Jump to content

Banner.jpg.b89429c566825f6ab32bcafbada449c9.jpg

Inexpensive f5 refractor improved by cardboard aperture mask


Mark at Beaufort

Recommended Posts

I recently bought a 80mm f5 Achro refractor from Opticstar mainly as a cheap travel scope. Although it can give very good views at low magnification when you start to increase the mag to 40x or above it becomes very soft.

I decided to make an aperture mask to fit over the objective to see if things could be improved. The mask which has a diameter just under 60mm changes the focal length from f5 to about f6.7.

What a difference!!

Last night I viewed the Comet and the whole of the Kemble Cascade with a 21mm Ethos (19x and a 5 degree FOV - no mask) and then inserted the mask to view a few doubles. Using the 6.5mm - 19.5mm Pentax zoom plus a Baader Hyperion barlow I was able to increase the mag to 138x and could split the double double in Lyra.

Today I inserted the Lunt Herschel Wedge, with the mask in place, and have had great views of the Sun at about 40x up to 60x with no softness at all. In fact I can see quite a bit of granulation.

So even though this scope cost less than £150 a simple piece of cardboard has made this little scops so much better.

post-1628-0-65578200-1408376641_thumb.jp

post-1628-0-86356500-1408376659_thumb.jp

post-1628-0-11881700-1408376679_thumb.jp

post-1628-0-58337200-1408378593_thumb.jp

Link to comment
Share on other sites

  • Replies 29
  • Created
  • Last Reply

Wouldn't buying the 80mm f/7.5 one instead have been a better option?

Yes in hindsight it would have been a better purchase. However, I wanted a very small length inexpensive scope to fit into my hand luggage. The f5 scope is only 310mm in length and costs less than £150. The f7.5 version comes with a dual focuser and is 510mm in length and costs £80 more. Based on this info I went for the f5 scope. I would add that this scope is very well made and has good metal baffles (not plastic) so great for solar viewing with a Herschel Wedge.

I was surprised that the f5 version had such a soft view with magnification of 30x or more.

With this piece of cardboard aperture mask I now have the best of both worlds - nice FOV with a 25mm plossl and reasonable mag with the Pentax zoom. I won't be taking the Ethos EPs on holiday!!

Link to comment
Share on other sites

e7ezubyd.jpg

I'm thinking of buying this new scope from opticstar.

I have exactly the same scope as above (80mm f5) and I've masked it too but only the solar film. As said it provides a sharper image as it's not very good at anything over 30x

But in masking we're losing detail? So with this in mind the new opticstar is 102mm aperture and a f6.7..........it's only £155. This combined with a lunt wedge should better it?

Link to comment
Share on other sites

That's a cracking scope. I'd use a semi apo filter with it. Stopping down the aperture will improve contrast and not lose your detail. There are some super scopes from Opticstar.

I use the 80mm Opticstar and am surprised to hear of softness at the higher end. I had no problems bouncing up the magnification to x80.

Gives spectacular views using a 2" diagonal, Baader semi apo and wide eps. Loves Hyperions and my 15mm SW Panorama.

Going to try a stop at the end of the dew shield,

Nick.

Link to comment
Share on other sites

e7ezubyd.jpg

I'm thinking of buying this new scope from opticstar.

I have exactly the same scope as above (80mm f5) and I've masked it too but only the solar film. As said it provides a sharper image as it's not very good at anything over 30x

But in masking we're losing detail? So with this in mind the new opticstar is 102mm aperture and a f6.7..........it's only £155. This combined with a lunt wedge should better it?

These new Achro scopes look very reasonable - I don't think they were on display when I visited the shop in Sale (Manchester) a few months back. If you are going to purchase this 102mm scope and use it with your Lunt HW I would make certain that the baffles are made of metal and not plastic. The Guys in the shop are very knowledgeable and the shop has lots of scopes and accessories to view - well worth a visit.

Link to comment
Share on other sites

Decided to make better aperture masks - one of 60mm and the other 50mm.

Set the Opticstar and PST together and I even used the 8mm Ethos to give me 50x.

Not a huge amount of activity but the image in the 80mm at f8 is very sharp with the 8mm Ethos.

Nice to have white light and Ha in a portable package.

post-1628-0-15259600-1408443450_thumb.jp

post-1628-0-91137200-1408443469_thumb.jp

Link to comment
Share on other sites

So what does this if by making different masks? Is it not best to have just one happy medium?

Because I had lots of black sided cardboard I just decided to make two masks to see if there was any difference between f6.6 and f8 whilst viewing the Sun. To be honest my first attempt (First thread photos) was not very good although it worked. Is there a difference - well not really other than both masks produce a sharp image. It will be interesting to view the Moon or Planets.

Pleased that the 102mm Frac has metal baffles - seems a very good buy.

Link to comment
Share on other sites

Very interesting report and result Mark.

Personally I have some doubts over the quality control of some of the Optistar scopes. Neil English had one, the same model as yours I think, and found that the objective lens in that was poor optically. He swapped it for the objective lens from a Skywatcher ST80 and found a significant improvement.

It's usually the outer portions of these lenses where the figuring errors occur so stopping them down as you have often does deliver quite startling improvements. Some of the less reputable manufacturers actually stop the scopes down themselves by using an overlong drawtube on the focuser. Unless you actually measure your effective aperture you would not be aware of this apart from slightly dimmer views of DSO's if you compare the scopes to others of similar aperture that are not stopped down. CA and overall correction would be improved of course so you could be quite happy although you could argue that you are not quite getting what you paid for.

The newer Optistar models (as in Skemobserver's link) seem to be re-branded Explore Scientific or Bresser models so may have better quality control.

Link to comment
Share on other sites

Forgive my lack of knowledge here...but aren't you effectively decreasing the aperture by masking a bit of it off, so you now effectively have a 60mm f/6.7 scope, not 80mm? If that's the case, would a "true" 60mm scope not be significantly lighter?

Sent from my iPhone using Tapatalk

Link to comment
Share on other sites

Forgive my lack of knowledge here...but aren't you effectively decreasing the aperture by masking a bit of it off, so you now effectively have a 60mm f/6.7 scope, not 80mm? If that's the case, would a "true" 60mm scope not be significantly lighter?

Sent from my iPhone using Tapatalk

Quite true but I guess you work with the scope you have :smiley:

You could argue that it's going to possible that a low cost 60mm will have similar issues and would need masking down to 50mm or 40mm.

Link to comment
Share on other sites

Fair enough I guess. My f/11 80mm has a cap with a hole in it to stop it down, but it doesn't seem to be needed to improve the views, guess I got lucky (or f/11 is low enough already!)

Sent from my iPhone using Tapatalk

Link to comment
Share on other sites

Fair enough I guess. My f/11 80mm has a cap with a hole in it to stop it down, but it doesn't seem to be needed to improve the views, guess I got lucky (or f/11 is low enough already!)

Sent from my iPhone using Tapatalk

Making an accurate F/11 objective lens is easier than making an F/5 one of the same aperture. At F/11 chromatic aberration will be much less in evidence anyway because of the nature of the achromat design.

Link to comment
Share on other sites

Interesting random table: 

http://www.cityastronomy.com/CA-ratio-chart-achro.jpg

As noted above, the problem may not just be false colour though... :p

But achromats have gone up in my estimation. Still thinking of "doing up" my

ST102 at some stage. Experimenting with narrow band filters even. Seen quite 

good H-Alpha images obtained with f/3 to f/4 60-80mm resp. "super-finders" even. :)

Wider fields with small (video) chips etc. Maybe white light solar with a continuum filter...  

Link to comment
Share on other sites

Interesting random table: 

http://www.cityastronomy.com/CA-ratio-chart-achro.jpg

As noted above, the problem may not just be false colour though... :p

Yep, CA is a fact regardless of how well the achromat optics are figured. With lower quality figuring comes spherical over or under correction and other issues. Some of the low cost fast achromats will have good objectives though, but some won't .....

Link to comment
Share on other sites

Please find attached a CA calculator for doublets achromats in excel, I wrote it as a quick reference, you can change the objective size and focal length to give a CA index figure, you are looking for over 3 for a good result and over 5 for a great one. As you can see as the objective increases so must the f number (not just the focal length) to achieve the same result.

NB green is good : orange ok : red bad

CA Index Calc (L.Bale).xlsx

Link to comment
Share on other sites

Making an accurate F/11 objective lens is easier than making an F/5 one of the same aperture. At F/11 chromatic aberration will be much less in evidence anyway because of the nature of the achromat design.

Ok, I guess I didn't realise that the objective lens is actually different.  Do you mean it's a lower quality lens?  Likely to have more imperfections?  If I was to take my existing telescope and a grinder and cut the barrel down to 400mm, making it into an f/5, would it simply not work?  (Newbie questions, sorry!).

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.