SingAstro

Hi All,

I'm planning to venture into AP, and need some advice on equipment. I've been doing some research lately and narrowed down my choice of telescope to the Celestron 925 HD with CPC or CGEM mount. What I'm not sure are:

1. Is CPC (dual fork) or CGEM mount more suitable for AP?

2. As Singapore is near the equator, do I still need a wedge for deep space AP with the CPC? The tracking modes for CPC are Altazimuth, EQ North and EQ South. For CGEM, it's EQ North and EQ South. If a wedge is needed, does it need to be customised for use in Singapore? What kind of customisation is required?

3. In terms of portability, CPC seems to be more bulky but lighter than CGEM. Which do you think is easier to transport around as well as storage when not in use?

4. Does anyone have experience with both 8-inch and 9.25-inch SC telescopes in terms of portability and imaging capability? I'm wondering how much I will be sacrificing in image quality if I opt for the smaller 800 HD version for the sake of portability.

Really appreciate spending your time to share your thoughts on this.
Btw, I would be interested if anyone have the above equipment for sale.

Hi Jchangcc. Welcome to the forum!

There are very generally 2 major types of AP - lunar/planetary and deep-sky. Some telescope design is better for one and not so good for the other. There is no perfect telescope design that can do both very and equally well.

You may want to take this factor into consideration when venturing into AP.

Hello!
There are many flavours of AP but I think generally when people think of AP they think of pretty Hubble Telescope pictures. Those are generally deep-sky pictures requiring long exposures totalling into the hours.
To answer your first question: You can image deep sky with a CPC 9.25 but because it is an Alt-Az mount, it will suffer from field rotation. One way to overcome this is to use a Fastar setup which converts your f/10 scope into an f/2 by replacing the secondary mirror. If not, an equatorial mount, such as the CGEM is a much better idea.
You will still need a wedge. In fact you will have to be prepared that your wedge will be nearly perpendicular.
Portability: Lets just say that lugging out either scope will be a physical workout. You will have to be very, very strong to handle the whole setup, not counting associated things such as batteries, camera, eyepieces needed to run the equipment. If you don't drive and don't have easy access to an observing site well within comfortable distance or in your house, think twice.
With regards to your scope: why a 9.25HD? Is it because you are concerned about the aperture providing picture quality? (as in vs. an 8 inch?) When it comes to imaging, aperture plays a very little role in the actual picture quality. Very often things such as mount stability, periodic error, wind, weather conditions and extremely importantly, alignment play a much greater role than aperture. In imaging, far more important is the focal ratio. The role of aperture is to reduce imaging times and to provide difference image scales. A bigger scope does not result in a brighter picture unless it has the same focal length (hence a faster f ratio)
Personally, I'd be very cautious in advising a complete beginner to start with a SCT. Because it is a big scope and has a very long focal length, any errors are magnified many times and required imaging times are long. A much better starting point is a short focal length refractor (or imaging newtonian, if you care to collimate) which will provide wide field images.
If you're still keen on an 8-9" instrument, may I humbly suggest if you don't mind buying used to consider my WTS Ad for the sale of Vixen VMC200L and Vixen SXD? The vixen mounts are a step up from the Celestron mounts, although not top notch like the Astro-Physics or Paramount mounts, although again I must caution that if the basics have not been mastered yet, the quality of the mount may not come so much into play as the learning factors.

Gary wrote:Hi Jchangcc. Welcome to the forum!

There are very generally 2 major types of AP - lunar/planetary and deep-sky. Some telescope design is better for one and not so good for the other. There is no perfect telescope design that can do both very and equally well.

You may want to take this factor into consideration when venturing into AP.

The reason why i narrowed down to the celestron edgehd is bacause its designed for AP with its flatfield. Does anyone have any experience with this telescope?

Hi Jchangcc,

Welcome to singastro

It will be good to meet up with some of the members during obs sessions/ public events to see and learn about imaging/astrophotography, what are the pros & cons , the challenges. Singapore's heavily light-polluted skies are a huge obstacle in AP, leading to groups venturing to dark sites like Mersing, or parts of JB like Sedili (i think) to take astrophotos.

Its not...really.. a good idea to dive straight into it by buying the gear first. And as cloud_cover mentioned, aperture of the scope is not as critical but the F-ratio is.

Im a visual observer by the way =) Have not used the 9.25" SCT before , my experience of a 8" SCT was during my JC years.

Regards,
Junwei

Hi cloud_cover,

Thanks for your advice. From what I gather, a wedge is a must for deep-sky AP, but what kind of modification is required for use in SG/MY? Since a wedge is required even for CPC, then it makes more sense to just get the CGEM mount.

By using a Fastar setup, the magnification will not be sufficient for deep-sky AP, and it won't be possible to use a DSLR mounted setup. Based on your setup, I figure that CGEM mount is stable enough for AP. Also, it would be difficult to store the OTA in a bag if it's CPC mounted.

It seems the f-stop for an 8 or 9-inch SCT is typically F10. I intend to invest in the right equipment the first time to avoid having to upgrade later.

In terms of portability, I'm happy as long as it's possible to move the equipment with the help of a trolley, and it can fit into the boot or back seat of my car.

cloud_cover wrote:Hello!
There are many flavours of AP but I think generally when people think of AP they think of pretty Hubble Telescope pictures. Those are generally deep-sky pictures requiring long exposures totalling into the hours.
To answer your first question: You can image deep sky with a CPC 9.25 but because it is an Alt-Az mount, it will suffer from field rotation. One way to overcome this is to use a Fastar setup which converts your f/10 scope into an f/2 by replacing the secondary mirror. If not, an equatorial mount, such as the CGEM is a much better idea.
You will still need a wedge. In fact you will have to be prepared that your wedge will be nearly perpendicular.
Portability: Lets just say that lugging out either scope will be a physical workout. You will have to be very, very strong to handle the whole setup, not counting associated things such as batteries, camera, eyepieces needed to run the equipment. If you don't drive and don't have easy access to an observing site well within comfortable distance or in your house, think twice.
With regards to your scope: why a 9.25HD? Is it because you are concerned about the aperture providing picture quality? (as in vs. an 8 inch?) When it comes to imaging, aperture plays a very little role in the actual picture quality. Very often things such as mount stability, periodic error, wind, weather conditions and extremely importantly, alignment play a much greater role than aperture. In imaging, far more important is the focal ratio. The role of aperture is to reduce imaging times and to provide difference image scales. A bigger scope does not result in a brighter picture unless it has the same focal length (hence a faster f ratio)
Personally, I'd be very cautious in advising a complete beginner to start with a SCT. Because it is a big scope and has a very long focal length, any errors are magnified many times and required imaging times are long. A much better starting point is a short focal length refractor (or imaging newtonian, if you care to collimate) which will provide wide field images.
If you're still keen on an 8-9" instrument, may I humbly suggest if you don't mind buying used to consider my WTS Ad for the sale of Vixen VMC200L and Vixen SXD? The vixen mounts are a step up from the Celestron mounts, although not top notch like the Astro-Physics or Paramount mounts, although again I must caution that if the basics have not been mastered yet, the quality of the mount may not come so much into play as the learning factors.

If you're going to go into dedicated AP, my suggestion is to go for a fast apochromatic refractor on a sturdy German equatorial mount. If you want to do a bit of visual, just get another optical tube assembly (OTA), such as SCTs, that your GEM can support.

I believe all APers will tell you, in AP, mount is (almost) everything. (The other is luck )