Secondary question for an expert

[Peter.M]

Hi Guys, I have a question about my telescope and its secondary. I plugged all my values into NEWT and got a 100% illuminated circle of 0mm. Now I have been using my telescope for a few weeks now set up like this and vignetting is not that bad. I assume the light that I am seeing is from the 75% light rays even at the center of the field.


I changed the diagonal axis value in newt up and down and saw what effect it had on the 100% illuminated circle. Above 80mm saw the 100% circle move up. So I went investigating and bintel sell 88mm secondarys. Plugging in this value gave me a 17mm 100% illuminated circle. My chip is 17X13mm approximately so I have ordered one of them.

Now I understand that I am going to lose some light from the secondary size increasing ( the area went from 12% to 17%) but am I right in thinking that my chip should see an approximate 20% gain in light from the 100% rays as apposed to the 75% that I think it gets currently.

[Merlin66]

Sound right to me!
Smalller diagonals favour double star and planetary observers where the object being observed is small and on axis.....

[Garyh]

The 88mm secondary sounds like a good size for your chip with just a little drop off in the corners, shouldn`t be too noticeable, flats will fix that. I wouldn`t worry about the larger central obstruction loss, like Merlin mentions small ones help with splitting doubles etc but if your eyepiece isn`t fully illuminated especially a shorter focal length one you are just throwing away too much light that your mirror has collected.
Your original one sounds too small even for visual.
cheers

[Shiraz]

hi Peter - what scope is this for? My 200mm f4 has a 62mm secondary and fully illuminates most of a QHY8. If it is your 200 f4, the 70mm secondary should be plenty big enough. Regards Ray

[Peter.M]

Usually I would agree with you but I have my colour filter wheel and an Off axis guider to contend with. I have moved the mirror up the tube to compensate for this and now I think the secondary is insufficant, I should be getting the larger one during the week so after it is attached and set I will test and see if I get more light.

[Shiraz]

OK. Must be fun to fit all that on the focuser (plus an MPCC). interesting to hear how it pans out. regards ray

[Peter.M]

To confirm to myself that I do need the larger secondary I looked at some flats that I have taken after I cut the tube. The center of the flat frame after cutting averaged 4500 as a 2 second exposure. With the same binning and same filter in before cutting a 1 second exposure had the same reading in the center of a flat.

I sure hope the larger secondary doubles signal collection! But I think that is a little optemistic :)