Red cat V1.5 51 APO

[turbo 2011]

After a little advice I’m wanting to purchase a red cat apo for a wider FOV I will be using a sx825 mono with filter wheel . I was thinking of using my main scope as the guide scope WO FLT 110 with a lodestar x2 .

so will the red cat be fine with the camera will I need any extension tubes .

also will the main scope work as a guide scope 

im just trying to get away with spending 2k + on a full frame or much larger chip than I have now and just change  imaging scope that way I have best of both worlds, and I can swap between the red cat as a guider when I’m using main scope to image .

Or would a WO zenith star 61 / 11 APO be best suited.

any advice  would be great cheers 

[Richard_]

The Redcat is a petzval design which means back focus distance isn't critical. I've maintained a back focus of ~55mm with a DSLR and my ASI533 and star shapes have been perfectly fine so I imagine this back focus with the sx825 would be OK. 

Looking at the specs of your camera online (https://www.teleskop-express.de/shop/product_info.php/language/en/products_id/11294), in terms of sensor size, it should work fine. Your sensor dimension is smaller than my ASI533 and I don't see any severe vignetting issues from the size of the imaging circle. However, the pixel size of 6.45um is quite high and undersampled for the Redcat. When I shoot with the 3.76um pixels of my ASI533 I'm already undersampled and I see blocky stars. This can be resolved with dithering and drizzle integration but I have no idea what would happen at 6.45um pixel size. Have a look on astrobin and see if you can find any example images of the Redcat with larger pixel size cameras and see if it's possible. 

I can't advise on use of the larger focal length FLT110 as a guidescope for the Redcat as I've never tried something like that before! 

[turbo 2011]

Great reply  . I think on pixel size alone I should look at a alternative scope or camera .

thanks for the help 

[david_taurus83]

Don't be put off. I use it with my Canon 6D with similar pixel size. Think of all the signal those pixels will soak up! By the time you've processed and saved as jpeg the stars will be fine. Currently have my Atik 460 on it. I will say though it is very sensitive to focus being f4.9. Not an issue as I have an autofocuser setup but something to consider.

 

[turbo 2011]

Cheers mate thanks for great advice it’s going to be either WO zenith star 61 / 11 APO or red cat I can’t afford a big chip camera or full frame mono I’m afraid, would love one to go with my WO FLT 110  then it’s the 2 inch filters also  more money 

[newbie alert]

22 hours ago, Richard_ said:

The Redcat is a petzval design which means back focus distance isn't critical. I've maintained a back focus of ~55mm with a DSLR and my ASI533 and star shapes have been perfectly fine so I imagine this back focus with the sx825 would be OK. 

Looking at the specs of your camera online (https://www.teleskop-express.de/shop/product_info.php/language/en/products_id/11294), in terms of sensor size, it should work fine. Your sensor dimension is smaller than my ASI533 and I don't see any severe vignetting issues from the size of the imaging circle. However, the pixel size of 6.45um is quite high and undersampled for the Redcat. When I shoot with the 3.76um pixels of my ASI533 I'm already undersampled and I see blocky stars. This can be resolved with dithering and drizzle integration but I have no idea what would happen at 6.45um pixel size. Have a look on astrobin and see if you can find any example images of the Redcat with larger pixel size cameras and see if it's possible. 

I can't advise on use of the larger focal length FLT110 as a guidescope for the Redcat as I've never tried something like that before! 

I've never seen blocky stars on any of my undersampled pics, I've got a couple of friends using a samyang 135, one uses his with a 383 and the other with his one 6.. I've only ever seen blocky stars by someone using a 130 pcs..  have you a couple of example pics?

[Richard_]

2 hours ago, newbie alert said:

I've never seen blocky stars on any of my undersampled pics, I've got a couple of friends using a samyang 135, one uses his with a 383 and the other with his one 6.. I've only ever seen blocky stars by someone using a 130 pcs..  have you a couple of example pics?

Sorry, when I was writing the above comment I was misremembering the pixelated stars with my Canon 600D (4.29um pixel size). I determined that drizzling with my DSLR images improved star shape which I continued doing with my ASI533 out of habit. Can't say I really checked star shapes on non-drizzled ASI533/Redcat images come to think of it. 

I do have some comparisons of "non-drizzle" versus "drizzle" stacked images using my Canon 600D and Redcat 51 (this is what I misremembered, I thought I did it with my ASI533). If you zoom in you can see that stars are a little pixelated but are improved with drizzling. This may not be a big deal to most people, hence my suggestion to view images on Astrobin with a similar combination as I'm not sure how the stars would look with bigger pixels again. I'm not sure how far you can push drizzling before you see no further improvement. 

[newbie alert]

Thanks for the comparisons

I'd always thought of people using drizzle were planetary imagers..