Fast Refractor for EAA - Suggestions

[glend]

Long story, but because of vision problems I am moving to exclusive screen based EAA operation. As I only have one useful eye ( macular degeneration in the other), and the useful one is not great, I am moving away from reflector type designs, which require collimation (hence my recent sale of my f5 Newt and my CC8). So i need some help with identifying a suitable wide field refractor, that can be used for EAA (screen based display).
This will be observatory based, on my CGX mount (So weight is not a big concern), but speed is a factor in EAA, so f5 -6 maybe, and reducer setups would be ok, even though flat field is not that important. I am not fussed about perfect colour rendition, so an ED type would be fine. Please don't point me at TAKs.
Looking for reasonable optics, bigger the better I guess in terms of aperture, Doublet is preferred for ease of handling.
If you have a suggestion let me know here. Thanks.

[astro744]

I found this article which has a lot of good info. Chapter 4 deals with telescope selection.

See https://agenaastro.com/articles/guides/agena-beginners-guide-to-choosing-equipment-for-deep-sky-eaa.html

I would think that if permanently mounted that an 8” SCT would not need frequent collimating and when combined with a Meade f3.3 reducer would give you 660mm focal length and a large 200mm aperture, ideal for EAA. The f3.3 reducer has been discontinued but comes up for sale occasionally on IIS.

If you are set on a refractor then these are mentioned in the article. I’ve used a TV-101 with a Watec 120n+ (mono camera) but this was for an occultation of Saturn one night. I do remember trying it on Omega Centauri and it looked nice but Omega Centauri looks nice in just about anything. I also tried Omega with my C9.25 at f10, f6.3 and f3.3 with the Watec camera and f3.3 is great to give a wider field with the SCT. I didn’t really notice the difference the faster focal ratio made or at least I don’t recall. I did pick up a used colour camera a couple of years ago but haven’t even tried it yet. I just haven’t quite caught the EAA bug yet.

[ChrisV]

Why did you sell you newt!!! Can't beat one for EAA. Nice and fast.

[JA]

Hi Glen,

To make suggestions easier, it may help if you could clarify if you have any preferences for any camera/s you were thinking of using in your EAA widefield setup (Perhaps some you already have)? Also do you have any preference for how widefield you want to go in terms of focal length? Then of course there's the question of how high to go in terms of cost. Perhaps that's already sort of answered in the no TAKs constraint.

Best
JA

[glend]

It would be a colour ASI camera, yet to be purchased. Focal Length likely no more than 760mm, not less than 400mm.

[JA]

Given those choices and also your stated "need for speed" one scope I'd consider would be the Vixen 140ss or 140ssf at f/5.7 it's one fast refractor and reasonably unique focal length/aperture combo with 4 element petzval construction that doesn't cost the earth (TAK, AP, etc..). For focal lengths below that and if high speed is a necessity I would consider high quality (used) telephoto lenses something like a 600mm f/4, 500mm f/4 or 400mm f/2.8 adapted to astrocam use. At their level of performance they tend to be reasonably well corrected for flatness and colour and they don't have to be crazy money if you stick to manual focus used lenses, shop around and don't mind the odd cosmetic imperrfection.

What ever you get it would be good if it could support a full-frame image circle so that you have more camera options: Full-frame, APS-c, ±reducers, ±extenders, etc....

Happy Hunting

Best
JA

[gregbradley]

Pentax 67 400mm EDIF F4 lens. You can probably get an extender as well.

Pentax 67 lenses have he most backfocus of any lens and they are quite sharp and relatively inexpensive for their aperture.

I have a Pentax 645 300mm EDIF F4 plus a 1.5 extender. I have used the 300mm lens extensively for imaging and its a great lens. It has double ED elements. What would that be worth in a regular telescope?

Greg.

[xelasnave]

Get another 115 mm I find it is great.
Alex

[glend]

Thanks Alex, yes it is a peach (as they say), and when I sold it I was not thinking ahead to EAA. From what I have found in my research, EAA probably does not require a Triplet APO with excellent glass (overkill). My investment will probably find its way to a larger aperture Doublet.

[xelasnave]

I had a 150 mm doublet and it was fantastic ..occasionally a colour fringe but sharp as.
I gave it away which I now regret as I could have given it to you.
Alex

[mldee]

As an aside to Glen's comments, I also am interested in EAA, having just completed my dark sky RO Obs in time for my 77th birthday and ongoing failing eyesight and general doddiness.

The idea of being able to watch Sharpcap or such do its thing from in the house on a large screen TV in real time is appealing, rather than hunching over a PC and starting hours of post processing. That can come later, if desired.

Anyway, I don't know the procedure for finding out how many IIS members are doing / or considering EAA, and if a separate EAA Discussion forum would be desirable. Personally, I'd like to see one.

[ChrisV]

Would be good. Howie & I asked for one a few years ago, but it didn't get a response.

[xelasnave]

I must admit I like the idea as I have been exploring various objects in LMC using Ha and very high gain and exposures longer and higher than if I was seeking to construct an image..just to see the thing..in fact that was what I did when into visual exclusively was to explore rather than go out to observe a particular object...thinking about it I could see two units set up one to photograph and one to observe..and I think the main reason I don't do visual like I once did was it requires standing and often strange contortions..I like the idea of a nice chair and a screen...a 120 to 150 doublet would do the job nicely I expect.
Alex

[Hemi]

I’ve been doing EAA for years now. There is a great forum on cloudy entirely dedicated to it. Full of excellent advice and information. I remember a very abusive thread on here when I first joined, about “video astronomy” not being proper astronomy etc, put me right off, and went elsewhere for a long while... glad to say the main proponent of the vitriol hardly posts now.

Anyway Glen, I use my ES102 with a 294 mc pro for EAA. It works well, although not as well as my c925 and same camera. I find the sct really versatile for EAA. It’s a great way to observe and compliments visual really well, as well as note keeping. It’s also an astronomy lifeline in the tropics where you can sit inside in air con for half the year instead of sweating at the eyepiece at 4am

Best

Hemi

[mldee]

Looks like there are at least a few who would be interested in an EAA thread, although to paraphrase an old saying - "A few EAA fans does not a forum make".

Perhaps a simple survey would be a start, or even a response from whomever looks after IIS issues these days. Comments and advice welcome.

Hopefully technology and associated costs have moved on enough that there is more incentive to try. I've always been a fan of mossieless AC viewing rooms while the imaging computers do their thing.

Thanks Hemi for the heads up on the CN site, sounds worth a look.

My apologies for hijacking the thread Glen, but spent my youth in Toronto/Coal Point, so back to you.

[lollywater]

I do videoastronomy . I have a Skywatcher Esprit 80 f5 with a Starizona Apex-Ed 0.65 focal reducer which takes it to about F3.3 .I virtually never do stacks more than 3 minutes in total with exposures around 10 to 15 secs.
I have trouble getting decent images with my sct .Heaps of coma and vignetting.I am happy with the rasa,the newt and the refractors.

The EAA CN site is not what it used to be. A lot of the star performers no longer post.Some of them now post on Deep Sky Imaging.
cheers
Paul

[glend]

I have been reading a few threads on the CN EAA forum, and ASI Studio software seems to be used by a fair number of folks just starting out. It is free from ZWO and has a number of modules, obviously for ASI cameras. Simplest setup seems to be a fast scope, an ASI camera connected by USB to a laptop and ASI Studio (ASI Live). Anyone here using ASI Studio for EAA?
Since I am observatory based, and never have to do a setup or alignment, I could in theory leave it all setup and just power up for any session. I have no real desire to save video or images, as I have many years worth of wonderful traditional astro images on my backup drives.

[lollywater]

I have an ASIair pro which is my grab and go setup without the need for a laptop.There has been very little going anywhere this year so I occasionally use it at home just to remind myself how to use it. I have used ASI studio at home also ,but only a few times as I still prefer sharpcap when using a laptop
cheers
Paul

[rustigsmed]

hi glen,
i know this might be a bit bigger than what you were thinking but perhaps worth a look, 6" f5.9, should be fairly big and bright.

https://www.teleskop-express.de/shop/product_info.php/info/p2229_TS-Optics-6--f-5-9-Refractor---2-5--R-P-Focuser---Ohara--Japan--Objective.html

cheers

[glend]

Haha, not at all Russell, you have read my mind. I have been looking very closely at that scope, in fact i had just mentioned it in a PM to Dunk. It is nice and fast. It will accept the TS 0.79 reducer corrector, dropping it from f5.9 to f4.6 @ 700mm fl. However, it might be too long for good wide field EAA use. And don't point it at anything too bright without a fringe killer on it.

Another possibility is the TS80 6-element Flat field APO at an amazing f4.4. It would seem ideal with great colour and a focal length of just under 400mm.

[Camelopardalis]

Sounds like a beast of a scope :D

A larger-pixelled camera would help disguise any shortcomings of the colour correction (just like in the good old days), depending on what semi-apo means :shrug:

At ~900mm native it'd be much more interesting on smaller objects than an 80mm scope, but the reverse would be true for larger objects of course...maybe mount one on top of the other and gouge on your cake ;)

[glend]

Which camera would be a good match for that 6" TS scope? Native 900 reduced to 700. Seems out of ASI294 comfort zone. Bigger pixel?

[JA]

The specs for that scope without specifying the image circle say that it supports full-frame, so to use a sensor any smaller than full-frame is potentially throwing away good light. If you really want wide field and the best light collection then at the 900mm native focal length use a full-frame sensor and at the 0.79x reduced focal length of ~700mm, you could consider potentially using an APS-c sized sensor, depending on what size the reduced image circle shrinks to. Depending on the true image circle size (all we know is that it is supposedly greater than or equal to 43.3mm), it may, with the 0.79x reducer fitted, reduce to a minimum of 79% of that (34.2mm), which would clearly be more supportive of APS-c sized sensors, given the potential for vignetting/light fall-off* with the reducer fitted.

As a ballpark calculation using a full-frame sensor it should be possible to achieve the following image scales at the stated focal lengths:
Full-frame sensors at ~900mm = ~1.1 to 1.4 arcseconds/pixel (4.9 to 6 µm pixels)
Full-frame sensors at ~700mm* = ~1.5 to 1.8 arcseconds/pixel( 4.9 to 6 µm pixels)
With APSc sizes the image scale should be smaller still (~33% lower) , given the reduction in the field of view. An APSc sensor would potentially therefore put the image scale below/around 1 arcseconds/pixel, probably on the too low side for all but the very best seeing.

Long story short / minimum effort: You could try a full frame DSLR if you already have one

Best
JA

[Camelopardalis]

I’d speculate that the 294 would be pretty competent well over 1m. The high conversion gain mode, deep wells and good dynamic range should make it pretty versatile. You’d probably want confirmation from someone who owns one that they calibrate correctly. Horrors stories and all (that may be just that).

One potential advantage to the QHY version is the potential to switch it into 47MP mode for shorter focal lengths. Apparently, the quad-Bayer array means that each 4.6um pixel is actually 2x2 2.3um pixels. (Or should that be 4.8 and 2.4?) and can be read as such. A trick the ZWO model doesn’t do.

[rustigsmed]

Great minds Glen, great minds :thumbsup:

[glend]

Progress is being made, at least in relation to scope choice, and I have decided that I will be adopting a two scope approach to EAA. Conventional wisdom says an effective focal length of 400-600mm at a good speed is ideal for EAA, this is ideal for wide field views. However, it does rather poorly on smaller objects where longer focal length is desired (albeit speed is still important). It was pointed out to me that piggy-backing a smaller fast refractors on the top of a longer scope, has the potential to solve the reach issue. It complicates a single camera choice to some extent, as pixel size has to be kept in mind.
I have now ordered the longer focal length base scope (and reducer), and I will hold off on revealing that until it arrives. I believe I have an ideal candidate for the shorter focal length top mounted scope, and really there are many choices, but there is more work to do there. The invisioned double stack will not trouble my observatory pier mounted CGX.
Where I do have problems is in camera choice, the simple reason being there is very little stock of what I see as preferred cameras available in the near term. That's it for now.

[Camelopardalis]

:question:

;)

[glend]

An update on the TS152 f5.9 Modified Achro, I had sent a question to the TS product specialist asking about the use of a reducer on that scope. The reply came in today, which was yes, a 0,8x reducer can be used and would not impact the Image circle up to 42mm. They link in a specific adjustable ED reducer corrector for that scope. That would speed that scope up to just under f4.8. However, it does not mention what the SPOT diagram would look like.

https://www.teleskop-express.de/shop/product_info.php/info/p12210_TS-Optics-REFRACTOR-0-8x-corrector-for-refractors-from-102-mm-aperture---ADJUSTABLE.html

[glend]

I promised an update on my purchase decision supporting a two scope (long and short fl) strategy for EAA, and here it is:

I decided to go with a Skywatcher ED150DX as the long focal length scope. While native f8, or 1200mm fl, I will be running it initially at f(6,8), or 1020mm fl, with the (0.85x Skywatcher reducer) but anticipate that I can get it down to f(6,3) with a TS reducer, or 948mm fl.
Why this particular scope, simply the Black Friday Special price I was able to get it for was a big factor. The DX, Esprit 3" focuser equipped model, was a big savings over the previous list price for the 2" focuser standard model.
It will be used both visually initially, and EAA eventually (when I sort out my ideal EAA camera), although my Nikon D5300 can shoot video now.

For the short fl scope I am aiming at a 500mm refractor, likely the new Evolux 82mm f6,4 at 530mm native focal length with potential for reduction. The Evolux can easily piggy back on the ED150DX, so they can both live on the CGX in the observatory.
That is all for now.

[Camelopardalis]

Nice one Glen :thumbsup: that’s a beast of a scope and should do well.

Look forward to hearing how you get on with it.

[rustigsmed]

yes,congrats on the purchase glen, it will actually also be quite good natively if you wanted to view some planets as well, think you have most bases covered :thumbsup:

[ChrisV]

Sounds interesting Glen. Look forward to seeing some screenshots of your live stacked images (post-processing absolutely forbidden...)

Cheers Chris