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Help me choose - Refractor or Reflector ?


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I am sure others have asked this before but here goes.

I have so far used my DSLR with a zoom lens to do my astrophotography using my ST80 with guidecam for guiding.

Now I have a good mount I am about ready to take the next step.:help:

I was planning on going for a 100/120 ED APO triplet refractor but am now having second thoughts  and been looking at the Celestron Edge 8" or 9.25".

My max budget would be £2K. 

It will be used almost entirely for astro photography of Deep Sky Objects I am not really into observing.

What are  the pros and cons ? 

I understand collimation for the reflector but what else should I consider.

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The biggest thing that I would be considering here is the speed of the SCT's - They are slllloooooowwwww as a dog for DSO imaging. I know, I've used one. With that in mind a DSLR is so not the camera for this. You'd benefit hugely from a sensitive camera and that will tend to be a cooled CCD (or the new CMOS looks interesting) You are going to need long subs as well with the SCT - Is your mount up to it?

Life is easier with a refractor and I say that having tried imaging with a refractor, SCT, RC and ODK....... refractors are the most plug and play you can get. 

Collimation of an SCT isn't difficult, but I'm still not convinced that they offer the sharpness of refractors. 

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Also bear in mind that with the C.925 SCT, although it is a very good scope, the field of view tends to be very small.  Fo r visual you can barely squeeze more than one degree out of it, so many objects will be out of reach.

If you go for an ED Triplet in the region of 900-1000mm FL and around 5 inch aperture, you can get well over 2 degrees - almost 3 degrees with the right EPs.  Hard to beat a triplet frac for sharpness of view too.

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My choices lead me to a refractor, and a CCD color camera. (For the cooling (TEC) of the sensor)

I fancy Nebulae as my objects of fascination. And I wanted something that could do very long exposures.

I came to my conclusion, as basic as it is, that the telescope would be primarily a lens. And I did not want the added fiddling of collimation in the overall quest for imaging. (Or the more there is to move and change the image, the more that could go wrong.)

I also stumbled onto structure and how temperature could give me focusing issues as the main tube might expand or contract as the nights temperatures change. So I "bit the bullet" and got a carbon fiber tube.

And finally, I figured I could use an APO triplet telescope as an observing tool, but that a doublet or other might leave me wanting.

So I'm a refractor fan. And a small refractor fan at that. I can carry my entire telescope/camera/guiding assembly in one hand out to my waiting mount. Then return and carry out my laptop. From there it's all plug and play.

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Well, I do have several friends who post some very amazing images. One with a Celestron 9.25" reflector, and another with a Celestron RASA telescope.

Both of them are far from my retirement budget though, I'm just casual.

http://www.celestron.com/browse-shop/astronomy/telescopes/rowe-ackermann-schmidt-astrograph-with-cgem-dx

http://www.celestron.com/browse-shop/astronomy/telescopes/cge-pro-925-hd-computerized-telescope

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I don't think many would argue that about the most popular imaging rig is an 80mm, f5/6 refractor with the KAF 8300 chip (the ASI 1600mm Cool is the new kid on the block though and mighty tempting)

Why? Because there is about a bazzilion targets that fit beautifully on the chip and it just works, short focal length so easy on the guiding, no collimation... no messing about. I've had mine for two years and I've only scratched the surface of what's possible.

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OK, I have to do this..... not for the first time i hasten to add or those who have seen this before. (OK, rubbish, but Im quite proud of this, considering ive never managed anything since :()

got a couple of lesser images of M81 around the same time - just not as good. TBH nowhere as good.

Skywatcher 200P-DS reflector, 0.9 coma corrector, Canon 70D, unguided on AVX mount, 

Reflectors can do good.

Larger aperture, more light, little chromatic aberation.

Yes, collimating may be required - practice makes perfect; i have seem people complete that i 5 minutes, and they do it every night out.

Next month i expect to do bettter.

 

 

IMG_0395.JPG

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8 hours ago, wornish said:

I am sure others have asked this before but here goes.

I have so far used my DSLR with a zoom lens to do my astrophotography using my ST80 with guidecam for guiding.

Now I have a good mount I am about ready to take the next step.:help:

I was planning on going for a 100/120 ED APO triplet refractor but am now having second thoughts  and been looking at the Celestron Edge 8" or 9.25".

My max budget would be £2K. 

It will be used almost entirely for astro photography of Deep Sky Objects I am not really into observing.

What are  the pros and cons ? 

I understand collimation for the reflector but what else should I consider.

As the owner of a beautiful C8 Edge SCT, I would have no hesitation in recommending a..... refractor. Despite my love for the C8, and the excellence of many images taken by members of this site with their SCTs, I'd get a decent frac for your needs. Takahashi 100mm doublet at the top of your budget, or an Explore Scientific 127mm V2 triplet? Cheaper 120mm doublets from Skywatcher? All very good telescopes.

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If you are going to use a DSLR a long focal length is a total waste of time. You have to have a reasonable match between pixel size and focal length. If you have an overly long FL with small pixels you are trying to resolve details at a scale which the seeing and, quite probably, the guiding will not allow. So all you get from your long FL is a reduced field of view. You don't get any more detail than you'd get at a shorter focal length with a wider field of view. 

Depending on your site and the accuracy of your mount under guiding I would think that something like 1.0 to 3.5 arcseconds per pixel would be 'reasonable.' There is a calculator here. http://www.12dstring.me.uk/fov.htm  For an easy life keeping it to something closer to the 2"PP or above might be best. 

DSLRs like fast F ratios. At exotically fast F ratios they can even rival CCDs. The budget way to a fast F ratio and shortish focal length is a small Newtonian with coma corrector.

Nutshell: for a DSLR, short focal length and fast F ratio.

Olly

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