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Altair Astro 8" F8 Ritchey Chretien Astrograph


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Just wondered if anyone has any experience with one of these scopes...

https://www.altairastro.com/altair-astro-8-f8-ritchey-chretien-astrograph-metal-tube.html

good points, not so good, peculiarities etc.

I know they're optimised for imaging but if you happen to use them visually on the odd occasion what are they like? Can you even?

Thanks

Pete

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Yes, you can use them for visual but they are not ideal for that purpose since the secondary obstruction is very large in proportion to their aperture which reduces image contrast.

I had one.  I enjoyed it for the time I had it but sold it on in to contribute to the costs of my TEC140 that gives a similar object scale.

They have an unwarranted reputation for being difficult to collimate.  Yo an collimate one with nothing more than a Cheshire EP.

 

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From all I've heard/read - if you're looking for primarily visual use, you'd be better off checking out other types of telescopes. The RC's are quite good for AP, which explains why their huge 'cousins' are used in large observatories.

It would help if you could tell us what you have in mind. There are better choices for a mixed-purpose scope available.

 

Meet 'Big-Blue' - my pet dust-collector.

1 001.JPG

 

All the best -

Dave

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13 hours ago, kirkster501 said:

Yes, you can use them for visual but they are not ideal for that purpose since the secondary obstruction is very large in proportion to their aperture which reduces image contrast.

I had one.  I enjoyed it for the time I had it but sold it on in to contribute to the costs of my TEC140 that gives a similar object scale.

They have an unwarranted reputation for being difficult to collimate.  Yo an collimate one with nothing more than a Cheshire EP.

 

I had one here with a guest, Steve, and he had your video on his PC along with all the Howie Glatter bits and bobs. Could we collimate it? Nope, it went round and round in circles for us. Something was not right but we never go to the bottom of it.

Olly

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Hi Pete:

These are good scopes, but seem to have somewhat mixed reviews (I've seen some reports of them needing extensive recollimation, including recalculating the focal length and adjusting the secondary height, and also of the fiocuser not being great). Other reviews have said they're great, with no real issues, so it's hard (not having one myself) to make a call.

As others have said, this is primarily an imaging scope, but of a particular type - they are not designed primarily for "aesthetic" astrophotography and there are probably better options. In addition to a large, contrast robbing secondary, they show more astigmatism at the edge of the field than a comparable SCT, and have about twice the field curvature. They also produce larger stars than SCTs or Newtonians.

Why put up with this? Because while the stars at the edge of the field are stretched and elongated, this is not coma. The elongation is symetrical (not fan shaped, like a comet tail) which is useful for photmetry and critical for accurate astrometry. Which is what these scopes are designed for. For astrophotography, an SCT, Newtonian or refractor will perform as well or better. For collecting accurate positional or photometric data, RCs are about as good as it gets and are what most modern professional observatorires rely on as their workhorse scopes.

Just seen Olly's post as I was writing this and remember seeing something to this effect before. Pretty sure Skipper Billy also had something on here recently about trying to collimate an RC and it did not sound like fun.

Billy

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Olly/Billy,

They are not fun to collimate, I'd agree with that.  I can collimate my SCT 9.25 in about 20 seconds.  It takes  much longer to do the RC but it is possible with just a Cheshire by getting the dots and rings centered and concentric and making sure the "hall of mirrors" when seen from the front is concentric - not exactly practical to do at night time.    It is an iterative process and once you have it nailed they maintain collimation for months.  I got so practiced at it I would get someone to deliberately screw up the collimation and I cloud get it back in about an half an hour of fiddling.   But they are fiddly there is no getting round that.

I cut down on my scopes and the RC8 had to go to make way for the TEC. I enjoyed the RC8 whilst I owned it.  But I enjoy the TEC 140 more :)  All my AP is done with refractors now (FSQ85 and TEC140).  For visual I use the Dob or the C925.

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Ask a dozen people how to collimate these scopes and you'll get a dozen different methods.  :smiley:  They do require some effort and it takes a fair bit of practice, that is for sure but once you've got it collimated, it tends to hold the collimation quite well.  I've tried various methods in the years that I've had my RC8, but the one that seems to work for me is to use a Cheshire to adjust the secondary then do a star test for the primary, iterating round if necessary. To help with star test I use the defocused star collimation viewer in CCD Inspector, which gives quantitative advice on which adjustment screws to turn and how far out your collimation is. 

The stock focuser is ok-ish but the scope certainly benefits from an upgrade, particularly if you get one that will enable you to screw your imaging paylod onto the focuser, rather than rely on a compression fitting, where there's potential for sagging.

Yes you can stick an eyepiece in them and see something but I wouldn't make a habit of it.  If you want a scope that you're going to want to use visually more once or twice year, I'd look elsewhere.  For visuals on the planets, the Moon, and the occasional bright DSO, I use my Mak: it's a much more satisfying experience.

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  • 1 month later...

I've got the Altair 6 inch RC. When I first got it I did some viewing through it. I had the best ever Andromeda Galaxy, M32 and M110 view I've ever had. It was a secondhand scope and arrived collimating correctly. I checked it and didn't need to do anything with it. 

I've also done image with it, M51 which I'm quite pleased with. Camera is Starlight Express H694 with a Lightwave 0.6 focal reducer to speed things up.IMG_0680.thumb.PNG.03b17fff7a4e4d58e7808897d135d9fc.PNG

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4 hours ago, Anne S said:

I've got the Altair 6 inch RC. When I first got it I did some viewing through it. I had the best ever Andromeda Galaxy, M32 and M110 view I've ever had. It was a secondhand scope and arrived collimating correctly. I checked it and didn't need to do anything with it. 

I've also done image with it, M51 which I'm quite pleased with. Camera is Starlight Express H694 with a Lightwave 0.6 focal reducer to speed things up.IMG_0680.thumb.PNG.03b17fff7a4e4d58e7808897d135d9fc.PNG

That's a good M51 but does the focal reducer speed things up? You get exactly the same number of M51 photons per minute without it. By putting the light onto fewer pixels you may be able to get the faint signal over the read noise more effectively but, in general, beware of the F ratio myth which applies to objects which fit on the chip with or without reducer. What matters most is the number of object photons you catch and a reducer doesn't change this. Only more aperture catches more object photons.

Olly

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1 hour ago, ollypenrice said:

That's a good M51 but does the focal reducer speed things up? You get exactly the same number of M51 photons per minute without it. By putting the light onto fewer pixels you may be able to get the faint signal over the read noise more effectively but, in general, beware of the F ratio myth which applies to objects which fit on the chip with or without reducer. What matters most is the number of object photons you catch and a reducer doesn't change this. Only more aperture catches more object photons.

Olly

isn't the object brighter at a shorter focal length? I've been using the same reducer on my refractor and took m45. I ended up using 2 minute exposures and still got lots of nebulosity. Ok the corner stars are elongated as the reducer chip distance wasn't right - I was short a m48 to t thread adapter. The one I had put me well over distance!IMG_0674.thumb.jpg.a375496bcbbd274736b970d462218f3f.jpg

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51 minutes ago, Anne S said:

isn't the object brighter at a shorter focal length?

Be careful with the terminology.  When you are talking about brightness you are talking about the magnitudes of the objects and their brightness doesn't change (a magnitude 3.2 star is always that).

I think what you are referring to is flux at the imaging plane and this isn't dependent on focal length but aperture (simply something twice the radius captures 4 times the flux).  Effectively to get good pictures you need a good signal per pixel compared to the noise of that pixel (termed signal to noise or S/N imaginatively).  The 'focal ratio myth' that is being referred to is that a faster (i.e. F5 to F10) gathers more photons and that's not correct; all other things being equal that simply relates to aperture (the larger the more flux you get).  The myth is partially 'true' because generally people are referring to the same equipment/camera etc and then add a reducer.  In effect this reduces the telescopes focal length and hence puts more flux per pixel (but at a cost of resolution).  For example suppose you had 4 boxes  in a 2x2 arrangement. In the first box there is one star, the second two stars and so on in your normal setup (assuming all the stars are the same brightness).  In this set up there will be the least amount of flux in the first and the most in fourth box.  Keeping all things the same you then reduce the focal length of the telescope.  What this does is now put the flux from all those ten stars onto, for this example, one pixel (now you have a one pixel with a lot more flux, and three with none).  Hence this means this pixel now has a much higher signal to noise.  You have not changed the flux at all, the only thing you have done is how much you have spread out that flux.  You can actually achieve exactly the same thing by binning these pixels on the camera from a 2x2 to a 1x1.  It's exactly the same effect (ignoring other sources of noise)

Now consider a larger aperture at the *same* focal length.  Now you *are* placing more flux on each pixel...

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1 hour ago, Anne S said:

isn't the object brighter at a shorter focal length? I've been using the same reducer on my refractor and took m45. I ended up using 2 minute exposures and still got lots of nebulosity. Ok the corner stars are elongated as the reducer chip distance wasn't right - I was short a m48 to t thread adapter. The one I had put me well over distance!IMG_0674.thumb.jpg.a375496bcbbd274736b970d462218f3f.jpg

The key difference between M51 and M45 for your setup is that when you use the reducer you are still filling the frame with nebulosity that you want. In effect you really are getting more light from the object because you can capture its wider parts as well. But with M51 all that changes is the amount of sky you capture - and you may not particularly want more sky.

This is the M51 situation:

THE%20F%20RATIO%20MYTH%20V2-L.jpg

The M45 situation is slightly different:

reducers%20used%20properly-L.jpg

In the second case you will get a given S/N ratio faster, meaning shorter exposure times for a given standard, though your resolution will be lower. This is compensated for by the wider field which you did want.

The F ratio rule of exposure going as the square of the F ratio comes from the camera world in which the F ratio is only ever changed because the aperture is changed. Focal reducers change the F ratio by changing the focal length, which is not at all the same thing.

Olly

PS Good M45 as well! A bit green, I think. Have you come across Hasta La Vista Green from the Deep Sky Colors website or SCNR green in Pixinsight? I'd give one of them a try. I think you'd get a pleasant surprize!

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I'm going to have to think about the above in the morning, I'm half asleep now! I agree m45 is a bit green. I'm only just beginning to get good enough subs to process. I was short of them as my eq6 was still jumping at the time and decided it had done enough after 40 minutes! I suspect doing my final process in Lightroom mobile added the green tint!

I'm going to try Pixinsight soon, just need to upgrade my desktop first as my XP one won't cut it.

I'm still trying to get images as good as the ones we took about 7 years ago. Maybe more clear nights would help, I haven't had a decent one since the end of Jan. That's Wales for you.

Anne

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Hi Pete,

I have the 8" altair RC as well as a 102mm Doublet Refractor, so here's my take on it.

1) RC's do need a bit of patience to collimate, but as long as you take your time, it's not a particualry a hard process, but you will need to factor in the cost of some good colimation tools and I would recommend the addition of a focuser tilt Ring.

2) At F8, it's a slow optical design, so you're going to need longer Sub lengths if Imaging or possibly consider a reducer.  I have a reducer, and have spaced mine to to give me a FL of around 1300mm @ F6, as it speeds it up a good bit whilst maintainging a fairly flat field and decent focal length.

3) Extra Focal Length = Harder to guide.

So by the time you have purchased tilt rings, and possibly a reducer they start to get a bit more expensive. So if i could afford to replace my RC with someething else would i do it ?  yes - probably    , What would i get, Probably a larger refractor or maybe a high quality imaging newtonian at around the 1000mm FL mark.

 

Rich.

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17 hours ago, Anne S said:

I'm going to have to think about the above in the morning, I'm half asleep now! I agree m45 is a bit green. I'm only just beginning to get good enough subs to process. I was short of them as my eq6 was still jumping at the time and decided it had done enough after 40 minutes! I suspect doing my final process in Lightroom mobile added the green tint!

I'm going to try Pixinsight soon, just need to upgrade my desktop first as my XP one won't cut it.

I'm still trying to get images as good as the ones we took about 7 years ago. Maybe more clear nights would help, I haven't had a decent one since the end of Jan. That's Wales for you.

Anne

Aha, I did wonder if Anne S was you! These forum names are a constant confusion!!

Olly

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You're a bit easier to identify!

I've got what you and Whirlwind were saying. You only get more photons if there's something else you want to include when using a focal reducer. Incidentally, I was binning my SX H694 for M51 as the resolution was down to sub 1 arcseconds per pixel even with the focal reducer.  I normally have 1.4 arcseconds with my refractor.

Anne

 

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Hi - I had the 8 RCT now a 12 so I like them- great  for high resolution  AP - collimation is important but once you get the hang of it, not that terrible - visually large central obstruction reduces contrast a lot and I found planets fairly dissapointing but compared to Mewlon 210 which I use ,with has a small central obstructuion, that is not suprising- fairly flat field and with reducer/ flattener that gets better- I have virtually no coma effect at the peripheral field of my 383 chip- would not agree that an SCT is better- tried that and wasnt happy  - they tend to have longer FL and you have to worry about the corrector - dew, temperature equilibration etc - these RCT scopes have no glass lenses apart from a reducer and cool relatively quickly and my experience has been good - the standard focuser is often poor however - Tony

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  • 1 month later...
On 3/17/2017 at 06:02, Whirlwind said:

Be careful with the terminology.  When you are talking about brightness you are talking about the magnitudes of the objects and their brightness doesn't change (a magnitude 3.2 star is always that).

I think what you are referring to is flux at the imaging plane and this isn't dependent on focal length but aperture (simply something twice the radius captures 4 times the flux).  Effectively to get good pictures you need a good signal per pixel compared to the noise of that pixel (termed signal to noise or S/N imaginatively).  The 'focal ratio myth' that is being referred to is that a faster (i.e. F5 to F10) gathers more photons and that's not correct; all other things being equal that simply relates to aperture (the larger the more flux you get).  The myth is partially 'true' because generally people are referring to the same equipment/camera etc and then add a reducer.  In effect this reduces the telescopes focal length and hence puts more flux per pixel (but at a cost of resolution).  For example suppose you had 4 boxes  in a 2x2 arrangement. In the first box there is one star, the second two stars and so on in your normal setup (assuming all the stars are the same brightness).  In this set up there will be the least amount of flux in the first and the most in fourth box.  Keeping all things the same you then reduce the focal length of the telescope.  What this does is now put the flux from all those ten stars onto, for this example, one pixel (now you have a one pixel with a lot more flux, and three with none).  Hence this means this pixel now has a much higher signal to noise.  You have not changed the flux at all, the only thing you have done is how much you have spread out that flux.  You can actually achieve exactly the same thing by binning these pixels on the camera from a 2x2 to a 1x1.  It's exactly the same effect (ignoring other sources of noise)

Now consider a larger aperture at the *same* focal length.  Now you *are* placing more flux on each pixel...

Err, sorry to resurrect the post but this techie stuff always interests me. So outside it is 100% total overcast conditions and at 5:00pm (8 minutes from official sunset), I just put my two 6" aperture scopes on two mounts side by side pointing at the same piece of total 100% overcast sky. I took my Canon 700d in full manual mode and slipped it into each OTA three times to average out in case the 100% total cloud cover changed a bit in the time I was swapping the cameras. All 3 times it was slipped into the 6" x 600mm f/4 the built in exposure meter says correct ISO100 exposure is 1/160th sec but all 3 times slipped into the 6" x 1500mm f/10 the correct ISO100 exposure is 1/15th sec? The sensor is fully covered by the lightcone in both cases (isn't it?). The grey almost black cloud is moving very slightly so if it were darker exactly where the f/10 was pointing then in one of the times I had swapped the 700d into it, it should have registered a time of less dark area and vice versa in the f/4 OTA. Yet it didn't? The f/4 consistently was letting more light through causing the camera to say correct exposure was 1/160th, while the f/10 consistently let less light through causing the camera to say correct exposure was 1/15th. Yet both were 6" aperture scopes. Signed ... confused!

Cheers if you willing to answer this old thread.  

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On 17/03/2017 at 20:14, tony210 said:

Also if you did decide to take the plunge and buy one please PM me as there are lots of little hints that I could share - best wishes Tony

Hi Tony,

I am a relative beginner at Astrophotography (well about 6 month serious) and I have a very good friend who is guiding me.

I'm looking to obtain a GSO Carbon Ritchey Chretien 8'' f/8 as I have read some very good things about them, however my budget is limited to about £650 second hand, can't really justify buying new at just over a £grand.

I'm running on a Hypertuned (well it is being collected today to be done by DarkFrames) HEQ5 Pro, thus my feeling about needing one of these in Carbon for the weight.

If I purchased a second-hand one, I would guess that there is a fair chance of it needing Collimating, what procedure/collimater do you use?

I hear various things about the focuser, some good, some bad, do you have any suggestions, I'm using a DSLR (N800)

Much appreciate your help, thanks.

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Hi -I prefer the metal tube ones as temperature equilibration is better and the weight is not that different- I know a dealer and astrophotographer who used the 8 inch CF version and found the CF tubes did not cool quickly enough- also the metal tube versions are cheaper - collimation I found on the 8 inch  is not that difficult even with a cheshire- the focuser s are not great and in the long run need replacing - for all that you get a pretty flat field , no false colour and great resolution on galaxies, so I am a fan - I sold my 8inch second hand for about £600 with a good focuser! -I had my 8inch on an EQ6 and coped with guiding running at F6 with  reducer-now have a 12 inch on a Mesu and use a Tak collimator on that that -does a fair job but for perfection you do collimation on a star- (may have under priced the 8 inch not sure) - for attaching a DSLR I think you would need a new focuser though in the long run as you will get flexing - they are good for high resolution astrophotography but not for visual work as contrast is not great -please let me know if you have any other questions or get one! - Tony.

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