website and on resellers’ adverts, I can’t find any mention of a focal length issue. There was no mention of it either in the Infinity review that was in the Sky at Night magazine a few months ago.
Perhaps someone at ATIK could clarify this once and for all, please.
Many thanks,
Peter

[Chris]

Not ATIK but I expect that what they mean is that the field of view is a bit small with a long focal length.  The camera will still work.

The narrow field will mean that the mount needs to be capable of tracking very accurately to get good results.  I've never used a LX90 so don't have an opinion on how this will work.

Something that will help is a focal reducer.  This reduces the focal length that the camera sees and gives you a wider field and more tolerance of inaccuracies in tracking.  The standard SCT reducer gives a reduction of 0.67 and will give a focal length of about 1300mm.  There's also a CCD focal reducer that will get you to a focal length of something like 660mm.

You can also get a reducer that screws onto the 1.25" camera nose.  These typically give about 50% so a focal length of 1000mm

You need to get the CCD to reducer spacing right to get the correct reduction, this should be described in the reducer documentation.

Chris

[JimH123]

Regarding focal length.  In addition to using the Atik Infinity with a telescope, I have been playing around a bit with older m42 lenses.  These are easy to adapt to.  I have tried 300mm, 200mm, 135mm, 50mm, and even 35mm and 28mm.  They work.  Since the sensor size is small compared to a camera such as APS-C for example, the results appear to be a greater focal length than a DSLR camera would provide.  And also, the wider angle lenses have a very small objective and do not gather much light.

 Would like to try my more modern lenses, but they aren't easy to do this with since proper adapters aren't made, and manual control of the aperature size is not available either.  So doing it with m42 type lenses is the only route.