5 replies to this topic

[tboss70]

Hey everyone,

 

How would you configure the secondary mirror of an 8" F/6 newtonian OTA if you were trying to find a compromise between visual and astrophotography (1.52", 1.83", 2.14")?

I would really like to find a middle ground here but not sure its possible or worth the effort.

Is there an online calculator somewhere that would help me determine camera chip coverage as well as ensuring proper focus for both visual and astrophotography using a secondary mirror size of, for example, 2.14"?

 

It would also be nice to use a coma corrector/reducer, but these usually require backspacing and I'm not sure how to ensure my focuser will come to focus if adding this.

Thanks for any insight you may have

 

Todd

 

[DaveB]

You can use this: https://stellafane.o...b/newt-web.html

 

If you are using it for both visual and imaging, I would err on the side of a larger secondary so that you fully (or at least nearly fully) illuminate your chip. 

 

For my 10" f/5, I went with 3.1" secondary, but I'm strictly imaging, I *may* have gone down to the next smaller size if it was for visual.

 

Just my opinion, you will see a bigger degradation in imaging if your secondary is too small vs. the degradation for visual if your secondary is too big. The secondary will still be smaller than the secondary on an 8" SCT.

 

This all assumes a 2" focuser. If it's a 1.25", then it will block the light of a larger secondary.

Edited by DaveB, 06 August 2022 - 02:49 PM.

[KLWalsh]

Many ‘T-adapters’, which allow you to attach a DSLR (for example) to a 2” barrel that slides into the focuser drawtube, are designed to provide a 55mm backfocus from the mounting surface to the camera focal plane.
On a DSLR you can typically find a Ø symbol that indicates the location of the sensor. The focal plane of the telescope needs to extend about 55 mm or so beyond the drawtube when the drawtube is fully racked in, to make sure the sensor can reach the telescope’s focal plane.
The Baader MPCCIII coma corrector maintains this 55 mm of backfocus when attached to a T adapter. Other coma correctors may have longer backfocus distances. You might do some research to find the best coma corrector for the f/ratio of your scope.
For visual use you’ll likely need some blank 2” barrels that screw into the eyepiece barrel, to move the eyepiece focal plane out to match the telescope’s focal plane.

[tboss70]

Thank you, Dave.

Are there any downsides to not fully illuminating the chip? I have a ZWO 2600MC which I use with my refractors but if the only downside is vignetting I am okay with that for now.

 

I have an 8" F/5 that when using a planetary camera (224 or 462), it does not come to focus without a Barlow, which, I never really use without one.  When I have used the 2600MC with it, it came to focus without the Barlow. I would like to think the same would happen with this F/6 if configured correctly and ensure the focus tube does not interfere with the light path when taking pictures.

 

Just out of curiosity, how does one visualize the light path on the chip? How would I know the coverage of light on a 1.52" vs 1.83" vs 2.14" secondary mirror?

 

In reading the link you sent, it sounds like "A telescope which is optimized for photographic use does not usually perform well for visual work."

 

But how well can a telescope optimized for visual, work for astrophotography? I think this goes back to 'what is the downside of not illuminating the chip 100%". Again if its just smaller field of view, vignetting, and some cropping, I may be okay with that since I'm not looking for wide field images.

 

"Photographic Use:
Generally, to attain the brightest image (and utilize the full potential of the telescope's light gathering ability), the film in the camera should be as fully illuminated as possible.  This requires a substantially larger diagonal mirror than does visual work.

In a 35mm camera, the short dimension of the film is 24mm (about 1 inch).  The camera body requires the focal plane to be moved farther out from the focuser as well. Adding 2 inches of focal plane height for the camera body, and requiring a 1 inch area of 100% illumination will call for a fairly large diagonal mirror.

The other components of the telescope must be redesigned to accommodate photographic work.  The focuser inside diameter must be larger to prevent vignetting of the light cone, and the diagonal mirror spider mount must be strong enough to prevent the heavier mirror from vibrating or sagging.

A telescope which is optimized for photographic use does not usually perform well for visual work."

[astrohamp]

This calculator may help.

 

My 222mm diameter f5.8 Newt clearly vignettes through the 2" focuser with a 2.14" secondary using my ASI294 camera.  If I trade up for imaging the 2.6" secondary size would be the one I choose.

[tboss70]

Yes this does help, thank you.

 

This is likely nothing new to most, but I needed to understand for my self and I think this helps with my decision as far as using a camera with this mirror.

"This will increase the probability of vignetting the light cone, because a larger conic section will have to fit through the focuser and other parts of the telescope to reach the focal plane.  For this reason, a telescope designed for camera use should have the lowest and widest possible focuser."

 

I'm starting to think I should just maximize this for visual use and in the future, I can find something more suitable for astrophotography.

 

Thanks for all the input Dave, KLWash, and astrohamp.