Hartman Mask

[ballaratdragons]

I attemped to use the factory supplied Hartman Mask but the whole is too small (51mm) for effective views. At this ratio on a 12" I think it works out to be an f23.

Does anyone know the formula to work out the best size hole for a 12" f5 for planetary viewing probably around f8 or f10 (or know the size hole I need). :)

[Starkler]

Ken theres no need to mask it at all. By masking down your scope your throwing away resolving power.

[janoskiss]

f8: 7.5 " ~= 188 mm
f10: 6.0 " ~= 150 mm
fx: 60/x " ~= 1500/x mm

With f8, the spider vanes might get in the way, which would defeat the purpose of using a mask.

[janoskiss]

Geoff, the mask will get rid of diffraction spikes from spider & help see the moon without making you go blind.

[ballaratdragons]

Thanks guys, I won't bother masking. Just trying to get better planetary views.

The GS factory one is useless. Don't know why they bother supplying it. At f23 even the moon is too dark.

<b>EDIT: </b>Sorry guys, just found out why moon was too dim at f23. I just packed my EP's away for the night and the 15mm I used still had the ND filter attached. OOPS!

[westsky]

Hi Ken, A Hartman mask is really for focusing a camera the hole size doesn't matter for this.An aperture mask will help as Janos said but should be larger than 50% of the main apeture so you could take it to 6" and get some better results.

David.

[ballaratdragons]

Thanks David,

I'll give it a try and see what results I get.

[ving]

my aperture mask is 80mm bringing mine to f15. it fits nicely in between the spdier vains so there is no obstruction. I always use it for my doubles, planets and moon observing as it gives better contrast on bright objects

[ballaratdragons]

Maybe because it is blocking a lot of stray light???

[ausastronomer]

Over time you get used to not seeing the diffraction spikes, you basically look straight past them. I don't have a problem with them. Something else to check is that your spider vanes are perfectly square to the optical axis of the tube. If they are canted or buckled at all this can greatly exaggerate the diffraction spikes and also create diffraction noise which will rob you of a lot of planetary detail. Another option is a simple off axis mask, but I prefer the brighter and more detailed views complete with invisible diffraction spikes, of full aperture.

CS-John B

[ausastronomer]

Something not quite right with your scope if thats the case. If we were talking Andrews 20" which uses a 7" off axis mask I would agree as the 7" unobstructed aperture has plenty of resolving power and will cut through the seeing a lot better than 20" of aperture if the seeing is poor. With an 8" scope full aperture should beat the masked view almost all the time.

Maybe your scope is not quite collimated correctly ? Are you using just a laser to collimate ? If not how are you collimating it? Have you centred the secondary under the focuser and squared the focuser to the optical axis ?

Have you star tested the scope ? What is the star test like ? Is any astigmatism present, the GS scopes are notorius for having overtight secondary holders which introduce astigmatism and this will affect your full aperture views.

I would be looking for another solution to the problem than an off-axis mask with an 8" scope.

CS- John B

[ving]

I use a cheshire for colimation. I star test it regularly. admittedly I use the aperture mask for double splitting mainly, but I do find that it does cut down on really glarey objects under some conditions. maybe its the seeing in my area?
theres no astigmatism that i can see.

[ausastronomer]

David,

Seeing shouldn't be that bad all the time. maybe its just a personal preference thing where you prefer the dimmer views of the stopped down scope.

CS-John B

[ving]

maybe... i dont know.
go figure hey...