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what are small refractors for?

Refractor Astrophotography
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#1 turings.enigma

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Posted 31 July 2021 - 08:57 PM

I have used William Optics Zenithstar 73 and Sharpstar 94 EDPH. Those have mid focal length. I also plan to buy Astrotech 130 EDT, which has 910mm focal length. Should I buy small refractors like RedCat? I feel I can always do Mosaic if want to have a larger field of view.

 

What additional benefits do I get from a small refractor with 230~250mm focal length?



#2 SpaceConqueror3

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Posted 31 July 2021 - 09:36 PM

An extreme wide field of view?


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#3 Enance42

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Posted 31 July 2021 - 09:36 PM

A few things that I can think of...
Wide field of view, depending on camera sensor of course.
Light weight
Easier with tracking/guiding
More portable
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#4 rj144

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Posted 31 July 2021 - 09:47 PM

Mosaics aren't trivial and take a much longer time.


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#5 turings.enigma

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Posted 31 July 2021 - 10:54 PM

Mosaics aren't trivial and take a much longer time.

Right. I used ASI air pro and telescopius.com plan mosaics, but the coordinates were a bit off. It also takes a long time to process mosaics.



#6 turings.enigma

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Posted 31 July 2021 - 10:56 PM

An extreme wide field of view?

I think only a few objects require very wide view. I feel my sharpstar 94 + 2600mm pro covers most of the DSOs. So my next step is to zoom in instead of zoom out.



#7 turings.enigma

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Posted 31 July 2021 - 10:57 PM

A few things that I can think of...
Wide field of view, depending on camera sensor of course.
Light weight
Easier with tracking/guiding
More portable

Yes, portable might be one benefit if I get a lighter mount. I have not taken my telescope out yet. I am using eq6-r pro, which is quite heavy.



#8 RichA

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Posted 31 July 2021 - 11:03 PM

I have used William Optics Zenithstar 73 and Sharpstar 94 EDPH. Those have mid focal length. I also plan to buy Astrotech 130 EDT, which has 910mm focal length. Should I buy small refractors like RedCat? I feel I can always do Mosaic if want to have a larger field of view.

 

What additional benefits do I get from a small refractor with 230~250mm focal length?

Most of these produce images that are just a conglomeration of small reddish smudges of hydrogen gas and stars.  Really not a lot of real detail owing to the small image-scale of most deep-sky objects. There are exceptions, some huge expanses of nebula though they are likely better-served with a lens of larger aperture.  But short focal length lenses permit modest mounts to perform pretty well for photography not having the stringent requirements of stability that mounts used for longer focal length instruments need. But given their slow photographic speeds (f4ish at 200mm or so), you have to wonder whether a good, used camera lens might not be better or at least as good? 



#9 turings.enigma

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Posted 31 July 2021 - 11:06 PM

Most of these produce images that are just a conglomeration of small reddish smudges of hydrogen gas and stars.  Really not a lot of real detail owing to the small image-scale of most deep-sky objects. There are exceptions, some huge expanses of nebula though they are likely better-served with a lens of larger aperture.  But short focal length lenses permit modest mounts to perform pretty well for photography not having the stringent requirements of stability that mounts used for longer focal length instruments need. But given their slow photographic speeds (f4ish at 200mm or so), you have to wonder whether a good, used camera lens might not be better or at least as good? 

the boundary between camera lenses and small refractor is vague.



#10 licho52

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Posted 01 August 2021 - 07:35 AM

From my POV the number of targets that are justifiably serviced by a short refractor is relatively low.  For that small number of objects I do mosaics (NINA makes it very painless) rather that buy small refractor where the resolution is terrible and the final outcome is unsatisfactory.

 

I have a short refractor at 275mm and it's wonderful at what it does but the number of targets for it compared to my 1200mm is very low.  The 1200mm RC has targets to last me a lifetime.

 

For me going deep is where the true reward is.


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#11 turings.enigma

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Posted 01 August 2021 - 08:32 AM

From my POV the number of targets that are justifiably serviced by a short refractor is relatively low.  For that small number of objects I do mosaics (NINA makes it very painless) rather that buy small refractor where the resolution is terrible and the final outcome is unsatisfactory.

 

I have a short refractor at 275mm and it's wonderful at what it does but the number of targets for it compared to my 1200mm is very low.  The 1200mm RC has targets to last me a lifetime.

 

For me going deep is where the true reward is.

What RC are you using? What’s the aperture? I agree a small refactor is not very useful to me at this point. I am currently using a 413mm refractor. Should I go for 900mm or 1200mm? I am curious about your collection.



#12 licho52

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Posted 01 August 2021 - 08:43 AM

I got 6" RC with 1000mm first and then added 8" version at 1200mm after reduction.  1200mm is very good for most objects, any more and you need some really expensive guiding.  I use superpixel in post-processing to get image scale to 1.3px.

 

6" and 8" RCs from GSO are very affordable but needed to be collimated.  One may want to also upgrade the focusers but it's not a huge priority for me.  8" is f/6 after reduction is surprisingly "bright" for that focal length.  It really gathers a lot of light.



#13 turings.enigma

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Posted 01 August 2021 - 08:50 AM

I got 6" RC with 1000mm first and then added 8" version at 1200mm after reduction.  1200mm is very good for most objects, any more and you need some really expensive guiding.  I use superpixel in post-processing to get image scale to 1.3px.

 

6" and 8" RCs from GSO are very affordable but needed to be collimated.  One may want to also upgrade the focusers but it's not a huge priority for me.  8" is f/6 after reduction is surprisingly "bright" for that focal length.  It really gathers a lot of light.

Is there a brand that requires minimum collimation? Can I use guidescope instead of off axis guiding for a 8inch RC with reducer?



#14 dswtan

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Posted 01 August 2021 - 12:36 PM

> Is there a brand that requires minimum collimation?

 

LOL, no. :-) The pervasive RC6/RC8s you see around all seem to be one manufacturer, and multiple brands. I personally love my AT8RC for galaxies. I do find the collimation to be painful, but a tri-bahtinov mask helped a lot, as well as the expensive Takahashi collimation scope. A basic Cheshire was a must. And previous practice with an SCT also helped. I feel a bit better about it all now, but it's still intimidating and there's no way I'm touching the primary.

 

> Can I use guidescope instead of off axis guiding for a 8inch RC with reducer?

 

Not if you want reliably tight round stars.



#15 turings.enigma

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Posted 01 August 2021 - 05:24 PM

> Is there a brand that requires minimum collimation?

 

LOL, no. :-) The pervasive RC6/RC8s you see around all seem to be one manufacturer, and multiple brands. I personally love my AT8RC for galaxies. I do find the collimation to be painful, but a tri-bahtinov mask helped a lot, as well as the expensive Takahashi collimation scope. A basic Cheshire was a must. And previous practice with an SCT also helped. I feel a bit better about it all now, but it's still intimidating and there's no way I'm touching the primary.

 

> Can I use guidescope instead of off axis guiding for a 8inch RC with reducer?

 

Not if you want reliably tight round stars.

Are you saying off axis guiding is a must for 8 inch RC with reducer. Off axis guiding is a pain for me. I have ZWO 290mm mini and ZWO M68 off axis guiding. I found the field of view is small and the stars are dim when I use Celestron C8 with f/6.3 reducer.



#16 turings.enigma

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Posted 01 August 2021 - 05:26 PM

Are you saying off axis guiding is a must for 8 inch RC with reducer. Off axis guiding is a pain for me. I have ZWO 290mm mini and ZWO M68 off axis guiding. I found the field of view is small and the stars are dim when I use Celestron C8 with f/6.3 reducer.

Does AT RC8 work well for nebulae? My main interest is in nebulae. I have a refactor for large scale structure. So I would use RC to shoot local structures and smaller nebulae such as cat’s eye and ring.



#17 RogeZ

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Posted 01 August 2021 - 06:56 PM

Lets also remember mosaics take 2 or n times more exposure time. Mosaics are not trivial.

A well corrected, mechanical rigid small refractor is a fantastic tool in this hobby.

#18 licho52

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Posted 01 August 2021 - 10:14 PM

Are you saying off axis guiding is a must for 8 inch RC with reducer. Off axis guiding is a pain for me. I have ZWO 290mm mini and ZWO M68 off axis guiding. I found the field of view is small and the stars are dim when I use Celestron C8 with f/6.3 reducer.

I have absolutely no problem getting multistar guiding in my RC with 290MM, it's not an issue whatsoever with the right settings.



#19 kel123

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Posted 01 August 2021 - 10:48 PM

Speed and portability. Mosaics projects can only be completed when the weather allows. You can easily backpack a small refractor and a portable mount on the go.
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#20 matt_astro_tx

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Posted 02 August 2021 - 10:55 AM

I run an AT60ED 60mm refractor at 288mm FL.  Great little scope for widefield work, and ultra portable.  I have it on a lightweight mount and tripod and never have to break it down.  It doesn't take up a lot of space in the house, and when I'm ready to image I can just carry the whole rig outside.  No setup required.  It's also small enough to fit everything inside a small pelican.


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#21 72Nova

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Posted 21 August 2021 - 04:37 PM

I also use a 60mm refractor for widefield imaging.  My little AZGTi mount can’t handle anything much larger and I think it is a great way to learn AP.  With an l extreme filter, my streetlights in my front yard are no longer a nuisance 

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#22 unimatrix0

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Posted 21 August 2021 - 05:58 PM

I have used William Optics Zenithstar 73 and Sharpstar 94 EDPH. Those have mid focal length. I also plan to buy Astrotech 130 EDT, which has 910mm focal length. Should I buy small refractors like RedCat? I feel I can always do Mosaic if want to have a larger field of view.

 

What additional benefits do I get from a small refractor with 230~250mm focal length?

At 230m-250 you'd be heading into camera lens territory.  Not sure if I would buy one of those (although they're out there not expensive, like the Askar guide APO/astrograph ) instead just add a focal reducer to a 350-450 mm telescope. 



#23 kel123

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Posted 22 August 2021 - 10:43 AM

At 230m-250 you'd be heading into camera lens territory. Not sure if I would buy one of those (although they're out there not expensive, like the Askar guide APO/astrograph ) instead just add a focal reducer to a 350-450 mm telescope.


A focal reducer wouldn't reduce the weight of a scope,which is part of the considerations for a portable setup and an appropriate low capacity grab-and-go mount. In fact, it adds to the weight of the setup.
And who says heading into camera focal length territory is forbidden? Some of the advantages that scopes have over camera lenses is not that it has longer focal lengths but that it has a smoother focusing mechanism and very sharp optics for the price. Scopes also don't limit you to using sampling challenged DSLRs and color cameras , except with cumbersome adapters and modifications which add to the complexity and the danger of something going wrong in the train.
Not because of the "fear" of heading into camera lens territory which in fact, doesn't mean a thing.
After all, there are camera lenses that have more than 600mm focal length. Canon has an 800mm focal length camera. So, even your scope is in camera lens territory.

#24 turings.enigma

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Posted 29 August 2021 - 09:09 PM

I got a small refactor "Askar FMA 230", which has 50mm aperture and 220mm focal length. I put it on skywatcher star adventurer 2i pro. I am not satisfied with the tracking accuracy of this small mount, so I put it to my eq6-r pro. So at the end, a tiny refractor on a big mount.



#25 kel123

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Posted 29 August 2021 - 10:16 PM

I got a small refactor "Askar FMA 230", which has 50mm aperture and 220mm focal length. I put it on skywatcher star adventurer 2i pro. I am not satisfied with the tracking accuracy of this small mount, so I put it to my eq6-r pro. So at the end, a tiny refractor on a big mount.


Well, they say you can never over-mount a scope. So, all's well.


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