Vixen VC200L

[Gogleddgazer]

Hi Folks, I am toying with the idea of taking the plunge on a long FL 8" OTA for getting up closer to DSO than my Tak 76DS and TS Photoline 130 APO afford and wondered if anyone with experience of the above Vixen could offer their thoughts.  It will be mounted on an AZ-EQ6 which should suffice for the potentially long exposures that seem necessary with such a scope. I've read a variety of online reviews but would particularly welcome members thoughts on the utility of the Vixen primarily for AP, but potentially also for occassional dedicated visual work. Thanks in advance

J

[Stephenstargazer]

I was a long time owner of a VMC200L for visual and occasional video live. I changed to a VC200L 18 months ago. Both are very well made and have longevity built in. Good 2nd hand buys but beware ones not cared for. Also Vixen accessories cost! Visually it is excellent. Collimation is well held due to fixed mirror, but if needed must be done with patience and care in correct order. My VC arrived spot on and still is untouched.

If you want to know more about capability for imaging, check out Ian Morison's Astronomy Digest. He rates it highly.

PS the focuser is adjustable and the micro focuser needed for AP. There is not a lot of back focus, especially with reducer,  so check that out.

Edited December 5, 2021 by Stephenstargazer

[JeremyS]

I remember reading that the coating of the main mirror coating on the VC 200L contributes to its figure so it has to go back to vixen in Japan for recoating

[Gogleddgazer]

Thanks to both of us for your insights, just the kind of input I was looking for to help inform my decision 

[ollypenrice]

Are you sure that you will really obtain more DS detail from a scope of this focal length than with your 130 refractor? I wouldn't bank on it. You will need very good, stable seeing and very good tracking accuracy.

I've found no significant difference between the final detail caught with a TEC140 at a metre FL and that caught with a 14 inch catadioptric at 2.4 metres FL (on a mount running at around 0.3"RMS.)

The Vixen would obviously give you a bigger image but would that bigger image hold up at 100% and, if it didn't, would it be any better than an image shot at lower nominal resolution? I wrote an article on this very comparison for Astronomy Now and concluded that I was limited not by FL but by seeing. My site is pretty good.

Pixels are now getting so small that the rationale behind long focal length is thrown into doubt.

Jeremy is right that the figure on this scope is finalized by having a thicker aluminium coat around the edge of the mirror, which does have implications long term. However, I think the main question is, Will this really give you more detail?

Olly

Edited December 6, 2021 by ollypenrice
typo

[vlaiv]

On 04/12/2021 at 19:44, Gogleddgazer said:

I am toying with the idea of taking the plunge on a long FL 8" OTA for getting up closer to DSO than my Tak 76DS and TS Photoline 130 APO afford

Wrong reason to choose such ota.

TS 130 APO is capable of getting you "really close" to your targets - as close as you can go with current cameras and their pixel sizes (in fact - it is likely that you'll be oversampling with about half of current cameras even with that scope).

Good reason to go for 8" OTA is to speed up your capture - but in order to do that - you need to set your working resolution.

Do you have any idea what working resolution you want to image at? Do you have idea of what your mount and skies are capable of?

[JeremyS]

Here’s one of the links about recoating the mirror:

https://www.opticron.eu/faq/index.php?action=artikel&cat=2&id=13&artlang=en
 

a very expensive exercise, not to mention the time taken to ship to Japan and back.

This put me off buying one, even though I hear they are excellent. Also, I would not by a used one unless I could be sure it had not been recoated in a conventional process.

[Gogleddgazer]

4 hours ago, ollypenrice said:

Are you sure that you will really obtain more DS detail from a scope of this focal length than with your 130 refractor? I wouldn't bank on it. You will need very good, stable seeing and very good tracking accuracy.

I've found no significant difference between the final detail caught with a TEC140 at a metre FL and that caught with a 14 inch catadioptric at 2.4 metres FL (on a mount running at around 0.3"RMS.)

The Vixen would obviously give you a bigger image but would that bigger image hold up at 100% and, if it didn't, would it be any better than an image shot at lower nominal resolution? I wrote an article on this very comparison for Astronomy Now and concluded that I was limited not by FL but by seeing. My site is pretty good.

Pixels are now getting so small that the rationale behind long focal length is thrown into doubt.

Jeremy is right that the figure on this scope is finalized by having a thicker aluminium coat around the edge of the mirror, which does have implications long term. However, I think the main question is, Will this really give you more detail?

Olly

Thanks Olly, really helpful additional food for thought and to be honest it adds a lot of weight to the argument for me to persevere with what I have rather than adding another OTA to the collection. 

[Gogleddgazer]

4 hours ago, vlaiv said:

Wrong reason to choose such ota.

TS 130 APO is capable of getting you "really close" to your targets - as close as you can go with current cameras and their pixel sizes (in fact - it is likely that you'll be oversampling with about half of current cameras even with that scope).

Good reason to go for 8" OTA is to speed up your capture - but in order to do that - you need to set your working resolution.

Do you have any idea what working resolution you want to image at? Do you have idea of what your mount and skies are capable of?

Thanks Vlalv, time for me to get into the technical maths and understand the things you talk about better so I can make a more informed decision. Every day is a school day! At least your post gives me some steers to do more targeted research, so thanks for taking the time to reply.

Edited December 6, 2021 by Gogleddgazer
Spelling mistake

[vlaiv]

1 hour ago, Gogleddgazer said:

Thanks Vlalv, time for me to get into the technical maths and understand the things you talk about better so I can make a more informed decision. Every day is a school day! At least your post gives me some steers to do more targeted research, so thanks for taking the time to reply.

To get the idea of what your "top" working resolution should be - take any of your old subs you shot with TS 130mm and measure star FWHM in arc seconds.

You'll get range of values - depending on how good seeing was at particular instance (and your focusing of course - pick subs with good focus) and how your mount performed at the time - divide that with 1.6 and that is pixel scale that is suitable for given image.

My guess is - more often than not you'll be above 1.5"/px - approaching 2"/px.

Larger aperture will give you slightly smaller FWHM - and hence possibility for higher working resolution - but not by much. Maybe shave off 0.1 to 0.2"/px from your 130mm when using 8" aperture if you leave everything else the same (mount and sky).

[ollypenrice]

1 minute ago, vlaiv said:

To get the idea of what your "top" working resolution should be - take any of your old subs you shot with TS 130mm and measure star FWHM in arc seconds.

You'll get range of values - depending on how good seeing was at particular instance (and your focusing of course - pick subs with good focus) and how your mount performed at the time - divide that with 1.6 and that is pixel scale that is suitable for given image.

My guess is - more often than not you'll be above 1.5"/px - approaching 2"/px.

Larger aperture will give you slightly smaller FWHM - and hence possibility for higher working resolution - but not by much. Maybe shave off 0.1 to 0.2"/px from your 130mm when using 8" aperture if you leave everything else the same (mount and sky).

...and another thing is that your 130 is a known quantity in terms of optical performance.  You don't say how happy you are with it but, all being well, it should be an easy scope to use with no time lost to collimation or other issues. Plus easy maintenance. (Wipe it down with a damp cloth! 😁)

Reluctantly I've come to the conclusion that Vlaiv is right in estimating 1.5 arcsecs PP as a probable minimum. I've been working at about 0.9 but I don't think I'm gaining anything over 1.5 or so.

What's your guiding RMS in arcseconds?

Olly