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My next scope: SCT or APO?

astroexplorer

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astroexplorer

astroexplorer

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It is time for a new scope! I enjoy astrophotography for 3 years now. I use a SW NEQ-6 Pro mount, an ATIK 460 EX mono camera, a ZWO 120MM camera for guiding/planetary, FW and LRBG/narrow band filters, 2.5x GSO barlow and 5x Powermate. I started with a SW 200pds to also use for observing but it was a difficult and not forgiving start. Since 2 years I use a Takumar 200: easy and forgiving with amazing results in narrowband but not in RGB. Regarding personal preference, I would say 60% DSO - 40% planetary.

Now I can afford another scope and my budget is about 1500-1700 pounds (scope and flattener). Initially I thought of an 70-80 mm APO (triplet ideally) but then with Mars coming closer I thought of a 9.25 SCT not to miss the opportunity. By the way, I will be spending summer in southern Europe with the equipment before the scope returns to light-polluted Netherlands. No permanent observatory hence ability to setup, and move around is critical hence the 9.25 and not something larger. 

A few questions come to mind:

1. will the 9.25 offer me much more than the 200 pds for Mars and other planetary?

2. With several APOs between F5 and F6 within my budget,how to choose?

any ideas that could help me make up my mind?

Many thanks,

astroexplorer

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Gerry Casa Christiana

Gerry Casa Christiana

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The 9.25 will be too heavy for the mount for DSO but ok for planetary. 

Why not get something that's dedicated more to wide field which you don't have. Use your 200 for planetary and get a nice Apo with FPL53 glass that's well corrected. More than likely though you may never use your 200 again as it will be so much hassle compared to a light apo. As to make I can't help. I have that quandry too. I go to astrobin look at pictures and see if it is what I want to achieve. 

Gerry 

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astrosathya

astrosathya

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The NEQ6-PRO has a payload capacity of 25kg visual and about 18kg for astrophotography. The C9.25 weighs in just under 10kg (20lbs). Along with a heavy DSLR/CCD with filter wheel and filters etc and maybe even a focuser, the load wont go beyond 15kg. I think you can safely go for a C9.25. No tot mention that the resolving power of the 9.25 will be far better compared to a triplet of 80-100mm. You could even add a hyperstar in the coming days and it'll work well with the 460EX. 

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wimvb

wimvb

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Do I dare mention the word doublet?

Since you do mono imaging, you could consider getting a good doublet rather than a triplet. Mono imaging is a little less sensitive to CA, afaIk. So you can get more aperture per €.

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ollypenrice

ollypenrice

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Talking only about DS imaging, I'd avoid the SCT. Turning theoretical resolution into details visible on the screen would lie between very difficult and impossible, given the need for perfect seeing and outstanding guiding. Your field of view would also be exasperatingly tiny in the 460.

The Esprit Pro 80 is listed by FLO at £989. You'd get 2.3 arcseconds per pixel which is entirely realistic even with a mobile setup and a FOV of about 1.8 x 1.4 degrees. That would be a nice DS setup.

Olly

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ollypenrice

ollypenrice

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4 hours ago, astrosathya said:

The NEQ6-PRO has a payload capacity of 25kg visual and about 18kg for astrophotography. The C9.25 weighs in just under 10kg (20lbs). Along with a heavy DSLR/CCD with filter wheel and filters etc and maybe even a focuser, the load wont go beyond 15kg. I think you can safely go for a C9.25. No tot mention that the resolving power of the 9.25 will be far better compared to a triplet of 80-100mm. You could even add a hyperstar in the coming days and it'll work well with the 460EX. 

I think that the mistake here is to concentrate only on payload. For DS imaging you also need to think about accuracy. Even reduced to F7 the C9.25 will be working at 0.57 arcsecs per pixel, meaning that you'd want a guide RMS of about 0.25* arcseconds. That is a tall order for any mount and effectively out of the question for an EQ6.

Binning 2x2 might make the exercise possible but you'd still be left with the tiny field of view and no more real resolution of detail than you could acheive with a far shorter focal length and vastly wider field. This strikes me as pointless.

Olly

*Corrected, thanks to Vim's vigilance, from 2.5" - which would never do at all! My apologies.

 

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wimvb

wimvb

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19 minutes ago, ollypenrice said:

the C9.25 will be working at 0.57 arcsecs per pixel, meaning that you'd want a guide RMS of about 2.5 arcseconds.

?? Guiding rms at 2.5 arcsecs with imaging at 0.57 arcsecs? What am I missing here? I thought that guiding should be better than image scale.

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newbie alert

newbie alert

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Totally agree with olly..the esprit is a superb tool and at just under £1000 pounds superb value , a friend is selling a 9.25 on astro buy sell for £750 ,you could use it for the mars opposition and sell it on afterwards..i wouldn't try if for dso..it can be done and know plenty that do use even up to 11 inch for deep sky but needs abit more money thrown at it to make it work..

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ollypenrice

ollypenrice

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1 hour ago, wimvb said:

?? Guiding rms at 2.5 arcsecs with imaging at 0.57 arcsecs? What am I missing here? I thought that guiding should be better than image scale.

My answer came in kit form: you were supposed to move the decimal place yourself! ?

Sorry, 0.25 arcsecs.

Well spotted.

Olly

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wimvb

wimvb

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29 minutes ago, ollypenrice said:

My answer came in kit form: you were supposed to move the decimal place yourself! ?

Sorry, 0.25 arcsecs.

Well spotted.

Olly

Well, just because I moved to the land of IKEA, doesn't mean I subscribe to their point of view. ??

Glad though for the clarification. Thanks

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  • 2 weeks later...
astrosathya

astrosathya

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On 12/06/2018 at 14:21, Gerry Casa Christiana said:

Really? I've always read the accepted wisdom was half the total observing weight. Can I ask where you got this from. Many thanks 

Gerry

Hi Gerry, I only took payload capacity into consideration and should've thought about the accuracy of the mount itself. However, answering your question, I got this number from several sources on the internet. There seems to be two schools of thought about loading it either 50% or 75%, I chose the latter. From personal experience, I've loaded my HEQ5-Pro mount upto 15 kg with an 8" F/5, 80ED, DSLR, Finder and rings to fit them all together and it did not suffer from problems. Now however I stick with about 10kg overall because i've sold my 8" newt as it was too big to carry to the countryside without a car.

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