Reducer? correctors?

[pequod]

Hi. Just joined the forum and looking for advice. Living in Spain so have about 300 days clear skies per year. Astophotography is my main interest.

I have decided on the CG-5 mount as it can handle a wide range of OTA's and has "go-to". I would like a 100mm APO refractor, but not yet as funds are tight. So I was thinking of a smaller, cheaper (70mm or whatever) which could be used as an autoguider at a later date. Current buget for this OTA is €700 (including IP's finder etc). I have a small DSLR and a microscope CCD which will be used. Any advice on this would be appreciated.

Some supliers list a reducer/corrector for photography. Why would I need this on an APO? Thanks, Ed

[Louis D]

7 hours ago, pequod said:

Some supliers list a reducer/corrector for photography. Why would I need this on an APO? Thanks, Ed

Most APOs have curved fields because the radius of curvature for most refractors is about one third of the focal length.  There are some astrographs (4 or more elements) and Petzvals with designed flat fields.  For the rest, you'll need to purchase a field flattener unless you use a really small imaging chip (planetary imaging) or buy a really long focal length refractor.  The shorter the focal length, the shorter the radius of curvature, the worse the corners will look.  Being an APO simply means really good control of false color.  It does not mean complete freedom from chromatism as in a reflector or flatness of field.

[Dr Strange]

Are you buying the CG-5 used? It is no longer made by Celestron. It has been replaced by the Advanced VX mount. I have owned both and both will serve you well and are good mounts at their price point. As to a refractor there are a couple of options in your budget. If you would like to do visual as well as AP (Astro Photography) then a 120mm Skywatcher Evostar would be a great option on either mount. It is a bit slow for an imaging refractor but it can still be used to image and it does very well visually. It is also very affordable and has very good glass for a doublet. In the middle of your price range would be the Williams Optics 2019 Zenithstar 73 II.  At the top end of your price range would be the Takahashi FS-60. It can be resold at about 80% of its new price if you decide you want to sell it. It is the very top shelf of refractors. I provided you links to all three below. 

A flattener will likely not be needed for these three telescopes if your DSLR is using a APS-C sensor. Anything bigger than that will likely require a flattener. There is a reducer for the 120 that, with the 120's price, is very affordable. My choice in your shoes would be the 120 because I can do both AP and visual with it. I had a great view of Mars with a friend's 120. Saw the ice caps and some detail on the mountain ranges. Small DSO were small but not too bad. 

 

https://www.firstlightoptics.com/evostar/skywatcher-evostar-120-ota.html

https://www.firstlightoptics.com/william-optics/william-optics-2019-zenithstar-73-ii-apo.html

https://www.firstlightoptics.com/takahashi-fs-60-series-refractor-telescopes/takahashi-fs-60cb-f5-9-doublet-fluorite-apo-refractor.html

[pequod]

Thanks Red Dwarf, that really helps me. Also thanks to Star Forming. I did see the Skywatcher and it would be a good one to start. However, the Williams and Takahashi are well out of budget for now as I would have to add EP’s etc to this price. Maybe a Takahashi next time.  I need to look at an alternative mount. The VX is more expensive. Maybe I will start from sctratch.

[Louis D]

On 22/07/2020 at 03:04, pequod said:

Thanks Red Dwarf, that really helps me. Also thanks to Star Forming.

Just an FYI, our user handles are Louis D and Dr Strange, respectively.  Red Dwarf and Star Forming are our member "rankings" based on number of posts.  Your ranking is Vacuum.

[pequod]

I'm OK with that. I always thought I sometimes live in a vacuum.