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Old 13-10-2020, 11:23 AM
Emuhead
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4" focusers, enormous imaging circles

Hi guys,

Checking out Takahashi's, as you do, noticed that FSQ106 & TOA130NFB have 4" focuers and enormous imaging circles (88mm and up).. what sensor in the consumer market could handle this kind of field? Guessing these are out of reach for most imagers. Putting a smaller sensor on a scope like that seems like a waste of photons right?

Last edited by Emuhead; 13-10-2020 at 11:49 AM.
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Old 13-10-2020, 11:30 AM
sunslayr (David)
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I think it's so you can do a significant reduction of focal length without vignetting on medium format cameras but I'm not sure. Still out of reach for me though
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Old 13-10-2020, 11:40 AM
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Yep - Tak still show a Pentax 67 format film camera in many of their system charts. 6x7cm film size is far bigger than a KAF 16803 sensor (36x36mm).

DT
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Old 13-10-2020, 11:49 AM
Emuhead
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..and what size filter would this need to not have vignetting? Biggest Astrodon 3nm i found is 65 x 65mm for 'only' 2500USD each. But i take your point on reducers, that could be beneficial.
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Old 13-10-2020, 01:34 PM
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Imaging performance slowly degrades the further you move off axis.

So it's not so much that you have to fill the 88mm image circle with a sensor, but more a case of having un-distorted stars out the edge of the current crop of sensors.

For example: the 60Mp IMX455 sits comfortably within the 88mm circle, but slight edge distortions can still be seen.

While a 4" focuser may not deliver perfection, it's a whole lot better than the 2" variety.

Expensive? Sure. But my experience is if you pay for the penny ride, that's usually what you get
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Old 13-10-2020, 02:08 PM
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Also interesting.. ok, so knowing the full imaging circle wont be utilised, it can be a good idea to go a 4" focuser over a 2.7" focuser to get a sligtly less distorted image, although at sensor sizes of Full Frame or under, its probably negligible im guessing..
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Old 13-10-2020, 02:23 PM
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I had a Tak FS152 with a 2.7 inch focuser. I found it could not handle a fullframe camera. I upgraded to a 4 inch focuser and no troubles.

My experience is 3 inch minimum or 3 inch + are needed for full frame.

Greg.
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Old 13-10-2020, 05:53 PM
Emuhead
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Ok noted, so full frame will benefit from the 4" focuser, and APSC and smaller should be fine for 2.7" focusers.
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Old 13-10-2020, 08:16 PM
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You also can't use the 67 Flattener with the TOA-130 if it only has the 2.7 inch focuser. I had to purchase a Feathertouch focuser to replace the 2.7 inch focuser on the TOA I had so that I could use the 67 flattener.

DT
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Old 14-10-2020, 02:02 PM
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I know that with the 10" f4 Newt that I went for has a 2" focuser as
anything else was going to be ridiculously pricey.
Large field flatteners, filters and the focuser with OAG equipment
and with focus drive motor, larger camera -
will all be extremely costly -
much more than the telescope itself.

OK - a 2" focuser will not allow for 100% illumination
of the entire field but
it will still image well at the center of the target.
Even if you did upgrade to a larger focuser then
you'd still be thinking about adaptive optics -
( if you had a telescope with the back focus to allow for it. )
Then you'd get fed up with setting it up every time
you wanted to use it
and need and observatory dome & pier.

Astronomy seems like a bottomless pit to throw money at.
Everyone needs to draw a line somewhere when they set their budget.


cheers
Allan
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Old 14-10-2020, 02:45 PM
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I couldn't agree more. Have to have a strict budget in place otherwise what should be a relatively small number can very easily blow out to a very large number in no time. And agree, the larger everything gets, the greater the cost of all the extras. Large filters & camera alone..
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Old 14-10-2020, 07:59 PM
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Quote:
Originally Posted by Emuhead View Post
I couldn't agree more. Have to have a strict budget in place otherwise what should be a relatively small number can very easily blow out to a very large number in no time. And agree, the larger everything gets, the greater the cost of all the extras. Large filters & camera alone..
It starts really with the sensor size you want to image with. Full frame or "medium format" as in the 16803 CCD requires a huge amount of optical and mechanical perfection.

But these days there are a large number of excellent sensors from smaller than 4/3rds to 4/3rds to APSc. APSc is often considered a large sensor these days.

The ASI/QHY183, 294, 1600 sensors are not demanding or require large illuminated circles so work from which sensor you are planning to use and take it from there. Factor in if you feel you are likely going to want to upgrade to larger sensors later though as filters, filter wheels etc need to be larger also with larger sensors. Everything is more costly with the larger sensors.

I have seen plenty of stunning images from these smaller sensors though so don't think you would be slumming it using one. Not at all.

Greg.

Greg.
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