Achromat Refractors... ?

[Stu1smartcookie]

Hi , I am seriously looking at buying an Evostar 102 f10 refractor ... the thing is ... i don't know why ?

let me explain . I currently only own one scope ... a fairly small SCT (127mm ) which i absolutely love and gives great albeit quite small views of the planets and moon .. and works quite well for photography of the brighter stuff in the sky . So , what would i gain with this 102mm ? Will i be able to see more Deep sky stuff ? I presume it may have a wider field of view than the SCT but by how much ? Will the Views of the planets be sharper ? Will CA be a problem , ( i know its an issue with shorter achromats) 

What i dont want to do is buy a scope and then realise iv'e bought something too similar to what i already have (viewing wise) . Of course the refractor will have an advantage of being ready to use a lot quicker than my SCT regarding cooling down  , so thats one plus point ! 

Also , There must be a market out there for these scopes ... but people tend to go for the more expensive Apochromats now .. Has the Achromat had its day ? ... personally i hope not

Any thoughts would be most welcome .

 

Stu 

[fifeskies]

The f10 you are considering is fairly close in performance to your SCT.  (I have guessed it might be a Skymax 127 for comparison.)

It would be better to go for a shorter tube to get a much wider FOV , there will be slightly more CA with this of course but not enough to be an issue in my opinion especially with wider DSO targets.

 

I have added a photo showing your existing SCT in orange , the f10 you mentioned in green , and a f5 tube scope in blue.

I would suggest the wider field is a better second scope. For photography the f5 will allow shorter exposure times as well.

 

Consider obtaining a second hand ED scope instead , much better performance for a similar price

(and available , new scopes are like hens teeth at the moment)

A good 80mm ED is a great photography scope. Quality of image even for visual makes the ED worth the extra effort to obtain.

 

Shown for Pleiades with a 10mm eyepiece.

 

 

Edited November 12, 2020 by fifeskies

[Stu1smartcookie]

Thanks for the answer ... i have to say i was expecting it to be quite similar , hence the post in the first place . I like the look of the Bresser 120/635 and the startravel 120T ... good aperture for a refractor . It seems i am destined along that path . I want a refractor rather than a reflector for portability . 

[fifeskies]

Both are good options

Both are out of stock almost everywhere at the moment however, I think there are a few SW about , but the Bresser is unlikely to be in stock anywhere unless you are very lucky.

Again , for that price you can get a much nicer 2nd hand ED, bought on here from a fellow amateur you will get almost new quality.

[Stu1smartcookie]

The bresser is in stock at one of the suppliers but its a lot more expensive than the SW ... have to say though , the quality of bresser is generally better than SW ( disclaimer = IMO). 

An ED is the way to go for imaging ... ive owned WO scopes that are superb . The little sw 72ed is not bad , but i am more visual than imaging and would prefer the additional apperture . 

[fifeskies]

Bresser scopes are more expensive but you are more likely to get a good one with Bresser.

The skywatchers can be just as good if you get a good one , but quality control from SW can be a bit erratic.

 

Bresser and your SCT compared on Bodes galaxy , you gain a much wider view.

[Stu1smartcookie]

Yes i've heard of quality issues with SW ... but they are good generally . I  (along with others) had an issue with the dedicated SW flattener which was well documented . 

Looking at the bresser again ... its almost £ 100 more than the SW ! ... got to sort out my book of excuses to my long suffering wife as to why i NEED this ... er , NOW ...lol 

[vlaiv]

I know why - the same reason I bought it

We simply want to have one to try it out . I have now total of 5 scopes - one of them being Evostar 102.

My initial thoughts were to test it out as beginner - all rounder scope. Here is how I reasoned:

- It's achromatic refractor and it will have some chromatic aberration but due to slow F/ratio - it will not be as bad (had ST102 earlier). With Baader Contrast Booster filter - most of CA will not be noticeable on planets.

This is confirmed - it is in fact good scope for the Moon without too much CA. Unfortunately, I did not get to test it on other planets so far (had it for half a year now). Baader Contrast Booster helps and makes image virtually color free - except on bright stars. In fact, I was expecting better performance on stars and worse on the Moon - but it turned the other way around. There is slight pale blue haze around bright stars and virtually no color around the Moon.

- It can be used as wide - ish field scope - compared to other scopes I own. Except for my 80mm f/6 Apo - this is shortest focal length scope I own (or rather - it is second shortest FL scope that I own) - other FLs being 1200mm, 1300mm and 1600mm.

With 2" Eyepiece I expect to fit almost whole Andromeda into FOV. Longer FL will have benefit over shorter achromats - less CA and darker light polluted sky due to exit pupil.

Here is what you can expect from widest EP:

I used TV plossl 55mm as it has max field stop for 2", but there are other eyepieces that have similarly large field stop so you don't necessarily need to use that eyepiece. 38mm 70° eyepiece for example will show similar piece of the sky

- It is relatively light weight so it can be easily mounted.

This is where I was somewhat wrong - it is a long scope and I don't particularly like how it sits on my AZ-4. It is wobbly scope on that mount and I think I'll have to upgrade to SkyTee2 or similar.

- It can be used for astro photography.

Most people dread idea of using achromat for AP - but it will work if done properly. Here is what I wanted to try out:

Take this scope and use x0.67 reducer to reduce it's focal length to ~670mm (could be even more - depending on sensor to reducer size). I have such focal reducer already.

Second would be to create aperture mask of about 80mm and/or use filter to filter out offending chromatic aberration. This will be enough to get one started in AP. I'm also planing to change focuser on mine as I have leftover Monorail 2" M90 that was stock focuser on my RC8".

I already did some AP with ST102 with aperture masks and filters, so I know that I can get color free result like this:

Excuse the lack of flats on this 3x3 mosaic. I used guide / planetary camera to do this image - hence the need for mosaic. This was on 500mm with 66mm of aperture.

In the end, I got it to serve as both white light solar with Herschel wedge (all my other scopes except for small AP are reflectors) and solar Ha scope one day when I get Daystar combo quark to go with it.

So you see - it is versatile scope and I think it can be very good beginner all rounder.

[Peter_D]

Why not look for a used Achromat? Here in Germany, used astro gear is generally quite expensive but I've bought a couple of Vixen achromats for good prices that are excellent performers (a little 60mm Tasco branded 9VR and an A81M). One advantage of the later is it has a 2 inch focuser - like the Evostar 102.

[Stu1smartcookie]

Vlaiv  many thanks for that detailed post ... its clear that scopes like this still have a place , If i'm honest i actually like the look of the scope ... its impressive and neat ... whether it beats my omni xlt 127 on views its another matter . 

The shorter wide field scopes are attractive to me also , because of that wider field of view ... but , i am still looking at the evostar and thinking .... maybe ... whether  Mrs Stu sees it that way is another matter lol 

One thing you mentioned was the use of wide EP's ... the widest i have is still only 25mm . I've read time and time that good quality EP's are so important ( i suppose its a no brainer ) but its something that i have neglected !! 

As for the photography side of things ... i use  a driven CG4 ( stainless steel tripod) which is very stable ... I took an image of M45 the other night with the XLT 127 and a canon 100d .. i was really surprised how good it was , especially as M45 was only just clearing our roof . At least with the evostar , looking at your FOV picture i could  get  targets fully into  view . 

I don't know ... its all a bit of a minefield ... lol . Great isnt't it  

[Stu1smartcookie]

1 hour ago, Peter_D said:

Why not look for a used Achromat? Here in Germany, used astro gear is generally quite expensive but I've bought a couple of Vixen achromats for good prices that are excellent performers (a little 60mm Tasco branded 9VR and an A81M). One advantage of the later is it has a 2 inch focuser - like the Evostar 102.

The one i'm looking at is from another forum member so , yep , you are right . A 2" focuser is paramount as far as i'm concerned .