Cel Evolution 6" issues - is it an old SCT construction?

[larsmn]

Past summer I bought a Nexstar Evolution 6" to be able to use a bit bigger tubes than possible with my old SkyWatcher AltAz Goto.  Have done a lot of astro-photo with the SkyW and a SkyW Evostar 80ED with a lot of good result, the Evolution to improve that (has a wedge planned for EQ operations). 

Due to getting back claims from a previous bankrupted employer,  I got enough for the combo + reducer and a Sony A7R. When looking for a suitable solution, I told the shop that I was to use the combo with a full sensor camera, my old Sony Nex-5R having past 50.000 pictures, showing a lot of wear. The tube also did have a 2" attachment at the back.  But my first tries have been a sordid disappointment.  An extreme amount of coma triangular stars (pointing to the center) from 1/3 out of the center, degrading outwards.  With the reducer is should be a f-6.5 tube, but my experience is that it is worse than my Evostar without it's reducer, f-8.

My astro club chairman tested the tube with his 1990 Cel SCT 0.65x reducer and his verdict was that the tube I have is an old construction, same as his 1990:es SCT. With his reducer and his ST-8300 camera, he gets good circular stars over 60% from center, but he says the tube is a dud for APC-S or above. 

You knowing anything about, is it correct that Celestron ships an older 6" tube with Evolution, compared to the 8" and 9.25" tubes, not allowing APS-C or above in sensors or do I simply have got a the wrong tube?  The shop is trying to get out of this unfortunately. 

If I am stuck with this, is there any other reasonably priced reducer/flattener suitable to correct some of this, so I haven't thrown all of my compensation down the drain?

[ronin]

The Evo 6 is an SCT, equally so are the 8" scope they produce, and a few others.

The design means that in reality the one you have will be the same basic item as the "old" 6" SCT's Celestron made, the 8" SCT's will be effectively the same as the "old" 8" SCT's they made. The SCT design being (I believe) 2 spherical mirrors and a corrector plate at the front, they come out as about f/10 and there is little or no variance. Change the mirrors and you have a different scope design, no longer SCT.

Reading the blurb the changes are things like Wifi, better coatings, reads like an internal rechargeable battery and whatever but the basic optics will not have altered since the early 2000's. The basic optics of the 8" will also not have changed either.

I would say the tube is not "older" just the design is fixed, the earlier tube is optically close to identical to the one now. Celestron actually developed the SCT for mass production so it has not undergone a significant design change.

Whatever 6" SCT Celestron bring out in 10 years time will be a 6" SCT - same diameter, same focal length, same optical construction.

An achro refractor is the same, 2 lumps of glass spherically ground.

Which is the same as an ED refractor.

There is not a lot that can be done.

I would I suppose disagree with the term "old construction" you have a "normal" "standard" Celestron SCT construction.

As to the sensor that they illuminate no idea, that is an issue for one of the imagers to answer.

Will say that a 6" SCT is not the usual item for DSO imaging.

[larsmn]

Thanks,

however, the key issue at hand is that most modern tubes today do work with at least APS-C sensors and 2" openings, ED or not ED, refractor or reflector. But apparently not this one, though the ocular fitting is 2". 

If I understood my club chairman right, this one is limited to max threefour sensors. That is what I meant with an old construction, while the 8" and 9.25" is adapted, this is not. It did work with his 1997 reducer, not with the one Celestron delivers today.  So there is some mismatch visa-vi the larger tubes.  Is it only the sized that makes it or is there some design differences causing this.  I had expected that if only having one reducer model, well, it to be working on all tubes it's intended for. The Cel's 8" and 9.25" SCT:s both are Fastar or EDGE SCTs tubes, which apperently can use this 2" reducer and give at least a APS-C sensor coverage. So is it only the smaller size that causing these issues.  

Also, due to mobility, an 8" or larger was not an option, the 6" Evo is the maximum my balcony observation solution can handle. 

[Uplooker]

Have you checked the collimation?

Sent from my Fone

[michael8554]

Quote  - "It did work with his 1997 reducer, not with the one Celestron delivers today"

If the Celestron Focal Reducer/Coma Correctors are like the Meade ones, then the correct FR to image chip spacing is different for the "old" and "new "designs.

Incorrect spacing makes the coma worse.

So your club chairman's FR may well give good results if he's got the right spacing on his setup.

Regarding the vignetting of full-frame sensors, I don't think the 6" has a 2" opening at the back - and putting a 2" attachment on it doesn't make that opening bigger.

Michael

[Merlin66]

All the non EDGE reducer designs were based on a 110mm backfocus requirement to give the design f6.3.

I'm guessing that the baffle tube construction on the 6" is smaller than the tubes used on the larger models and this may limit the FOV.

[larsmn]

Thanks for all answers.   To answer some of the questions:

- No, I haven't tested the collimation, but my club mate says it's ok. 

- The attachment fixture takes 2" attachments, like the reducer and backfocus extension, therefore I guessed it was made to work with 2". My mistake.  

- We have both used the exact same backfocus between the reducer and the sensor, the 105 mm recommended by the shop for this tube.  110 mm is a new value for me, first they said 97 mm, then 105. The shop is my country's biggest and regarded as a knowledgeable astro shop.  

My conclusion is the tube is for less than APS-C sensors and the reducer I received is not the correct one.  Have to live with that I got incorrect information about the tube visa-vi camera options, but also have to try to get the reducer changed.