6 replies to this topic

[divers]

When a company/person/website says a mount is acceptable for XXX focal length.

 

Example.  I have a Stellarvue 80 access with native 560mm focal length.  When I use my Canon t7i with a 1.6x crop factor I get 928mm equivalent (35mm).  So, when statements are made like a Skyguider pro is good to about 200-400mm focal length is that native (full frame) or crop or is this all just hogwash?

 

I have a 0.8x focal reducer on my 80 which mathematically means I now have 448 focal length.  But....with a 1.6 crop does that mean I am back to 716mm?  In this example the Skyguider Pro would not be a good fit. 

 

 

Just musing....

[kathyastro]

Forget crop factor "equivalent" focal length.  There is no such thing.  Focal length is focal length.  Your scope is 560 mm focal length, regardless of what camera you hang on it.

 

"Crop factor focal lengths" are a marketing gimmick for DSLRs.  They are not used or useful in astrophotography.

Edited by kathyastro, 20 January 2021 - 03:31 PM.

[the Elf]

In astro photography we look at things differently. When it comes to small objects where you crop the image and present it at full scale the parameter we use is scale in arcsec/pixel. Anyone a bit above noob level should say that a mount is capable of tracking within <any number> of arc seconds. Then you can check how many pixels that is in your scale.

All that crop stuff does not matter in AP. It is meant to give people aged 50+ (my generation) an idea about the field of view because the field of view a particular focal length yields on 24x26mm film is so deeply burned into our minds. A better way is simply to state how much field in degree you have. If you image a large object (filling the full field of view) and present it on screen or a small print you cannot zoom to pixel scale not even on a 4k display.

 

The thing to worry about is picking up plenty of signal. You use the reducer to make the scope faster. The math tells you that you loose resolution but that is not quite true. As the noise eats up the fine detail first you can often see more detail when you use a reducer due to the stronger signal than you can see at a higher resolution with less signal. This is totally counter intuitive to a daylight photographer.

Edited by the Elf, 20 January 2021 - 03:37 PM.

[AgilityGuy]

Regarding the relevance of acceptability of a mount - there are two factors that are most critical, weight and length of the tube (with refractors it relates directly to physical focal length).  

 

When a manufacturer states a mount is acceptable for a focal length of xxx mm it implies the mount can control the moment of inertia generated by that length of tube.  It's all physics.  Even if a telescope is well within the weight range of a mount, if it's too long the mount won't be able to control the rotational motion reliably for it's intended purpose.  A rule of thumb for weight capacities, if using a mount for astrophotography is to stay under half of the maximum weight capacity.  The maximum weight is expressed as the weight of the telescope and not the weight of scope plus counter weights.  The maximum length is a bit more variable.  Someone else might be able to tell you if there's a rule of thumb measurement of that as well but it's probably safe to say that staying within the maximum recommended physical length of the telescope - expressed as focal length in mm is the goal.  Many people use longer scopes and are successful but that's probably been figured out by trial and error.

 

Good luck!

[divers]

So crop factor is relevant to the extent it will reduce my field of view as opposed to a full frame camera with the same telescope? This is of course assuming the telescope can illuminate a chip of that size.

[the Elf]

That is technically correct. In daylight photography full frame (24mmx36mm) has been _the_ standard for many years and the crop factor refers to that standard. In astro photography sensor sizes have always been very different and the full frame has never been any sort of reference. That is why crop factor is not used.

 

Also note that there is no standard what a load capacity supposed to refer to. A bad guy might refer to the weight you can load onto it without the tripod legs breaking, a good guy might refer to the maximum for decent photo. Other than that many factors play a role, e.g. a belt drive often behaves better than a gear drive when guiding. Ask here what others load onto a specific mount before actually ordering one.

[AgilityGuy]

So crop factor is relevant to the extent it will reduce my field of view as opposed to a full frame camera with the same telescope? This is of course assuming the telescope can illuminate a chip of that size.

Or you can buy a Nikon full frame camera that can be switched between full frame (FX) and crop frame (DX) modes and you get the best of both worlds.  I use both modes regularly and each has it's own advantages.