Dear Mike,
Try this link,
http://www.astrotarp.com/Focal_Reducer_Distance_to_Sensor_Page.html
I had the same or similar problem. I recently set up my focal train with
focuser, focal reducer, filter wheel, then Nikon D5600 DSLR camera. My images
were horrible. Stars on the outside were severely bloated and looked more like
shuttlecocks. To correct the problem I had to remove the filter wheel and
change camera adaptor to a shorter one I found in my parts bin. This brought my
camera about 2 inches close to the focal reducer.
I also found out I need to live with some elongation of stars at the perimeter
as I am using a Meade 10-inch LX200 SCT with a large format sensor (as found in
DSLR cameras). I found the following on the Starizona web site:
Schmidt-Cassegrain Aberrations
Spherical aberration is corrected by the Schmidt corrector lens. The main
aberration present in commercial SCTs is coma. Like the Schmidt camera, placing
the corrector plate near the center of curvature of the primary mirror would
eliminate coma, but the tube length becomes impractical. For example, a
standard 8$B!m(B SCT would be 26$B!m(B long instead of 16$B!m(B.
Alternatively, one of the mirrors (most likely the secondary) could be
aspherized to eliminate coma. This would increase the cost of the system, and
for most purposes, the inherent coma is not problematic, so the aberration
remains in favor of cost and ease of manufacture. However, for imaging with
larger-format cameras, coma can be an issue. Some designs, such as
Meade$B!G(Bs RCX and LX200R series, use an aspheric secondary to remove coma
but retain a compact system. Like most Cassegrain designs, field curvature is
an issue. This can likewise be problematic for large-format imaging. Field
flatteners are sometimes available to reduce this aberration. The Schmidt
corrector does introduce some chromatic
aberration<http://starizona.com/acb/basics/equip_optics101_longcolor.aspx>, and
in the interest of cost, the corrector lens is often designed for minimal glass
use rather than optimal color correction. However, the amount of chromatic
aberration is very small in either case and is not normally an issue.
Astigmatism is
negligible.<http://starizona.com/acb/basics/equip_optics101_astigmatism.aspx>
Sent from Mail<https://go.microsoft.com/fwlink/?LinkIdU0986> for Windows 10
________________________________
From: az-observing-bounce@xxxxxxxxxxxxx <az-observing-bounce@xxxxxxxxxxxxx> on
behalf of Michael McDonald <mikemac@xxxxxxxxxxx>
Sent: Sunday, June 2, 2019 6:32:43 PM
To: az-observing@xxxxxxxxxxxxx
Subject: [AZ-Observing] Re: Playing last night
Does anyone know what the $B!H(Bback focus$B!I(B distance is for these
reducers? What$B!G(Bs the proper distance from the male threads to the focal
plane? Or don$B!G(Bt I understand how they work/should be connected?
Starizona$B!G(Bs SCT Corrector II specifies a 90.3mm +/- 2mm. My EFW to
ASI1600 sensor back focus is 37.5mm. Means I$B!G(Bd need a ~2$B!I(B adapter
for the Starizona. But the Antares/Celestron/Meade f6.3 reducer doesn$B!G(Bt
say anywhere I$B!G(Bve found.
Mike McDonald
mikemac@xxxxxxxxxxx
On Jun 2, 2019, at 4:09 PM, Jim Sellers <jsellersaz@xxxxxxxxx> wrote:
Mike, one experiment you my try is using a different focal reducer. I have a
Meade and Celestron f6.3 reducer/flattener. Both will fit directly to the
visual back threads of a Celestron telescope. I also have a Starfield 0.8x
reducer/flattener with a 2$B!I(B / 48mm internal female thread on one end
and a 42mm external male thread on the other end. Your welcome to borrow them
if you would like to see if they impact vignetting?
- Jim Sellers
Sent from my iPhone
On Jun 2, 2019, at 3:38 PM, Michael McDonald <mikemac@xxxxxxxxxxx> wrote:--
I went to find the specs for my Antares f6.3 reducer. They$B!G(Bre hard to
find. But I found one spot where it claims the reducer has a 1.5$B!I(B
image circle. That should more than cover my 21mm diameter of my image
sensor. So I$B!G(Bm not sure why I$B!G(Bm getting such severe vignetting.
I don$B!G(Bt know if the spacing between the components of my image train
are correct. (I don$B!G(Bt even know how I hooked it all up for that
test!) I$B!G(Bm wondering the vignetting could be caused by either the
tube connecting the reducer to the EFW or maybe the filters themselves. I
doubt the latter as that spacing is the same as I always use it.
I don$B!G(Bt know how to determine if it$B!G(Bs the tube (Celestron
#93633-A).
I$B!G(Bm open to experiments that might help me determine where the
problem is.
Mike McDonald
mikemac@xxxxxxxxxxx
On Jun 1, 2019, at 2:47 PM, Michael McDonald <mikemac@xxxxxxxxxxx> wrote:
Hmm, I thought coma would exhibit as a radial effect. The stars on the left
side look like that could be the case. But the ones on the right look like
they$B!G(Bre more vertically oriented. Could it be collimation? Or a
combo of the two? Bad enough polar alignment so that rotation is showing
up? Operator error? Equipment mismatch? Lots and lots of possibilities. :-(
My reducer/$B!H(Bcorrector$B!I(B bites! If you don$B!G(Bt remember
(and no reason you should), it gives me images like this:
http://www.mikemac.com/Astrophotos/M42_L_2019-03-22_20_Bin3x3_60s_reducer.png
I wish I could find one that was 0.5X and would cover my ASI1600$B!G(Bs
17.6mmX13.3mm surface. That would give me an effective focal length of
around 700-800mm. And that would open up a lot of other targets! A
Hyperstar$B!G(Bs 290mm is way too wide. Plus I$B!G(Bd have to get a
color camera. Kaching!
Mike McDonald
mikemac@xxxxxxxxxxx
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