Choosing the best astrograph

[Slawomir (Suavi)]

Originally Posted by Stefan Buda

Yes, that is better but the 8" RASA would fall off range. I think wee need to start lower. Also let's agree on the theory first.

Please forgive me for giving a specific example, but when looking at what is currently being used by astrophotographers, for the maximum wide-fields low resolution purposes, I believe it’s hard to improve on 106 FSQ + KAF16803 combo.

Therefore at the lower end of focal lengths range, petzval seems to be the most optimal astrograph.

[Stefan Buda]

Originally Posted by Slawomir

Please forgive me for giving a specific example, but when looking at what is currently being used by astrophotographers, for the maximum wide-fields low resolution purposes, I believe it’s hard to improve on 106 FSQ + KAF16803 combo.

Therefore at the lower end of focal lengths range, petzval seems to be the most optimal astrograph.

I don't dispute that. It may be true, but as I said in an earlier thread, the FSQ106 does not play ball when it comes to theory - Greg will be pleased to hear that - and I have no idea how to deal with it, even though I indicated that I know the reason. I'm convinced that that the FSQ106 produces larger spots than an equivalent FL APO with flattener. But it does it in a nice uniform way, across a large field.

[gregbradley]

Originally Posted by Stefan Buda

I don't dispute that. It may be true, but as I said in an earlier thread, the FSQ106 does not play ball when it comes to theory - Greg will be pleased to hear that - and I have no idea how to deal with it, even though I indicated that I know the reason. I'm convinced that that the FSQ106 produces larger spots than an equivalent FL APO with flattener. But it does it in a nice uniform way, across a large field.

I see the usual tighter stars in the centre and slightly weaker further out in their spot diagrams for the FSQ106EDXiv (the various EDX models have identical optics and its only various mechanical changes to overcome focuser sag in some earlier models).

https://www.takahashiamerica.com/tak...r-telesco.html

Greg.

[Slawomir (Suavi)]

From what I have seen a quality APO will have tighter stars that Petzval (like FSQ) but since one of the requirements for an outstanding astrograph is a larger corrected circle and the reason to use a short focal length is to take wide fields low resolution images, I still think petzval is the optimal solution for a short focal length wide field astrograph.

Having said that, if we are willing to give up on some of that wide field, then a well-corrected APO might be a better astrograph in terms of data quality, but not the speed and wide-field.

I really look forward to seeing what Greg will think about his new 105mm hand-aspherised oil spaced APO. It is promised to cover a full-frame at native f/6.

[gregbradley]

Originally Posted by Slawomir

From what I have seen a quality APO will have tighter stars that Petzval (like FSQ) but since one of the requirements for an outstanding astrograph is a larger corrected circle and the reason to use a short focal length is to take wide fields low resolution images, I still think petzval is the optimal solution for a short focal length wide field astrograph.

Having said that, if we are willing to give up on some of that wide field, then a well-corrected APO might be a better astrograph in terms of data quality, but not the speed and wide-field.

I really look forward to seeing what Greg will think about his new 105mm hand-aspherised oil spaced APO. It is promised to cover a full-frame at native f/6.

FSQ is a hard to beat widefield instrument. It has a few downsides though.
One is it is very sensitive to focus shift due to temperature change. The other is the flat colour it tends to have (EDX versions). The black paint used in the tube has a green bias. The coatings on the lens I am not 100% sure I like them. They look mustard in colour and no other scope I have had that.

The focuser has been a weak point in the EDX series hopefully finally fixed with the iv model although I would not be surprised to hear of the odd one here and there that flexes still. Replacing the focuser with a Moonlite or Feathertouch would be the safe option with a temperature compensation electronic focus option.

F5 and 106mm is a good imaging platform. The flat field is great as is the sharp to the corner stars. Its not wind affected on a decent mount which was a great advantage for me at my dark site. If it were too windy it was FSQ time!

A lot of the all time best images are from the various FSQ models. The EDX models optics as far as I know have not changed only the mechanicals like the infamous Captains Wheel and slight changes to the focuser.

The focus lock mechanism does not work and the micro focus adjustment is only 1:6 which is not fine enough for a scope like that with a small critical focus zone.

So its not all roses but its definitely the most famous and popular widefield imaging scopes of all time for a reason.

I can see another one in my future at some point. A TV127i also looks to a nice substitute for the $26,000 FSQ130 which seems outrageous for a 130mm APO scope but hey maybe its amazing (I doubt that its $26,000 amazing though).


Greg.

Greg.